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39C o u n t e r m e a s u r e s

Chapter 6

An Overview ofEmerging Missile State Countermeasures

As we discuss in the previous chapter, the five originalnuclear weapon states have in the past invested sub-stantial effort and money in developing countermea-sures to ballistic missile defenses and continue to doso. However, the question is often raised whetheremerging missile states will have both the capabilityand incentive to deploy effective countermeasures tothe US NMD system.

Some argue that the deployment of a US nationalmissile defense will deter the development and deploy-ment of missiles by emerging missile states because itwould cast doubt on the effectiveness of such weap-ons.1 This is only plausible if the steps emerging mis-sile states could take to counter the defense were tech-nically difficult or prohibitively expensive relative toacquiring ballistic missiles in the first place. As we dis-cuss in this and subsequent chapters, this is simply notthe case. Thus, if the United States deploys a nationalmissile defense, it must expect that any adversaries in-terested in acquiring long-range ballistic missiles willcontinue to do so, and that countries that have acquired(at considerable expense and effort) long-range ballis-tic missiles to threaten the United States would alsotake steps to counter the defense by deploying counter-measures.

Any country that has both the technical capabilityand the motivation to build and potentially use long-range ballistic missiles would also have the technicalcapability and motivation to build and deploy counter-measures that would make those missiles useful inthe presence of the planned US NMD system. More-over, it must be assumed that a country that is develop-ing long-range missiles with the intent of using or

threatening to use them would have a parallel programto develop countermeasures.2 This is especially true inthe current environment in which the US plan to buildan NMD system is headline news.

The 1999 National Intelligence Estimate (NIE) ofthe ballistic missile threat to the United States, whichwas prepared by the US intelligence community,reached the same conclusions, stating that3

• “We assess that countries developing ballisticmissiles would also develop various responses toUS theater and national defenses. Russia andChina each have developed numerous counter-measures and probably are willing to sell therequisite technologies.

• “Many countries, such as North Korea, Iran, andIraq, probably would rely initially on readilyavailable technology—including separating RVs[reentry vehicles], spin-stabilized RVs, RVreorientation, radar absorbing material (RAM),booster fragmentation, low-power jammers,chaff, and simple (balloon) decoys—to developpenetration aids and countermeasures.”

• “These countries could develop countermeasuresbased on these technologies by the time theyflight test their missiles.”

1 BMDO Fact Sheet, “National Missile Defense Program,”no. JN-99-05, March 1999, p. 2, available online atwww.acq.osd.mil/BMDO/bmdolink/pdf/jn9905.pdf.

2 If a country purchases a long-range ballistic missile ratherthan developing its own, the United States must assume thatthe country selling the missile would be willing to sellcountermeasures as well.3 National Intelligence Council, “National IntelligenceEstimate (NIE): Foreign Missile Development and theBallistic Missile Threat to the United States Through 2015,”unclassified summary, September 1999.

40 C o u n t e r m e a s u r e s

In this chapter, we provide an overview of coun-termeasures to the planned NMD system that would beavailable to an emerging missile state capable of de-ploying a long-range ballistic missile. Most of thesecountermeasures would be useful against any defensethat used exoatmospheric hit-to-kill interceptors.

Some of the countermeasures discussed belowwould be effective for an attack using one missile, whereothers would be most effective if the attack involvedmore than one missile. As we discussed in Chapter 1,we will consider a limited attack of tens of missiles.

Some of the countermeasures we discuss in thischapter would be effective against one type of sensorbut not against all of the planned NMD sensors. (Thefull defense will include ground-based radars that op-erate in the X-band and the UHF band, and satellite-based infrared and visible sensors. In addition, the killvehicle will use visible and infrared sensors to homeon its target. See Chapter 3 for more details.) We donot limit this discussion to countermeasures that areeffective against the full suite of planned sensors fortwo reasons: different countermeasures can be com-bined together into packages that would be effectiveagainst all the sensors, and there are situations in whichdefeating only one type of sensor will defeat the defense.

We do not claim that the discussion here is com-prehensive in that it includes all of the countermea-sures that are both useful to an attacker seeking to pen-etrate the planned NMD and feasible for an emergingmissile state to implement. Rather, this chapter is in-tended to give an idea of the range of techniques thatmight be employed by an emerging missile state seek-ing to defeat an exoatmospheric ballistic missile de-fense and to suggest those that might be most promis-ing from the perspective of an attacker.

In the following chapters, we will focus on three ofthese countermeasures in much greater detail. Thesethree countermeasures were chosen because they ap-pear to combine high effectiveness against the plannedNMD system with ease of deployment. For this rea-son, we will use the examples discussed in the nextthree chapters as a baseline threat for assessing the likelyoperational effectiveness of the NMD system. We be-lieve the administration and Congress should also takethese examples into account in their assessment of thesystem’s effectiveness.

To best structure the discussion in this chapter, wegroup the countermeasures according to the generalstrategy they employ to defeat the defense. We discusseach countermeasure in more detail in the rest of the

chapter, but first describe them briefly here. An emerg-ing missile state could

• Overwhelm the defense by deploying too manyreal targets for the defense to intercept. For anemerging missile state, this strategy is feasiblefor chemical or biological weapons delivered bysubmunitions.

• Overwhelm the defense by deploying too manyfalse targets, or decoys, for the defense tointercept. The decoys are designed so the defensesensors are unable to discriminate them from thereal warheads. There are several classes ofdecoys: (1) replica decoys, which replicate thewarhead as closely as possible; (2) decoys usingsignature diversity, where the decoys are made toappear slightly different from each other and thewarhead; and (3) decoys using anti-simulation, inwhich the warhead itself is disguised to mimic adecoy. Using anti-simulation, the attacker candisguise the warhead in several ways: forexample, by enclosing it in a radar-reflectingballoon, by covering it with a shroud made ofmultilayer insulation, by hiding it in a cloud ofchaff, by using electronic decoys, or by usinginfrared jammers (e.g., flares).

• Reduce the radar signature of the warhead. Doingso could reduce the range at which defense radarscould detect the warhead and thus reduce thetime available to the defense, and could makeother countermeasures more effective.

• Prevent hit-to-kill homing by the kill vehicle, ormake it more difficult, by reducing the infraredsignature of the warhead. Doing so would reducethe range at which the infrared sensors on the killvehicle could detect the warhead, leaving it lesstime to change course in order to hit the warhead.The attacker could reduce the infrared signatureof the warhead by covering it with a low-emissivity coating or by using a shroud cooled tolow temperatures by liquid nitrogen.

• Prevent hit-to-kill homing by hiding the exactlocation of the warhead. The attacker could hidethe warhead by enclosing it in a very largemetallized balloon or in one of a large number ofsmaller balloons tethered together. Doing sowould prevent the defense sensors from


determining the location of the warhead, in whichcase the kill vehicle could only hit it by chance.

• Prevent hit-to-kill homing by making thewarhead maneuver.

• Launch preemptive attacks on ground-basedcomponents of the defense system using cruisemissiles or short-range ship-launched missiles,small airplanes, or special operations forces.

Overwhelming the Defense: Submunitionsfor Biological and Chemical WeaponsHere, the goal of the attacker is simply to present thedefense with so many real targets that it is unable tointercept them all.

For missiles armed with biological or chemicalwarheads, an attacker can defeat a limited missile de-fense simply by packaging the biological or chemicalagent in up to more than one hundred small warheads—called submunitions—rather than in one large unitarywarhead. If we assume that an emerging missile statehas only five long-range missiles, an attack could eas-ily involve 500 submunitions. In this case, even if thedefense expended all 250 of its interceptors, it could atbest intercept half of the incoming submunitions, andthus reduce the amount of agent that reached the groundby a factor of two. However, doing so would not nec-essarily reduce the number of people killed or injuredby a factor of two.

Using submunitions would not only overwhelm thedefense, but would be a more effective way of dispers-ing the agent. Therefore an attacker would have a strongincentive to use submunitions to deliver these agentseven in the absence of missile defenses. The use of sub-munitions to deliver chemical or biological agents isdiscussed in more detail in Chapter 7.

Since nuclear warheads cannot be subdivided intoarbitrarily small parts, this strategy cannot be used formissiles carrying nuclear warheads. In this case, themost straightforward response to a limited defense de-ployment would be to deploy large numbers of war-heads to overwhelm it. This could be done either bydeploying a large number of missiles with single war-heads or by deploying a smaller number of missileswith several warheads per missile. As discussed inChapter 5, the United States, Russia, Britain, and Francehave all deployed multiple warhead missiles, largelymotivated by concerns about the potential deploymentof Soviet or US strategic missile defenses.

However, an emerging missile state is unlikely tobe able to use this strategy to overwhelm the defense.Such states would have a limited capability to producethe fissile material needed for nuclear warheads, andtheir nuclear arsenals would thus likely consist of asmall number of warheads. Moreover, deploying a largenumber of intercontinental missiles—whether theycarry one or more nuclear warheads—would be a rela-tively expensive way of overwhelming a limited de-fense and may well be beyond the financial means ofany of the emerging missile states.

Instead, an emerging missile state seeking to de-liver a nuclear weapon via long-range missile wouldlikely conclude that deploying a relatively small mis-sile force with countermeasures is a more feasible andcost-effective approach to defeating a limited NMD sys-tem. Decoy warheads are one type of countermeasurethat also relies on overwhelming the defense; we dis-cuss these next.

Decoys: Overwhelming the Defense withFalse TargetsOne important class of countermeasures uses a largenumber of decoys, or false targets, that the defense sen-sors cannot discriminate from the nuclear warhead. Thedefense then has to shoot at all the targets—real andsimulated—to avoid letting the nuclear warhead pen-etrate unchallenged. But a limited defense would sim-ply run out of interceptors if the attacker uses enoughdecoys.

As discussed in Chapter 3, the defense plans to firemultiple interceptors at each target to achieve a highprobability of intercepting the warhead. If time per-mits, the defense plans to use a “shoot-look-shoot” strat-egy in which it will fire one or more interceptors, as-sess whether the target was intercepted, and then, ifnecessary, fire additional interceptors. The final sys-tem planned for deployment would have up to 250 in-terceptors deployed at two sites—one in Alaska andone in North Dakota.

To avoid wasting interceptors (and potentially run-ning out of them), the planned NMD intends to dis-criminate decoys from warheads. However, even if itssensors are not able to discriminate the warheads fromthe decoys, the defense could still choose to fire all itsinterceptors to intercept as many of the incoming ob-jects as possible. In this way, the defense would havesome chance of intercepting the warhead and prevent-ing any damage on the ground. However, the effec-tiveness of the defense system would be greatly reduced


if the attacker deploys a large number of decoys. Forexample, if an emerging missile state with only tenmissiles deploys a total of two nuclear warheads and500 decoys, a defense with 250 interceptors will haveless than a 50 percent chance of intercepting each war-head, and less than a 25 percent chance of interceptingboth warheads. Thus, the attacker will have at least a75 percent probability of getting a warhead through thedefense.

Of course, the defense might not want to use all itsinterceptors at once, but would likely reserve some forlater use. For example, the defense might be concernedthat the ten missiles launched were only carrying de-coys, and that the attacker would launch more missileswith nuclear warheads a short while later. If the de-fense had launched all its interceptors against the firstten missiles, there would be none left to intercept thenuclear warheads on the remaining missiles.

Decoys are a particularly attractive strategy againstexoatmospheric defenses. Decoys designed to defeatan exoatmospheric defense take advantage of the factthat there is no atmospheric drag in the vacuum of space,so that lightweight objects travel on trajectories identi-cal to that of a much heavier warhead. Because the de-coys can be lightweight, the attacker can use a largenumber of them. (Because both the size and range of amissile depends on the weight of the payload it is car-rying, there is in general an incentive to limit the pay-load weight to achieve a greater range and/or to limitthe overall size of the missile.)

As such lightweight decoys and the warhead beginto reenter the atmosphere, the decoys would be sloweddown more rapidly by atmospheric drag, allowing thewarhead to be identified. However, depending on thealtitude at which such slowing and warhead identifica-tion occurs, it might be too late for an above-the-atmo-sphere interceptor to intercept the warhead before itpassed below the interceptor’s minimum intercept alti-tude. Moreover, for attacks against targets far from theinterceptor deployment site, the defense would need tolaunch its interceptors before the lightweight decoyscould be discriminated. This in itself would cause prob-lems for the defense, since it would need to commit itsinterceptors before it knows whether timely discrimi-nation is even possible. The attacker could exploit thisuncertainty by using a mix of lightweight and some-what heavier decoys. In general, the heavier a decoy is,the lower in the atmosphere it would go before the de-fense could discriminate it.4 Moreover, if the defense

has to intercept high within the atmosphere rather thanabove the atmosphere, it would not have time to assesswhether the first interceptors missed the target beforelaunching additional interceptors and would be unableto use its planned “shoot-look-shoot” strategy.

Several different decoy strategies are possible.Below, we discuss three categories of decoys: replicadecoys, decoys using signature diversity, and decoysusing anti-simulation. Although these are presented asdistinct approaches, in actual practice there are likelyto be overlaps between them.

Replica Decoys. Perhaps the most obvious ap-proach would be to deploy large numbers of decoysthat are intended to be indistinguishable in appearancefrom the nuclear warhead (indistinguishable to the de-fense sensors, but not necessarily to the human eye),but are much lighter in weight. Such decoys are knownas replica decoys. If successful, the use of replica de-coys would leave the defense with the choice either offiring at every possible target, which, depending on therelative number of interceptors and decoys, may not bepossible, or of letting the warhead penetrate unchal-lenged. While replica decoys are probably what mostpeople imagine when they think about decoys, they arenot necessarily the most effective decoy approach, aswe discuss below. Figure 6-1 is a photograph of a USreplica decoy that was deployed in the 1960s.

Given the high measurement resolution of the NMDX-band radars, a replica decoy would need to be verysimilar in shape to the warhead and have a similar ra-dar cross section. It might also need to mimic any dy-namical characteristics of the warhead, such as the ro-tation about its axis and any wobbling in this rotation.In order to be effective against SBIRS-low, a replicadecoy would also need to have a similar temperatureand emit a similar amount of infrared energy as thewarhead in the wavelengths used by the defense sen-sors.5 Doing so might require putting a heater in thedecoys.

It should be possible for an emerging missile stateto construct and deploy credible replica decoys that aremuch lighter than a nuclear warhead and that could bedeployed in significant numbers by a long-range bal-listic missile delivering such a warhead. The AmericanPhysical Society’s Directed-Energy Weapons study

where W is the weight, CD is the drag coefficient, and A isthe cross-sectional area.5 The requirement that the decoy emit a similar amount ofinfrared energy means that the product of the decoy’ssurface area and emissivity must be similar to that of thewarhead.

4 The altitude at which discrimination could occur woulddepend on the ballistic coefficient of the decoy, β= W/(CDA),


concluded that such decoys might weigh as little as a“few kilograms including dispensing and erection hard-ware.”6 This figure presumably was an estimate for theSoviet Union, but given the relative simplicity of suchdecoys, this figure seems plausible even for an emerg-ing missile state.

Decoys Using Signature Diversity. A potentialattacker considering the use of replica decoys may beconcerned that the defense will be able to identify andexploit some small observable difference between thewarhead and the decoys. One way to address this issuewould be to modify the decoy strategy to exploit thefact that while the defense might know the general char-acteristics of the warhead, it would not know the exactcharacteristics. Thus, rather than trying to exactly rep-licate the warhead, the decoys would be made to haveslightly different signatures from the warhead and fromeach other. This would prevent the defense from pick-ing out the warhead as the one object that was differentfrom the rest.

For example, the attacker could use cone-shapeddecoys with the same shape as the nuclear warhead,but of slightly varying lengths and nose radii of curva-ture. Such decoys would have slightly different radarcross sections from the warhead and each other.Because they would have the same shape, several ofthese decoys could be stacked over the warhead inside

the nosecone of the missile. Small weights on the innersurface of the cones could be used to control their mo-ments of inertia so that each one would wobble in amanner similar to (but slightly different from) the war-head. The attacker could also diversify the infrared sig-nature of the decoys by using small heaters or, for day-light attacks, different surface coatings that would re-sult in different decoy temperatures.

Decoys Using Anti-simulation. With anti-simu-lation, the attacker takes the deception one step fartherby modifying the appearance of the warhead. Ratherthan making a decoy simulate the warhead, the attackerdisguises the nuclear warhead. By introducing variabil-ity into the warhead appearance, a wide range of decoycharacteristics can be made compatible with those ofthe warhead, thus greatly complicating the decoy dis-crimination problem for the defense. Indeed, when thepossibility of altering the warhead appearance is takeninto account, it is clear that there is no need for thedecoy to resemble a bare warhead at all. The attackercan either use decoys that are similar in appearance tothe disguised warhead, or exploit the advantages of sig-nature diversity by using decoys that vary in appear-ance, differing from the warhead and each other.

Anti-simulation techniques can also be used to de-feat a defense strategy commonly used to deal with largenumbers of potential targets—“bulk filtering.” In thistechnique, objects with characteristics that are a poormatch to those the defense expects the warhead to haveare either not observed because of sensor filters or ob-served very briefly and immediately rejected withoutthe need for a detailed examination. This approach al-lows large numbers of false targets to be screened outrapidly, but is vulnerable to being deceived by anti-simulation techniques. If the attacker disguises the war-head, this could lead the defense to reject the warheaditself as a possible target. The attacker could also de-ploy at least one decoy that would have observed char-acteristics similar to what a bare warhead would have.

The attacker can modify the appearance of thenuclear warhead in many different ways. By changingits shape, the attacker can change the radar cross sec-tion of the warhead as measured by an X-band radar byseveral orders of magnitude. By changing its surfacecoating, the infrared signature of the warhead canchange by more than an order of magnitude. Or, as wediscuss in more detail below, the attacker can disguisethe warhead by enclosing it in a radar-reflecting balloon,by covering it with a shroud made of multilayer insula-tion, by hiding it in a cloud of chaff, or by using elec-tronic radar jammers.

Figure 6-1. Photograph of a US replica decoy for the MARKIV reentry vehicle that was used on some Titan ICBMs.US Air Force photo 113217 USAF, dated 19 October 1961,reprinted from Chuck Hansen, “Swords of Armageddon,”CD-ROM (Sunnyvale, Calif.: Chukelea Publications,undated) Vol. 7, p. 560.

6 American Physical Society Study Group, “Science andTechnology of Directed Energy Weapons,” Reviews ofModern Physics, Vol. 59, no. 3, Part II, July 1987, p. S153.


Metallized Balloons. One anti-simulation strategywould be to enclose the nuclear warhead in a metal-lized mylar balloon, similar to but larger than thosesold at supermarket checkouts. This would be releasedalong with a large number of empty balloons. Becauseradar waves could not pass through the thin metal coat-ing, the radars could not determine what was insideeach balloon. However, a nuclear warhead gives offheat and could thus heat the balloon enclosing it. Toprevent discrimination by infrared sensors, the attackercould control the temperature of each balloon by equip-ping it with a small heater. Alternatively, for attacksduring daylight, the thermal behavior of the balloonscould be controlled by passive means: the attacker couldset the temperature of each balloon by choosing a sur-face coating with a specific solar absorptivity and in-frared emissivity. For attacks during nighttime, the tem-perature of the balloons will not depend on the surfacecoating, but can be varied by varying the shape of eachballoon (see Appendix A).

Although each balloon could be made similar inappearance, it might be even more effective to makeeach balloon different in shape and to design them toachieve a range of different temperatures. In this case,each balloon—including the one with the warhead—would look different to the NMD sensors, and none ofthem would look like a bare warhead. We discuss thismetallized balloon countermeasure in more detail inChapter 8.

Shrouds of Multilayer Insulation. Alternatively,the attacker can conceal the nuclear warhead in a shroudmade of thermal multilayer insulation and release italong with a large number of empty shrouds. Thus, theanti-simulation decoys are simply empty shrouds witha lightweight frame and of a size and shape that couldcover a warhead. The frame could be collapsible (likean umbrella). Alternatively, several decoys could bepacked over a conical warhead, Dixie-cup style, whichwould also avoid crushing the insulation.7

Multilayer insulation consists of many layers ofmetallized plastic (such as aluminized mylar) with verythin spaces between the layers.8 It is a very effective

insulator commonly used to maintain an object at a lowtemperature in a vacuum.9 A shroud made of this ma-terial would effectively conceal the thermal effects ofthe warhead, so that there would be no need to cool (orheat) the warhead to match the temperature of an emptyshroud. Moreover, because radar waves could not pen-etrate the metallic covering of the shrouds, the defenseradars could not determine which shroud containedthe warhead.

To prevent discrimination by the X-band radars,the attacker would also need to prevent the emptyshrouds from behaving differently from the shroudedwarhead. Because the empty shroud may not be rigid,it may begin to wobble or spin around a stable axis.However, the attacker can avoid this behavior by prop-erly weighting the frame to which the insulation isattached.

Chaff. Rather than hiding the nuclear warheadwithin a balloon, the attacker could hide it within a cloudof radar-reflecting chaff strands, while also deployingchaff clouds without warheads. Since the radar wouldnot be able to detect the presence of the warhead withinthe chaff cloud, each of the chaff clouds not containinga warhead would in effect act as a decoy.

A piece of chaff is simply a conducting wire cut toa length that maximizes its radar reflections, which isone-half the radar wavelength. For the planned NMDX-band radars, the appropriate length of a piece of chaffis about 1.5 centimeters (0.6 inches), whereas chaffeffective against the early-warning radars would be0.35 meters (1.1 feet) long.10 Assuming that the warheadhas been properly shaped to reduce its radar cross sec-tion (see Appendix C) and is oriented with respect tothe radar so as to maintain this low radar cross section,each chaff wire would have a radar cross sectioncomparable to that of the warhead.11 Since one pound

7 This is presumably the approach used by a warheadshaped decoy named “Dixie Cup” that was investigated byPhilco-Ford Corporation for the Air Forces in the mid-1960s.See “Filter Center,” Aviation Week and Space Technology,28 November 1966, p. 94.8 The layers are largely prevented from touching oneanother by small plastic spacers at intervals large comparedto the spacer size.

9 The vacuum between any two layers greatly reduces theheat transfer by conduction, and the highly reflectivemetallization reduces the heat transfer by radiation with aneffectiveness that increases geometrically with the numberof layers. Multilayer insulation is punctured with manysmall holes to permit the air to escape quickly in a vacuum.10 In practice, the chaff strands would be cut to a number ofslightly varying lengths to account for the ability of the radarto operate over a span of frequencies.11 The attacker would chose the orientation of the warheadaccording to the location of the defense radars, whichwould be known to the attacker. While this orientationmight not be the optimal one for reentry, since the attackerwould not be trying (nor able) to achieve high accuracy, thisis unlikely to be a serious concern. On some trajectories, itmay be possible for several radars to simultaneously observe


of chaff could contain millions of chaff wires, the at-tacker could deploy numerous small chaff dispensersthat would create many chaff clouds, only one of whichwould contain a warhead. The radar reflections fromthe chaff strands would prevent the X-band and earlywarning radars from determining which cloud containedthe warhead. Because the chaff strands would be spread-ing radially outward from the dispenser, each dispenserwould emit strands continuously over the roughly20 minutes it is traveling through space to maintain ahigh density of chaff strands near the dispenser (wherethe warhead, if there was one, would also be located).

Because chaff clouds would only prevent discrimi-nation by radar, the attacker would need to use othermeans to prevent the SBIRS-low satellite-based infra-red sensors from discriminating the chaff cloud withthe warhead from the empty chaff clouds. One possi-bility would be for the attacker to use flares in eachchaff cloud to generate a large infrared signal that wouldoverwhelm that of the warhead. Or the attacker coulddeploy a plastic balloon, possibly with a small heaterinside each of the chaff clouds that did not contain thewarhead.

Electronic Decoys. Another anti-simulation strat-egy is to drown out the reflected radar signals from thenuclear warhead by placing an electronic radar sourceon the warhead; this technique is known as “jamming.”The decoys would then simply be electronic radar jam-mers without the warhead. Thus, jammers can be usedboth to produce false targets and to disguise thewarhead.

Because modern missile defense radars, such as theplanned X-band radars, can operate anywhere within awide frequency range and can change frequency rap-idly, a simple broad-band jammer (like those used inWorld War II) that would drown out the radar over allthe possible frequencies it could be operating at wouldneed to be very powerful.12 For this reason, the attackeris likely to prefer electronic decoys that return a signal

at the same frequency the radar uses and can thereforebe very low power.

As the 1999 NIE noted, low-power jammers arereadily available technology. Electronic radar jammerscan be made using commercially available transpon-ders to return identical signals from both warheads anddecoys.13 Small antennas on the nose of the warheadand decoys would receive the radar signals sent by thedefense radars; the signals would then be amplified andstretched in time, by a variety of methods, to last some-what longer than the radar signal reflected by the barewarhead, and returned to the radar. The defense radarswould thus receive identical returns from the transpon-ders on the warhead and the decoys, which would over-whelm the smaller signals that are reflected from thewarhead and decoys themselves. The attacker could alsouse signature diversity: by designing each transponderto emit somewhat different signals so that every poten-tial target had somewhat different characteristics, theattacker would prevent the defense from searching forthe one target that is slightly different from all the others.

Because commercially available antennas and am-plifiers have a very wide frequency response,14 the at-tacker would not need to know the precise frequencyof the defense radar, nor could changes of the radarfrequency within its operating range reveal which tar-get is the warhead and which is a decoy. Moreover,antennas of the type needed, particularly “spiral” typeantennas, can be made very small, as small as a centi-meter in diameter. Lightweight electronic decoysweighing no more than a few kilograms could be madeusing such antennas and lightweight amplifiers andpower supplies, allowing large numbers of such decoysto be deployed along with the actual warhead. Becausethe electronic equipment is small, the decoys could alsobe packaged into small conical shapes with relativelyhigh ballistic coefficients. This would permit the de-coys to penetrate deeper into the atmosphere than someother types of lightweight decoys. (See Figure 6-2 fora schematic drawing of a US Navy electronic reentrydecoy.)

Since antennas are available that are essentially iso-tropic in their response over a wide range of angles, thethe warhead from widely different directions; in this case it

might be difficult or impossible for the attacker to shape thewarhead or its shroud so that it simultaneously has a lowradar cross section as viewed by each of the radars.12 For example, consider a radar able to operate over a1 GHz range of frequencies. The jammer would have tospread its energy over this entire band of frequencies. But aradar pulse with a length of 1 µsec (or chain of coherentlyintegrated pulses) would have a bandwidth of only 1 MHz,and only 0.1% of the jammer’s energy output would bewithin this bandwidth.

13 Sherman Frankel, “Defeating Theater Missile DefenseRadars with Active Decoys,’’ Science and Global Security,Volume 6 (1997), pp. 333–355, and Sherman Frankel,“Countermeasures and Theater Missile Defense,” SurfaceWarfare, July 1996, pp 38–40.14 See for example, antenna catalogues from MarconiAerospace Electronic Systems, Inc., 305 Richardson Road,Lansdale, Pennsylvania.


attacker can prevent any nutation and other motions ofthe warhead and decoys about their spin axis from pro-ducing detectable changes in the transponder’s signal.Moreover, by varying their amplification with time, thetransponders could also simulate such nutations elec-tronically. In addition, since modern radars can storeand analyze sequences of signals, to hide any possiblecorrelations between successive return signals, the tran-sponders (including the one on the warhead) could sendback signals that differ from radar pulse to radar pulse.

More generally, the use of modern microchip tech-nology could permit even emerging missile states todeploy a whole new class of “intelligent decoys” thatcould improve on these simple transponder decoys.

These electronic decoys would prevent discrimi-nation by the defense radars. The attacker would needto take additional steps to prevent discrimination bythe SBIRS-low infrared sensors.

Late Deployment of Decoys. When attemptingto defend a country as large as the United States withinterceptors at a few sites, there is a great premium onbeing able to launch interceptors as early as possibleafter the launch of an attacking missile, both to allowthe greatest time for the interceptor to reach its targetand, ideally, to permit firing multiple interceptors atdifferent times in a shoot-look-shoot strategy. Depend-ing on the relative location of the missile launch point,the target against which the missile is launched, andthe interceptor launch site, the attacker could attemptto exploit long interceptor fly-out times by withhold-ing the deployment of decoys until after all the inter-ceptors have been committed. In this case, the defensewould have committed its interceptors before it knewhow many decoys would be deployed and whether it

could discriminate them from the warhead. A NorthKorean attack on Hawaii might be one scenario wherethis tactic could be effective. One disadvantage of thisapproach is that decoy deployment would likely occurin full view of the X-band radars, raising the possibil-ity that the defense could discriminate the decoys byobserving their deployment.

Reducing Radar SignaturesBy reducing the radar signatures of the targets, the at-tacker could decrease the range at which the targetwould be detected by the defense radars, and hence thetime available for the defense to act. This would makethe job of the defense more difficult and could makeother countermeasures possible or more effective. Forexample, an attacker would almost certainly need toreduce the radar cross section of a nuclear warhead ifchaff is to be used to hide the warhead.

The attacker could reduce the radar cross sectionof the nuclear warhead by shaping the reentry vehicle(or a shroud around it) to minimize radar reflectionsback to a radar and/or by using radar-absorbing mate-rial on the surface of the reentry vehicle or shroud. Theattacker might choose to use a shroud if the shape ofthe warhead itself did not make a low radar cross sec-tion easy to achieve. For example, as discussed in Ap-pendix C, the attacker could give the warhead the shapeof a sharply-pointed cone with a rounded back end (acone-sphere), which would reduce its nose-on radarcross section for the X-band radars by a factor of about10,000 relative to a cone with a flat back, to roughly0.0001 square meters. While the radar cross sectionwould be lowest if such a warhead was viewed nose-on by the radar, it would also be significantly reducedover a wide range of angles around nose-on, at least±60 degrees. Thus, the attacker would need to use somedegree of orientation control to keep the warheadpointed in the general direction of the radars, which isfeasible. The 1999 National Intelligence Estimate statedthat “RV reorientation” is a technology that is readilyavailable to emerging missile states.15

Shaping the RV would not be as effective againstthe early warning radars, since their wavelength ofroughly 0.66 meters is comparable to the dimensionsof the warhead. Nevertheless, by using a cone-spherethe attacker could reduce the observed radar crosssection by a factor of ten or more—to roughly 0.01 to0.1 square meters.

15 National Intelligence Council, “NIE: Foreign MissileDevelopment,” p. 16.

Figure 6-2. A schematic drawing of a US Navyelectronic reentry decoy from a Naval SurfaceWeapons Center briefing (1984).


By reducing the warhead’s radar cross section inthis way, the attacker may be able to significantly de-grade the range at which a given radar could detect thewarhead. However, depending on the trajectory of thewarhead, the radar detection range might be limitedmore by the horizon. For some trajectories, the war-head would not rise over the horizon until it was closeenough to the radar that it could be detected with a re-duced radar cross section.

What is likely more significant is that by reducingthe radar cross section of the warhead and decoys, theattacker would degrade the ability of the X-band ra-dars to discriminate different objects from one another.Moreover, the attacker would need to reduce the radarcross section of the warhead to implement other pos-sible countermeasures, such as the use of chaff clouds.

Prevent Hit-to-Kill by Infrared StealthBy reducing the infrared signature of its nuclear war-head, the attacker could reduce the detection range ofboth the SBIRS-low infrared sensors and of the killvehicle’s infrared seeker. Even if the warhead’s infra-red signature could be reduced sufficiently to preventdetection by SBIRS-low infrared sensors, this wouldnot necessarily defeat the defense since the warheadcould still be tracked by the defense radars (and possi-bly by the SBIRS-low visible-light sensor). However,the smaller infrared sensors on the kill vehicle wouldnot have as great a range as those on SBIRS-low, andthe performance of the kill vehicle would depend criti-cally on how much time it has to maneuver to hit itstarget and thus on how far away it can detect the target.By reducing the infrared signature of the warhead, theattacker might be able to reduce the detection range ofthe kill vehicle’s infrared seeker enough so that the killvehicle either could not detect the warhead or did nothave enough time to home on the warhead after detect-ing it. In this case, the defense would fail catastrophi-cally, even if the warhead could be tracked by the de-fense radars and SBIRS-low. We discuss two ways anattacker could reduce the infrared signature of awarhead.

Low-Emissivity Coatings. One way to reduce thesignature of the warhead would be to cover it with alow emissivity coating, since the infrared signature ofthe warhead is determined by its temperature and theproduct of its emissivity and surface area. A warheadcovered with a carbon-based or wood ablative cover-ing would have an infrared emissivity of about 0.9 to0.95, while a warhead with an outer surface of unpol-ished steel would have an emissivity in the range of 0.4

to 0.8. If the warhead was instead covered with a thinpolished gold coating (with an emissivity of about 0.02),its emissivity would be reduced by a factor of about 20to 40.

Since a gold-covered warhead would tend to warmup to well above room temperature in sunlight (seeAppendix A on the thermal behavior of objects inspace), this approach would be best suited to trajecto-ries that were completely or largely in the earth’sshadow. On such nighttime trajectories, a heavy war-head would slowly cool below its initial temperature,which we assume is room temperature (300 K). How-ever, the attacker could reduce the infrared signatureof the warhead even further by instead enclosing thewarhead in a thin, gold-plated balloon that was ther-mally insulated from the warhead (see Chapter 8 for adiscussion of how this could be done). Such a balloonwould quickly cool to nearly its nighttime equilibriumtemperature of about 180 K. If the balloon reached anequilibrium temperature of 200 K, its infrared signa-ture would be further reduced by a factor of about 10(for infrared sensors in the 8 to 12 µm band) to 200 (forsensors in the 3 to 5 µm band) relative to that of a bal-loon at 300 K. Thus, by using this entirely passive ap-proach, an attacker could reduce the infrared signatureof the warhead by a factor of 200–400 (8 to 12 µmband) to 4,000–8,000 (3 to 5 µm band). This wouldcorrespond to a decrease in the kill vehicle detectionrange by a factor of from 14–20 (8 to 12 µm band) to60–90 (3 to 5 µm band), which would significantly re-duce the time available for the kill vehicle to maneuverto hit the warhead.

As we discuss in Chapter 9, the attacker would needto orient the warhead to make sure that earth infraredradiation reflected from the warhead would not reachthe infrared sensor.

Cooled Shroud. Using low emissivity coatingsor passive cooling may not reduce the range at whichthe warhead could be detected enough to prevent thedefense kill vehicle from detecting and homing on thewarhead. The attacker could obtain a much greater re-duction in detection range by enclosing the nuclearwarhead in a cooled shroud. Such a shroud could beisolated from the warhead by commercially availablesuperinsulation material and be cooled by a smallquantity of liquid nitrogen. Cooling the shroud to liq-uid nitrogen temperature (77 K) would reduce theinfrared signature of the warhead by a factor of at leastone million relative to its signature at room tempera-ture.16 The warhead would then be effectively invis-ible to the kill vehicle. Again, the attacker would need


to take care to prevent reflected radiation from reach-ing the infrared sensor on the kill vehicle. This coun-termeasure would work even if the warhead were de-tected and tracked by the defense radars; however, theshroud could also be shaped to reduce its radar crosssection against the X-band radars. This cooled shroudcountermeasure is discussed in more detail in theChapter 9.

Prevent Hit-To-Kill Homing by Hiding theWarheadAnother set of countermeasure strategies would exploitthe fact that a hit-to-kill interceptor must hit its targetdirectly to destroy it.

For example, the attacker could enclose the war-head in a large metallized balloon, with a radius of,say, 5 meters or larger. If the kill radius of the hit-to-kill interceptor is much smaller than the balloon, itwould be unlikely to hit the warhead inside the ballooneven if it hits the balloon itself. In fact, the attacker canmake the kill probability as small as desired by increas-ing the radius of the balloon. The attacker might beconcerned that the balloon itself would be destroyedby the impact of the interceptor (which would dependin part on how the balloon was constructed), thus leav-ing the warhead exposed for a second interceptor tohit. In this case, the attacker could pack additional bal-loons around the warhead to be sequentially inflated astheir predecessors were destroyed.

As another alternative, rather than using a singlelarge balloon, the attacker might use a cluster of per-haps dozens of closely spaced tethered balloons, onlyone of which contains the warhead. These would bespaced closely enough so that SBIRS-low could notassist in discrimination, and if necessary (for example,at night) the balloons without the warhead might con-tain heaters to simulate the heat radiated from the war-head. In this case, each kill vehicle would at best beable to destroy a few of these many balloons, makingsmall the odds of destroying the warhead.

Warhead ManeuversAnother countermeasure strategy would be for the war-head to make unexpected maneuvers to confuse theinterceptor or disrupt the kill vehicle’s homing process.As discussed in Chapter 5, Russian countermeasures

reportedly include warheads that make midcourse ma-neuvers,17 and China’s recent test of a spacecraft in-tended for manned flight demonstrated a low-thrustrocket propulsion system that reportedly could be usedto make warheads maneuver to defeat an NMDsystem.18 Emerging missile states could also use thisstrategy.

To maneuver outside the atmosphere (where theexoatmospheric NMD interceptors would intercept theirtargets), the warhead would need to use thrusters. Al-though maneuvering continuously using thrusters wouldrequire too much fuel to be practical, one maneuver ora series of several preplanned maneuvers could disruptthe defense.

For example, an attacker could also use a series ofpreprogrammed warhead maneuvers as a complementto lightweight decoys that the defense could discrimi-nate below a given altitude. In this case, the warheadwould make a series of maneuvers to bridge the gapbetween the altitude at which the decoys would bescreened out and the minimum intercept altitude of theNMD interceptor.

Preemptive Attacks on Defense ComponentsSome of the defense components, particularly theground-based radars and the in-flight interceptor com-munications systems (IFICS), could be quite vulner-able to attack. It is unlikely, for example, that theplanned NMD system could even attempt to defend itsradars in Britain against a missile attack from Iran orIraq. Other forward-based radars, such as those in theAleutians, on Greenland and on the US coasts, couldbe vulnerable to short-range ship-launched cruise mis-siles or radar-homing missiles, attacks delivered by ci-vilian or military aircraft, or even by attacks by agentsor special operations forces using shoulder-fired rock-ets. If such attacks succeeded in eliminating several oreven one of the radars, it would leave gaps in the radarcoverage so that the defense would be dependent onlyon SBIRS-low for interceptor guidance againstincoming missiles on certain trajectories. WithoutX-band radar coverage, the defense’s ability to discrimi-nate decoys from warheads would be severely degraded,putting the defense at a great disadvantage. If an attackdestroyed one of the IFICS, this could prevent the de-fense from communicating with its interceptors.

16 For an infrared sensor that operates at a wavelength of10 µm, the infrared signature would be reduced by a factorof a million; for a sensor that operates at 5 µm, the reduc-tion would be a factor of a trillion.

17 David Hoffman, “New Life for ‘Star Wars’ Response,”Washington Post, 22 November 1999, p. 1.18 Associated Press, “Space Technology Could Beat USDefences, Scientist Says,” South China Morning Post,22 November 1999, p. 1.


Chapter 7

Emerging Missile State Countermeasure 1:Submunitions with Biological or Chemical Agents

As we have seen in the previous chapter, there are manytypes of countermeasures that an emerging missile statecould use.

We believe the planned NMD program has seri-ously underestimated the effectiveness of the simplecountermeasures that would be available to an emerg-ing missile state and has overstated the technicaldifficulties in developing and building such counter-measures.

In this chapter and the following two, we describein detail three such countermeasures that could defeatthe planned NMD system. These are: (1) biological orchemical weapons deployed in submunitions that wouldoverwhelm any limited NMD system, (2) nuclear weap-ons deployed with numerous balloon decoys using anti-simulation techniques that would overwhelm theplanned NMD system, and (3) nuclear weapons de-ployed with a cooled shroud that would prevent theplanned hit-to-kill interceptor from homing on it.

It is essential that the United States accurately de-fine the baseline ballistic missile threat from emergingmissile states; otherwise, any assessment of the opera-tional effectiveness of the planned NMD system willbe meaningless. The question “Will it work?” can onlybe asked in the form “Will it work against what?” Thethreat that the NMD system appears to be designedagainst is simply not realistic. At a minimum, the base-line threat should include the three delivery options andcountermeasures discussed in this and the next twochapters.

Should the Baseline Threat Include Chemicaland Biological Weapons?Discussions of the potential threats from emerging mis-sile states tend to focus on ballistic missiles armed withnuclear warheads. That focus may not be justified,however.

The three emerging missile states of greatest con-cern to the United States—North Korea, Iran, and Iraq—are all reported to have programs to weaponize chemi-cal and biological agents. Once successful, these coun-tries could presumably produce large amounts of theseagents and have a far larger stockpile of these weaponsthan of nuclear weapons. North Korea, for example, isbelieved to have enough fissile material to producepossibly two nuclear weapons and is not believed tocurrently have the capability to produce significant ad-ditional quantities. It may, therefore, see its few nuclearweapons (assuming it is able to weaponize its fissilematerial) as too scarce and valuable to fire on a rela-tively untested ballistic missile of unknown reliability,preferring instead to deliver them by a more reliablemethod, such as by ship. Arming missiles with chemi-cal or biological warheads, which would be more plen-tiful, would therefore make sense.

If the United States is concerned about ballisticmissile attacks from emerging missile states, then it mustinclude biological and chemical warheads in the base-line threat the NMD system would need to defendagainst.

SubmunitionsThe most effective method for delivering chemical orbiological (CB) weapons by ballistic missile is to di-vide the missile’s payload into 100 or more bomblets,or submunitions, each carrying up to a few kilogramsof CB materials.1 Shortly after the missile boosterburns out, these bomblets would be released from the

1 For chemical weapons, all of this material would be theactive agent. For biological weapons, the active agent mightonly be only a fraction of this quantity, with the rest beinginert materials such as anti-caking substances if the materialis in powder form or a liquid if it is in slurry form.


warhead in a way that makes them spread out in a cloudas they travel through space toward the target. Each ofthese bomblets would then land at a slightly differentlocation, thereby dispersing the agent more effectivelythan would be possible if delivered in one large “uni-tary” warhead. A warhead using bomblets could easilybe designed to disperse several hundred kilograms ofCB materials over a region 10–20 kilometers indiameter.2

For biological weapons, even the small quantity ofagent carried in a bomblet can be extremely lethal. Forexample, the M143 bomblet developed by the UnitedStates carried only about 6 grams of anthrax spores in aslurry, but this corresponds to 300 million lethal doses(in the hypothetical situation in which it is adminis-tered as an aerosol with no loss to the atmosphere).3

The analysis presented in this section shows that,if a country has developed chemical or biological agentssuitable for delivery by ballistic missile, there wouldbe no technical barriers to that country delivering thoseagents in bomblets rather than a single, large warhead.We show below that the chemical or biological agentsin the bomblets can be protected from reentry heatingusing standard heatshield materials that were developedthirty years ago, and that this heatshield would alsoprotect the agent from heating or cooling of the bomb-let during its 30-minute flight. We also show that theatmosphere would slow bomblets to aircraft speeds atlow altitudes, and that this has a number of advantagesfor the attacker. For example, it makes dispersal of theagent easier than for a unitary warhead, and it allowsmore thorough testing of the bomblets since testing canbe done from aircraft.

It is clear that if the attacker successfully deployssubmunitions this measure would defeat the defensesince there would simply be too many targets for thedefense to intercept. Thus, there is no need to test theNMD system against submunitions. Instead, the Penta-gon should make clear that the planned NMD system isneither designed to nor capable of defending against

chemical or biological agents delivered by missiles us-ing submunitions.

US missile defense programs provide a strongincentive for countries to develop and deploy submu-nitions since they would be highly effective counter-measures to the planned NMD system, as well as tomany US theater missile defense systems. If the sub-munitions are released from the missile shortly after itsboost phase ends, they would overwhelm any missiledefense system designed to intercept its targets afterthe boost phase (such as the planned NMD system).4

However, regardless of US missile defense plans,a country planning to deliver chemical or biologicalweapons by ballistic missile would have a strong moti-vation to divide the agent into a large number of smallbomblets rather than to use a single large warhead, sincebomblets offer a number of important advantages tothe attacker.

The most important advantage is that bombets candisperse CB agents more effectively than a unitarywarhead, for several reasons. The first is the problemof oversaturating a small area with agent by using aunitary warhead. A unitary warhead delivers a largeamount of agent to the impact point, and relies on aircurrents to spread it over a larger area. The concentra-tion of agent will be highest near the impact point andwill decrease as the agent spreads away from that point.Making the concentration large enough to deliver a le-thal dose far from the impact point means that the con-centration at the impact point is much larger than re-quired to give a lethal dose, and any agent beyond whatis required for a lethal dose is simply wasted. Deliver-ing smaller concentrations to many points using sub-munitions reduces this overcontamination problem.

The importance of spreading out chemical and bio-logical agents using submunitions to avoid simply over-contaminating a small region was recognized early and

2 The Pentagon has also voiced concern about the possibil-ity of countries developing radiological submunitions, inwhich a small conventional explosive could be used toscatter radioactive materials such as cobalt 60 or strontium90. (David Fulghum, “Small Clustered Munitions May CarryNuclear Wastes,” Aviation Week and Space Technology,11 October 1993, p. 61.)3 A lethal dose of anthrax is reported to result from inhaling10,000–20,000 spores (See, for example, SIPRI, CB Weap-ons Today, p. 67.

4 We note that there have been reports that the ERINTinterceptor of the PAC-3 theater missile defense system wassuccessfully tested against submunitions carrying simulatedchemical agent. This refers to an intercept test on 30November 1993 in which ERINT intercepted a target missilecarrying 38 canisters filled with water intended to simulatechemical weapons submunitions. (David Hughes, “ArmySelects ERINT Pending Pentagon Review,” Aviation Weekand Space Technology, 21 February 1994, p. 93.) However,in this test the submunitions were not dispersed early inflight; instead the canisters were all clustered together in asingle package, which makes no sense from the point ofview of an attacker facing a missile defense. So this test didnot demonstrate that submuntions could be defeated by aterminal defense system.


grew out of work on mustard gas during World War II.The first development was of cluster bombs for air-craft, but submunitions for missiles were soon designedas well.5

The second advantage of bomblets is that they canbe distributed in a pattern that covers a greater portionof a city with the agent than is possible with a unitarywarhead. When the agent is dispersed from the impactpoint of a unitary warhead, the wind carries it in a long,narrow plume, which cannot cover a city effectively.

In addition, by spreading out the bomblets over alarge area, an emerging missile state can help compen-sate for the poor accuracy of its ballistic missiles. Mis-sile inaccuracy could easily be several kilometers ormore, especially under the assumption that the missilewould undergo only a limited flight-test program.6

A final advantage of bomblets is that, at low alti-tude, atmospheric drag slows them to much lowerspeeds than unitary warheads. Since bomblet speeds atthese altitudes are typical of aircraft speeds, some meth-ods of dispersing the agent from the bomblets may bepossible that are not possible with unitary warheads.7

The total mass of the casings, heatshields, and dis-pensing mechanism for a chemical or biological war-head using bomblets would be expected to be greaterthan the mass of the casing and heatshield for a unitarywarhead. Thus a missile equipped with bomblets wouldbe able to carry less agent than would a unitary war-head. Such a trade-off is sometimes referred to as a“payload penalty.” However, for bomblets this shouldnot be considered a penalty, since the net result is moreeffective delivery of the agent. This is precisely whatled the United States to develop bomblets for chemicaland biological agents on aircraft and short-range bal-listic missiles (see box for more details).

Would an emerging missile state encounter any

technical barriers to using submunitions? Below weexamine the key technical issues a country would facein building and deploying submunitions and find thatan attacker would face no such technical barriers.

The development of submunitions of various typesbegan in the 1940s and effective heatshield materialsfor ballistic missiles existed in the 1960s. Technicalinformation about both of these is widely available inthe open literature. Much of the technical informationabout heatshields resulted from nonmilitary research,particularly research related to spacecraft. Although thecalculations we perform in this study are not highlydetailed, information for considerably more detailedanalyses than we do here is readily available.

The level of technology required to develop sub-munitions is simpler than that required to build long-range ballistic missiles. So if a country has developedlong-range missiles, it could also develop submu-nitions.8 If a country received foreign technology and/or expertise to assist its missile program, it is likelythat foreign assistance would also be available to de-velop submunitions to deploy on the missiles. Even if acountry simply purchased its ballistic missiles, it wouldalso be able to purchase submunition technology, sincea country willing and able to sell long-range missileswould presumably also be willing and able to sell sub-munition technology for those missiles.9

Indeed, the 1998 report of the Rumsfeld Commis-sion stated

All of the nations whose programs we examined thatare developing long-range ballistic missiles have theoption to arm these, as well as their shorter rangesystems, with biological or chemical weapons. Theseweapons can take the form of bomblets as well as asingle, large warhead.10

5 Stockholm International Peace Research Center (SIPRI),The Problem of Chemical and Biological Warfare, Volume I:The Rise of CB Weapons (New York: Humanities Press,1973). pp. 106–107.6 Executive Summary, Report of the Commission to Assessthe Ballistic Missile Threat to the United States (RumsfeldCommission Report), 15 July 1998, and National Intelli-gence Council, “Foreign Missile Developments and theBallistic Missile Threat to the United States Through 2015,September 1999. Both discuss the limited testing programsof emerging missile states.7 A less commonly discussed advantage of using bomblets isthat a combination of different agents could be used in anattack—for example, fast-acting agents and persistentagents—by putting different agents in different bomblets.(“A New Generation of CB Munitions,” Jane’s DefenceWeekly, 3 April 1988, p. 852.)

8 In its January 1996 report about theater missile defense,the Defense Science Board concluded that the United Statesmust expect emerging missile states to deploy “advanced”submunitions for chemical and biological weapons on theirtheater missiles, and noted that its own “red team” efforthad designed, built, and flown versions of such submuni-tions (Report of the Defense Science Board/Defense PolicyBoard Task Force on Theater Missile Defense, January 1996,pp. 14, 16).9 The Soviet Union was reported in the late 1980s to bedeveloping new types of chemical and biological submuni-tions for a variety of delivery systems, including short-rangeballistic missiles. (“A New Generation of CB Munitions.”)10 Rumsfeld Commission Report, p. 7.


Moreover, according to a 1995 news report in AviationWeek and Space Technology,

US intelligence officials are predicting the capabil-ity to release submunitions from ascending ballisticmissiles could be on the world market within fiveyears. They believe that China and North Korea willhave the capability to build fractionated warheads.Such weapons could dispense up to 100 5–10 lbsubmunitions at altitudes of 36 mi [60 km] or less.… US planners here are worried that China or NorthKorea will produce and sell the weapons to militarypowers such as Iran, Syria, Iraq or Libya.11

A number of countries have developed submuni-

tions for short-range missiles. Iraq apparently devel-oped and deployed submunitions to deliver chemicalweapons on its Scud missiles prior to the 1991 GulfWar. According to a Pentagon official, following thewar UN inspectors found that Iraq had “designed andprepared for firing” a chemical warhead for a Scud mis-sile, “which basically consisted of a bunch of little con-tainers.” The official also stated that developing amechanism for dispersing such bomblets early in amissile’s flight would not be difficult for North Korea,China, and Iran, either.12 The dispersal mechanism forlong-range missiles could be quite similar to that forshorter range missiles.

In addition, North Korea is believed to have devel-

12 Aviation Week and Space Technology, 29 April 1996,p. 23.

Early in its development of chemical and biologicalweapons in the 1940s and 1950s, the United States rec-ognized that using unitary warheads for delivery wouldoversaturate a small region with the agent and that windswould subsequently spread the agent in only a narrowplume. That led the United States to research ways todisperse the agent more effectively and, in turn, to de-velop submunitions.a

In fact, the United States developed chemical andbiological submunitions for several of its short-rangemissiles and for B47 and B52 aircraft in the 1950s and1960s.b These bomblets were small, carried smallamounts of agent, and had simple dispersion mecha-nisms to spread the agent once the bomblet was at ornear the ground. For example, the M139 bomblet wasan 11.4-centimeter-diameter sphere that carried 0.6 kgof GB nerve agent or liquid biological agents. It en-tered the US inventory in the early 1960s and was used

US Programs for Delivery of Chemical and Biological Weapons

a See, for example, Dorothy L. Miller, “History of Air ForceParticipation in Biological Warfare Program 1944-1951,”Historical Study 194, Wright-Patterson Air Force Base,September 1952, p. 81.b These short-range missiles were designed to release thebomblets late in flight rather than soon after boost phase.For information on these bomblets see Stockholm Interna-tional Peace Research Center (SIPRI), The Problem ofChemical and Biological Warfare, Volume II: CB WeaponsToday (New York: Humanities Press, 1973). p. 84, andSherman L. Davis, “GB Warheads for Army BallisticMissiles: 1950-1966,” Historical Monograph AMC 51M,Edgewood Arsenal, Maryland, July 1968.

on several short-range missiles: Little John (16 km range),Honest John (38 km range), and Sergeant (140 kmrange). It disseminated the agent on impact with anexplosive charge and was reported to have an 86 per-cent agent dissemination when used with the HonestJohn.

The M143 bomblet was a 8.6-centimeter-diameterspherical bomblet designed to carry liquid biologicalagents. It used 0.5 grams of explosive charge to dis-seminate the agent on impact. It was said to release8 percent of the slurry as inhalable aerosol. This bomb-let had a mass of only 0.34 kilograms when filledwith 190 milliliters of slurry containing about 6×1012

anthrax spores. It entered the US inventory in the mid-1960s and 750 such bomblets were carried in the M210warhead on the Sergeant missile.

The United States also developed other methods torelease and disseminate agents. The E95 bomblet wasa 7.6-centimeter-diameter sphere designed to carry drybiological agent for anti-crop use, delivered by planeor missile. It was designed to burst open in midair todisseminate the agent over a large area. The E120 bomb-let, a 11.4-centimeter-diameter sphere being developedin the early 1960s, carried 0.1 kg of liquid biologicalagent. Vanes on the outside of the casing caused it torotate as it fell, so that it would shatter and roll aroundon impact, spraying the agent from a nozzle.

Note that all of these bomblets are small enough tofit inside the heatshield in the 20-centimeter-diameterspherical configuration considered in this chapter.

11 Aviation Week and Space Technology, 24 July 1995,p. 19.


down so that they would hit the ground with speeds of75–150 meters per second. As we show below, it isstraightforward to develop a heatshield to protect theagent within the bomblet from the high temperaturesthat occur during reentry.

The final step in the flight of the bomblet is to dis-perse the agent in the bomblet when it is at or near theground. As we discuss below, methods for dispersingthe agent are well known.

For our analysis, we assume that each bomblet hasa total mass of 10 kilograms and carries up to a fewkilograms of CB materials. The dispensing mechanismwill add perhaps 50–100 kilograms and the shroudroughly 50 kilograms to the payload,14 allowing85–90 submunitions of this size and mass to be deployedon a missile capable of carrying 1,000 kilograms. Ourestimate below of heatshield requirements, along withthe sizes of the US bomblets discussed in the abovebox, suggests that the bomblets could be made smallerand lighter than we assume here, which would allowthe missile to carry more.

In this analysis, we consider submunitions of twoshapes: a sphere with a diameter of 20 centimeters(roughly soccer-ball sized), and a cone with a length of20 centimeters and a nose radius of 5 centimeters (seeFigure 7-1).15

There would be sufficient room in the payload sec-tion of a long-range missile for at least 100 bombletsand the dispensing mechanism. Even if the last stage ofthe missile were as small as the North Korean Nodongmissile, with a diameter of 1.3 meters,16 a cylindricalpayload section 1.5 meters long, capped by a conicalsection one meter long, would have a volume of twoand a half cubic meters.17 One hundred bomblets would

oped submunitions for its 300- and 500-kilometer-rangeScud missiles. And the Ballistic Missile DefenseOrganization said in 1997 that Syria was only monthsaway from producing chemical bomblets for its500-kilometer-range Scud-C missiles.13

The Design, Construction, and Use of Submu-nitions: Key Technical Issues. In this section we con-sider the key technical issues a country would face inbuilding and deploying submunitions to determine howdifficult it would be to use this means of delivery. Theseissues are (1) how to dispense the bomblets after burn-out and (2) how to design a heatshield for the submuni-tions so they will withstand the heat of reentry. We alsobriefly discuss the issue of dispersing the agent fromthe bomblet. Consistent with the conclusion of theRumsfeld Commission quoted above, we find that theseissues would not be difficult for an emerging missilestate to address.

For our analysis, we assume that the ballistic mis-sile used to deliver the attack can carry a payload of1,000 kilograms or more a distance of 10,000 kilome-ters. The payload would consist of a large number ofbomblets and a dispensing mechanism.

A missile of this range would burn out at an alti-tude of 200–300 kilometers—well above the atmo-sphere. At launch, a shroud would cover the bombletsto protect them from atmospheric heating. The missilewould drop this shroud before burnout, once it was at ahigh enough altitude. (North Korea has demonstratedits ability to perform this step, since it successfully re-leased a shroud that covered the third stage of its Taepo-dong-1 missile during its launch in August 1998.)

Shortly after the booster burns out, the warhead sec-tion would release the bomblets, kicking each one outwith a slightly different speed so that while travellingto the target they would spread out in a cloud of prede-termined size. We discuss below two ways this couldbe done. Note, however, that developing a dispersingmechanism is not demanding on the scale of the tech-nology required to build a long-range ballistic missile.Moreover, a dispersing mechanism could be extensivelytested on the ground and would not require flight test-ing.

The bomblets would then fall through the vacuumof space for about 25 minutes. They would begin toreenter the atmosphere at a speed of roughly 7 kilome-ters per second, but atmospheric drag would slow them

14 Assuming the shroud is a cylinder 1.3 m in diameter and1.5 m long, capped by a conical nose section 1 m long, itwould have a surface area of about 9 m2. If the shroud ismade of aluminum alloy (with a density of roughly 2,800kg/m3 and has an average thickness of 2 mm, then the masswould be roughly 50 kg. Note, however, the shroud can bedropped well before the end of boost phase, so that theupper stage of the missile does not have to accelerate thismass. As a result, the amount by which the mass of theshroud reduces the payload that could be devoted tobomblets would be considerably less than 50 kg.15 This shape was used for calculating the heating of thecone, but there is no special significance to these particulardimensions. The shape could be varied to improve theaerodynamic stability of the cone, for example.16 Some people assume that the Nodong missile will serve asthe second stage of North Korea’s long-range Taepo-dong 2missile.

13 Paul Beaver, “Syria to Make Chemical Bomblets for ‘ScudCs’,” Jane’s Defence Weekly, 3 September 1997, p. 3.


occupy only a third of this volume, leaving plenty ofroom for the dispensing mechanism.18

Details of Dispensing Bomblets. It is useful to com-pare the trajectories of the bomblets with the trajectorythat a unitary warhead would follow if launched by thesame missile. The dispenser that releases the bombletswould follow roughly the same trajectory as a unitarywarhead; this trajectory lies in a vertical plane contain-ing the launch site and the point on the ground wherethe dispenser would impact (see Figure 7-2.)

The warhead section of the missile, including thedispenser and all the submunitions, will be travellingat a speed of roughly 7 kilometers per second when thebomblets are dispensed. Consider what happens if abomblet is released with a push that gives it a smallspeed with respect to the dispenser in some direction.There are three directions to consider:

(1) If the bomblet is given a speed perpendicular to theplane of the trajectory, it will drift in thatdirection until impact. The greater the speed thebomblet is given, the farther it will travel from itsoriginal impact point. Making the bomblet land

17 On the Taepo-dong 1 missile that North Korea launchedin August 1998, the shroud enclosed a cylindrical payloadsection that housed the third stage of the missile.18 If we estimate the volume that a spherical bomblet wouldoccupy (including the space between bomblets) by a cubewith sides of length 20 centimeters, then 100 such bombletswould occupy a volume of only 0.8 cubic meters.

10 kilometers from the original impact point aftera flight time of 25–30 minutes would require giv-ing the bomblet a small speed of 5.5–6.5 metersper second (12–15 miles per hour) relative to thedispenser.19

(2) Giving the bomblet a push in the plane of the tra-jectory and in a direction tangent to the trajectoryis equivalent to changing the burnout speed of thebomblet relative to the dispenser. Thus the trajec-tory of the bomblet will lie in the plane of theoriginal trajectory but will have a slightly longeror shorter range. A speed of 2–5 meters per second(5–10 miles per hour) would change the range by10–30 kilometers.20

19 If δv is the additional speed imparted to the bomblet bythe dispenser, the bomblet will land roughly a distance δv×tfrom the impact point it would have if δv were zero, in adirection perpendicular to the plane of the original trajec-tory, where t is the flight time after the bomblet is released.Spinning of the bomblets could affect their dispersion; thiscould be compensated by adjusting the speed of release.20 This is easily verified using the standard “hit equation”governing missile dynamics.

Missile LaunchPoint

ImpactPoint

Plane ContainingMissile Trajectory

Local TangentPlane toTrajectoryat Burnout

Figure 7-2. The trajectory a bomblet would have if itwas given no additional δδδδδv after burnout of themissile. The tangent plane to the trajectory at burnoutis also shown. Giving the bomblets small velocitychanges δv by adding velocity vectors lying in thisplane will spread the impact points of the bombletsaround the δv=0 impact point.

Length = 20 cmBase Diameter = 15 cmNose Radius = 5 cmMass = 10 kg

9.5 degrees

Figure 7-1. The configuration used for calculating theheating of a conical bomblet. It has a nose radius of5 cm, a base diameter of 15 cm, a length of 20 cm, acone half-angle of 9.5 degrees, a mass of 10 kg, and aballistic coefficient of 12,000 N/m2 (250 lb/ft2).


(3) Giving the bomblet a push in the plane of the tra-jectory but perpendicular to the trajectory changesthe impact point of the bomblet very little, if themissile is on a standard maximum-range (“mini-mum-energy”) trajectory.21

Thus, to spread out the impact points of the bombletsover a large area of roughly 10–20 kilometers diam-eter, the dispenser would give the bomblets differentspeeds (ranging from zero to a few meters per second)in directions lying in the local tangent plane of the tra-jectory. This can be done in several ways.

A particularly simple method of dispersing thebomblets would be to use springs to give the bombletsthe required speeds. Consider a set of tubes having di-ameters just larger than that of the bomblets, lying in aplane, with each tube pointing in a slightly differentdirection in that plane (differing by perhaps 10 degrees).One could arrange a stack of such planar layers of tubessuch that all the layers were parallel to the tangent planeof the trajectory; this orientation could be controlledby the guidance system of the missile during boostphase. The tubes in each layer would point in a differ-ent set of directions from those in other planes. Insidethe tubes would be a line of bomblets with compressedsprings between them. The number of layers and thenumber of bomblets in each tube would be determinedby the size of the payload section of the missile.22

When the bomblets were released, they would shootout of the tubes with a range of speeds determined bythe stiffness of the springs, and in exactly the direc-tions that would result in a dispersed set of impact pointssince the tubes would lie in the tangent plane to thetrajectory. The shape of the impact pattern could becontrolled by proper choice of the spring constants;springs of different stiffness (i.e., with different springconstants) could be used in the sets of tubes lying indifferent directions. (However, it would not be neces-sary for the attacker to carefully control the impactpattern.)

A second method of dispersing the bombletswould be to arrange the bomblets in a cylindrically

symmetric pattern around the axis of the dispenser,which originally would be aligned with the axis of themissile and would thus lie along the direction of thevelocity. After burnout, small thrusters would rotate theaxis of the dispenser in the plane of the trajectory sothat the axis was no longer aligned with the velocity.Another set of small thrusters would then be used tocause the dispenser to spin around its axis. Such thrust-ers are standard technology for missiles.

Each bomblet would be attached to the dispenserby a wire that would be released once the dispenserwas spinning. In this way, the dispenser would releasethe bomblets in many different directions in the planeperpendicular to the rotation axis. Moreover, the speedof each bomblet would be different and would dependon how far the bomblet is sitting from the axis of dis-penser.23 To give the bomblets the range of speeds thatare needed to disperse them over an area with a widthand length of 20 kilometers, the dispenser would onlyhave to spin at a rate comparable to the rate at whichwarheads are typically spun after burnout to stabilizethem during reentry: a bomblet released at a distanceof 1 meter from the rotation axis would require a spinrate of less than one revolution per second to give it aspeed of 5 meters per second.

One could control the shape of the impact patternof the bomblets by controlling the angle through whichthe dispenser was rotated before it was spun, but evenwithout controlling this angle precisely, this methodwould result in a dispersed pattern at impact.

Details of Heat Shielding of Bomblets DuringReentry. A key difference between the bomblets de-signed for short-range missiles and those designed forlong-range missiles is that in the latter case, a substan-tial heatshield would be required to protect the agentfrom the much higher levels of heat generated duringthe high-speed reentry through the atmosphere.

It is important to keep in mind that calculationsshow that a nuclear weapon delivered by long-rangemissile would experience higher heat loadings than abomblet (see Appendix F for calculations of the heat-ing on various reentering bodies). Thus, if an emergingmissile state poses a threat of nuclear attack by long-range missile, it has mastered a level of heatshield tech-nology that is adequate for bomblets.

21 A push in this direction essentially results in a smallrotation of the burnout velocity of the bomblet within theplane of the trajectory. But on a maximum-range trajectory,the range varies only to second order in a change in theangle of the burnout velocity.22 If the tubes point only in directions within 90 degrees ofthe missile’s velocity, then the dispenser could remainattached to the upper stage of the missile, which wouldmake it easier to maintain its orientation until the bombletswere released.

23 The speed δv of a particular bomblet would be r×ω,where r is the distance the bomblet is sitting from the axis ofrotation and ω is the angular speed of rotation of thedispenser.


One might think that bomblets require more de-manding heatshield technology because (1) they areslowed in the atmosphere more than nuclear reentryvehicles and therefore take longer to reach the ground,so that the heat has longer to conduct to the interior ofthe bomblet, or because (2) chemical and biologicalagents are extremely sensitive to heat. We show belowthat neither of these is a problem.

How sensitive are CB agents to heat? As Table 7-1shows, many of the common chemical and biologicalagents are not highly heat sensitive: they can survivemuch longer than the several minutes of reentry time attemperatures greater than room temperature (300 K).

The bomblets reenter the atmosphere at about7 kilometers per second and are slowed by atmosphericdrag. In the process, the original kinetic energy of thebomblet is converted to heat in the air around the bomb-let, and some fraction of this heat is transferred to thebomblet itself. Two factors must be considered in de-signing a heatshield for the bomblet: the heating rate atthe surface of the bomblet and the length of time theheat has to diffuse into the interior of the bomblet. Forbomblets that slow down relatively quickly as they fallthrough the atmosphere, there will be a longer time forthe heat that has been absorbed by the bomblet to dif-fuse into the interior (this process is known as “heatsoak”).

As noted above, we consider two types of bomb-lets. The first is a sphere with a total mass of 10 kilo-grams and a diameter of 20 centimeters. This bombletcould be made to spin on reentry to spread the heatingout over its surface. The second is a conical bombletwith a length of 20 centimeters and a nose radius of5 centimeters, again with a mass of about 10 kilograms.This design has the advantage that it falls faster, so that

the heat-soak time is shorter. It also lends itself to asimple fusing and dissemination method, since it canbe oriented aerodynamically so that it hits the groundnose-first, which allows a disseminating charge to blowthe agent out the back of the cone.

We find that it is straightforward to produce ad-equate heatshields for these designs that are consistentwith the size and mass of these bomblets. Indeed, itappears that the bomblets could be made smaller andlighter than we consider here, which would allow moreto be delivered on a given missile.

For this study, we have only considered relativelysimple heatshield materials that were developed

30–40 years ago. Not only are these materi-als relatively simple, but considerable infor-mation about them is available to anyone, in-cluding an emerging missile state. However,considerably more advanced materials are incommon use today and are commerciallyavailable.24

The primary heatshield material we con-sider is silica phenolic or “refrasil phenolic,”which is roughly 35 percent by weight phe-nolic resin impregnated into a fabric rein-forced with high-purity glass fiber. Heatshieldmaterials based on phenolic resins were con-sidered state of the art in the 1960s becauseof their thermal, mechanical, and chemicalproperties. These materials reduce the heat

transferred through them by ablating the outer surfaceaway.

For the spherical bomblet with a heatshield madeof silica phenolic, the thickness of material ablated fromthe surface is only about 3 millimeters. (See Appendix Ffor details of the heating and ablation calculations.) Inaddition, a shell of this material that is 2 centimetersthick will keep the temperature increase at the inside ofthe heatshield to less than 50o C by the time it hits theground, and a 2.5-centimeters-thick shell will keep thetemperature rise to less than 20o C. For a bomblet witha diameter of 20 centimeters, a shell of this materialwith a thickness of 2 or 2.5 centimeters would have amass of 3.3 or 4.0 kilograms, respectively.

We also consider other standard heatshield materi-als of the same vintage as silica phenolic. For example,by using nylon phenolic, which has a low density

24 As one example, there is a material called Thermasorb, inwhich heat is absorbed with no rise in temperature by aphase transition in a material that could be used to fill a thinshell at the inner edge of heatshield (see www.thermasorb.com).

Source: Sidney Graybeal and Patricia McFate, GPALs and Foreign SpaceLaunch Vehicle Capabilities, Science Applications InternationalCorporation (SAIC) Report, February 1992.

Table 7-1. Stability of common CB agents to heat exposure.

tnegA erutarepmeTneviGataefil-flaH

stnegAlacigoloiBseropsxarhtnA 001([email protected] o )C

stnegAlacimehC)BG(niraS

)DG(namoS

XV

051([email protected] o )C

031(K304@sruoh4 o )C

051(K324@sruoh63 o )C


25 SIPRI, CB Weapons Today, for example, references anumber of US patents granted in the 1950s and 1960s forfusing and dispersal mechanisms, which give detaileddescriptions and technical diagrams.

relative to silica phenolic, a greater volume of materialwill be ablated but a lighter heatshield can be used.Using nylon phenolic for the spherical bomblet wouldresult in a surface ablation of about 9 millimeters, butrestricting the temperature rise at the inner surface ofthe heatshield to 20o C would require the original thick-ness of the heatshield to be only 2 centimeters. A2-centimeter-thick shell of this material would only havea mass of 1.2 kilograms, compared with the 4 kilogramsneeded for a silica phenolic heatshield that restricts thetemperature rise to 20o C.

In practice, other simple things would improve thedesign and reduce mass. For example, it would makesense to use a thinner shell of ablating material andback it with a lightweight layer of highly insulatingmaterial. In addition, if the bomblet had a metallic shellfor structure inside the heatshield, the metal would actas a heat sink and could reduce the amount of heat-shield required.

For the conical bomblet using the same silica heat-shield material considered above, about 1 centimeterof material would be ablated at the nose, where the heat-ing is most severe, and about 3 millimeters of materialwould be ablated at a point on the wall a distance of 10centimeters behind the nose. The calculations show thatat the nose a thickness of less than 3 centimeters ofmaterial is required to keep the temperature rise at theback of the heatshield to roughly 20o C. On the sidewalls of the bomblet, 2 centimeters of material wouldkeep the temperature rise at the inside surface of theheatshield to less than roughly 20o C, and 1.5 centime-ters of material would result in a temperature rise of70o C. A conical heatshield that had 5 centimeters ofmaterial at the nose and 1.5 to 2 centimeters of shield-ing on the walls would have a mass of about 1.7 to 2kilograms. And, as above, in practice a country coulddo things to make the heatshield thinner and lighter thanthis.

These calculations demonstrate that effective, light-weight heatshields can easily be made for bombletsusing even simple materials developed decades ago.Thus, even if a spherical or a conical bomblet of thisshape were not used, it is clear that an adequate heat-shield could be developed for a different design.

Heating or Cooling of the Bomblets DuringMidcourse. There seems to be a common mispercep-tion that the temperature of bomblets would drop dra-matically during their roughly 25-minute flight betweenrelease from the missile and the beginning of atmo-spheric reentry and that this could harm the CB agentcontained in the bomblet. As shown in Appendix A, if

the bomblet is in the sunlight its temperature can eitherincrease or decrease from an initial temperature of300 K (room temperature), depending on the surfacecoating of the bomblet. Thus the attacker can easilydesign the bomblet so that its equilibrium temperaturewill be close to 300 K.

If the bomblet is in the dark, its temperature willdrop, but will do so only slowly as it radiates awayheat. Appendix F considers the case of the 10 centime-ter-radius spherical bomblet with a 2-centimeter-thickheatshield made of silica phenolic. The appendix showsthat if the bomblet were in the dark along its entire tra-jectory, after 30 minutes the temperature of the bomb-let would drop by less than 20 K from its initial tem-perature of 300 K. So in neither case would the tem-perature change of the bomblet during the midcoursephase present a problem for the chemical or biologicalagent.

Releasing the Agent. The final step in delivery isto release the chemical or biological agent from thebomblet and disperse it. Of course, this would also needto be done for CB agents deployed in a unitary war-head, so if a country has weaponized these agents, itcould apply these techniques to bomblets.

Several methods of fusing have been discussed inthe open literature, including contact fuses that woulddetonate upon hitting the ground and barometric fusesthat would release the agent at a preset altitude.25 Notethat at low altitudes, the speed of the bomblets wouldbe very low: the spherical bomblet would impact theground at 75 meters per second and the conicalbomblet at 150 meters per second, corresponding to170–340 miles per hour. These speeds, which are typi-cal of aircraft, make dispersal easier than do very highspeeds. Given these speeds, it would even seem pos-sible to use a small sprayer to release the agent. Thiscould be very efficient because sprayers can release theagent in the droplet sizes that are optimal for infectingpeople.

For the conical bomblet, an easy fusing methodwould be to have a contact fuse in the nose of the bomb-let, which would ignite a dispersing charge when thenose hit the ground that would blow the agent upwardsinto a cloud. Designs for this type of dispersion mecha-nism have been around for 40 years.


26 The braking force on a body in the atmosphere is propor-tional to the square of its speed. Thus the faster bomblets ona long-range missile will experience much stronger brakingforces than slower bomblets. Bomblets having the sameballistic coefficients as those considered above releasedfrom a 500-km-range missile would have speeds of 90 to150 m/s at impact.

Because bomblets on a long-range missile under-go severe deceleration in the atmosphere, bombletsreleased from a 500-kilometer range missile like theSyrian or North Korean Scud-C, would have the samerange of speeds at low altitudes as would these bomb-lets on long-range missiles.26 As a result, the dispersalmechanisms developed for bomblets on short-rangemissiles could also be used for bomblets on long-rangemissiles.

Finally, it is important to note that because of thelow speeds and altitudes the bomblets would have atrelease, dissemination of chemical or biological agentscould be tested by dropping small bomblets containingsimulated agents from aircraft. In this way, clandestinetests can be done to achieve the optimal particle size.


Chapter 8

Emerging Missile State Countermeasure 2:Anti-Simulation Balloon Decoys for Nuclear Warheads

According to the September 1999 National IntelligenceEstimate on the Ballistic Missile Threat to the UnitedStates, balloon decoys are a “readily available technol-ogy” that emerging missile states could use to developcountermeasures to the US NMD system.

In fact, the first two intercept tests of the NMDsystem included one balloon decoy along with the mockwarhead. This test configuration, together with state-ments that it is representative of the threat, indicatesthat even the Ballistic Missile Defense Organizationbelieves that such balloons are within the technical ca-pability of an emerging missile state. In this chapter,we consider a countermeasure that would be onlyslightly more difficult to implement because it usesnumerous such balloons, but would be much more ef-fective against the planned NMD system because it alsoputs the warhead in a balloon. Of course, making anddeploying such balloons would be technically muchsimpler than building and deploying a long-range mis-sile and a nuclear warhead, which is the level of tech-nology the United States assumes an attacker wouldhave.

We conclude that an attacker seeking to deliver anuclear warhead by a long-range ballistic missile coulddefeat the planned NMD system by enclosing the war-head in a metal-coated balloon that is inflated in spacewhen the warhead is deployed, while at the same timereleasing large numbers of similar, but empty, balloons.The attacker could prevent the planned NMD systemfrom being able to discriminate the balloon containingthe warhead and thus prevent the defense from reliablyhitting the warhead.

The balloons could be either free-flying or teth-ered together. Above the atmosphere, where the attackerwould use anti-simulation to make the warhead looklike a balloon decoy, objects of different weights would

follow the same trajectory. The thin metal coating ofthe balloon would prevent radar waves from penetrat-ing the balloons to determine which contained awarhead.

The balloons could be identical in size and shape,or they could be designed so that each one wasdifferent from the others. They could be spherical orirregular in shape. In addition, the temperature of theballoons could be easily manipulated so that each onewas at a different temperature (over a range of tem-peratures plausible for the balloon containing the war-head) to prevent the NMD system’s heat-detecting in-frared sensors from being able to determine if a bal-loon contained a warhead.

As we describe below, for attacks on daytime tra-jectories (i.e., those in sunlight), an attacker could setthe temperature of each balloon anywhere in a span ofseveral hundred degrees centigrade simply by choos-ing the appropriate surface coating. By choosing sur-face coatings that would produce balloon temperaturesnear the initial temperature of the nuclear warhead, theattacker would essentially eliminate any thermal effectthat a nuclear warhead would have on its balloon. Thus,if the warhead were initially near room temperature (300K), the attacker could paint the balloons so that theirtemperatures would vary slightly around 300 K. In thisway, the attacker would prevent all of the NMD infra-red sensors—those on the SBIRS-low satellites andthose on the kill vehicle—from discriminating the bal-loon with the nuclear warhead from the empty ones.

For attacks on nighttime trajectories (i.e., those inthe earth’s shadow), the balloons would all cool to alow temperature. The temperature of the balloons wouldnot depend on their surface coating, but only on theirshape. If all the balloons had the same shape, the emptyballoons would cool to a somewhat lower temperature


(of about 180 K, or -93 degrees Celsius, for sphericalballoons) than would the balloon containing the war-head. To prevent the infrared sensors from discrimi-nating the warhead, the attacker could use small bat-tery-powered heaters to bring the temperature of theempty balloons up to that of the balloon with the war-head. Alternatively, the attacker could use entirely pas-sive means to mask the presence of the warhead. First,to reduce the heat transfer from the warhead to the bal-loon, the attacker could cover the nuclear warhead withsuperinsulation or a low-emissivity coating such asshiny aluminum foil or polished silver. Then the at-tacker could use balloons of different shapes, so thatall the balloons would have different equilibrium tem-peratures that varied over a range of a few degrees.One of these balloons would contain a nuclear war-head but again, none of the NMD infrared sensors—those on the SBIRS-low satellites and those on the killvehicle—would be able to discriminate the balloon withthe nuclear warhead from the empty ones.

The attacker could also design balloons that wouldbe effective regardless of whether the trajectory was insunlight, or earth’s shadow, or some of each. For ex-ample, the balloons could be of different shapes andhave a surface coating that would give an equilibriumtemperature near room temperature in the sunlight. Ifthe attacker then covered the warhead with superinsu-lation or a low-emissivity coating, each balloon wouldhave slightly different equilibrium temperature. Thus,the NMD infrared sensors could not discriminate theballoon containing the warhead from the empty oneson any trajectory, regardless of how much of it was insunlight or the earth’s shadow.

The NMD system would also attempt to discrimi-nate the empty balloons from the balloon containingthe warhead by using its X-band radars to observe anymechanical interaction between the nuclear warheadand its balloon. However, the attacker could also readilyprevent such discrimination. If the attacker attached thewarhead to its balloon using strings or spacers of theappropriate length, the balloon would move along withthe warhead, whether or not the warhead was tumblingor spinning. The attacker could also make the emptyballoons tumble and spin. And the attacker could use asimilar string structure inside the empty balloons, sothat all the balloons would have similar surface fea-tures where the strings were attached.

Thus, by placing a nuclear warhead in a balloonand releasing it with other empty balloons, an emerg-ing missile state would prevent the planned NMD

system from being able to discriminate the balloon con-taining the warhead in midcourse.

The NMD system would then be confronted with alarge number of potential targets, no two of which wereidentical in appearance, and any one of which couldcontain a warhead. Thus, to permit a midcourse inter-cept, the NMD system would either have to fire inter-ceptors at all of the balloons or risk letting the ballooncontaining the warhead go unchallenged. Because thenumber of balloons deployed per missile could belarge—up to 50 or more—the use of this countermea-sure would quickly exhaust the NMD’s supply of in-terceptors. The NMD system is intended to defendagainst an emerging missile state with tens of missiles;yet a state using only, say, five missiles could deployone or more nuclear warheads in balloons and hundredsof empty balloons in an attack on a US city.

Although the NMD system is designed to interceptin midcourse, the defense could—as a last-ditch effort—attempt an intercept after the warhead and decoys be-gin to reenter the atmosphere, where air resistancewould slow the lightweight balloon decoys relative tothe heavier balloon containing the warhead. Unless theattacker took steps to prevent it, the X-band radarswould be able to make very accurate velocity measure-ments using the Doppler shifts of the radar waves re-flected from an object. At altitudes low enough that thevelocity difference between light decoys and the heavywarhead could be measured, the radars would then beable to discriminate the balloon with the warhead fromthe other balloons. To implement this strategy, the de-fense would need to launch several interceptors at apredicted intercept point just above the minimum in-tercept altitude of the kill vehicle. However, to haveenough time to reach the intercept point, the defensewould need to launch these interceptors well before theballoons begin to reenter the atmosphere and then di-vert them in mid-flight if the X-band radars were ableto discriminate the balloon with the warhead from theother ones.

This would present the defense with a significantproblem because it would have to decide how manyinterceptors to launch before it knew how many of theballoons it could discriminate. If the defense was plan-ning to discriminate during reentry and saw dozens ofballoons deployed from each missile, it would need toassume that some of these balloons would be heavyenough to prevent discrimination above the killvehicle’s minimum intercept altitude. To achieve thehigh effectiveness and confidence levels planned for


it, the defense would need to fire a large number ofinterceptors at the balloons deployed by each attackingmissile. Thus, for an attack of tens of missiles, the de-fense would still be in the position of choosing betweenletting the warhead penetrate unchallenged or runningout of interceptors.

Moreover, as we discuss in detail below, the at-tacker could take various steps to further complicatethe job of the defense by lowering the altitude at whichthe X-band radars could discriminate a balloon con-taining a warhead from the other balloons. And, evenif the X-band radars could determine in time which bal-loon in a cluster of numerous closely-spaced balloonscontained a warhead, it may not be able to convey thisinformation to the kill vehicle in a useful fashion. Theability of the radars to determine the angular positionof the balloons (as distinct from their range) is some-what limited. Thus, if the balloons are spaced closelytogether, the NMD system may be unable to pass anaccurate enough map to the kill vehicle to allow it tohome on the target using its own sensors.

We thus conclude that an attacker could preventthe planned NMD system from intercepting its nuclearwarhead with high confidence by using anti-simula-tion balloon decoys.

As we discuss in more detail below, such balloondecoys would be easy to fabricate and deploy relativeto building an intercontinental ballistic missile or anuclear weapon. In fact, in the late 1950s the UnitedStates designed and built small metal-coated balloonsto measure the density of the atmosphere, and placedseveral of these balloons into orbit in the 1960s. Theseballoons, which were 3.7 meters in diameter, are quitesimilar to ones that could be used as a missile defensecountermeasure. (See Figure 8-1.) Detailed informa-tion on the design, construction, and deployment ofthese balloons has been publicly available for over thirtyyears; some of this information is provided in Appen-dix G on the NASA Air Density Explorer series of in-flatable balloon satellites.

In the rest of this chapter we first discuss how suchballoon decoys could be built. We then consider in de-tail how the attacker could prevent the NMD systemfrom using any of its sensors to discriminate a ballooncontaining a warhead from empty balloons in mid-course, where the system is designed to intercept itstargets. Finally, we discuss several measures theattacker could take to prevent the defense from usingatmospheric drag to discriminate the target and then tomake a last-minute intercept during reentry.

Design, Construction and Deployment ofBalloonsThe balloon decoys could be built in a way similar tothe way in which NASA built its Air Density Explorerballoon satellites. These satellites were made of a lami-nate1 of two layers of aluminum foil and two layers ofmylar, with the outer layer being aluminum. Each layerwas 0.0005 inches thick, for a total thickness of 0.002inches. The balloon satellites weighed roughly 4.5 ki-lograms (10 pounds).

An attacker could construct balloon decoys usinga two-ply laminate of aluminum foil and mylar (or amylar-polyethylene composite), with each ply havinga thickness of 0.0001 to 0.001 inches. Both of thesematerials are widely available commercially. In fact, itmay not even be necessary to make the laminate: alu-minized mylar is commercially available for use inpackaging.

To make a spherical balloon, the laminate wouldbe cut into strips and glued together over a hemispheri-cal mold, with the aluminum on the outside. However,there is no need for the balloons to be spherical. Othershapes may be easier to fabricate and fold and, as dis-cussed below, may have other advantages as well.

The air would then be pumped out of the balloon,and the balloon folded into a small volume. (The 3.7-meter-diameter NASA balloons were folded into a cyl-inder 18 centimeters (7 inches) in diameter and 28 cen-timeters (11 inches) long.) The balloon that was to con-tain the warhead could be cut open and resealed once

1 A laminate is a material made by gluing or otherwisebonding together two or more thin layers.

Figure 8-1. A photograph of one of the NASA AirDensity Explorer inflatable balloon satellites.


the warhead was placed inside. The air could then bepumped out of the balloon, causing it to collapse downon the warhead. To keep the warhead positioned withinthe balloon once it was inflated, the balloon could beattached to the warhead either by several rigid spacersmade of a material with low thermal conductivity orby strings. Alternately, the warhead could be left to floatwithin the balloon.

When deployed, the balloons could be inflated inone of several ways. Here we will assume that theballoon is inflated with nitrogen (or another gas) to apressure of 0.1 pounds per square inch (PSI).2 Thispressure would be more than adequate to inflate theballoons; it was the pressure used to inflate the NASAballoons. This gas pressure would stress the aluminumfoil to its yield strength to give the balloon its maxi-mum structural strength.3 Once the balloon has beenstressed to its yield strength, it will be stronger and willalso have its wrinkles and folds removed, even if thegas is then vented out. Until the balloon was releasedand inflated, the nitrogen could be contained in a smallsteel bottle. After the balloon was inflated, the gas bottlecould be made to detach and fall off outside the bal-loon, as was done for the NASA balloons.

We will consider two balloons that differ from eachother in weight. The heavy balloon is spherical in shape,has a diameter of three meters, and is made of a lami-nate of a 0.001-inch-thick layer of aluminum foil and a0.001-inch-thick layer of mylar (which gives it the samethickness as the NASA balloons). Balloons of roughlythis size and thickness, and the gas used to inflate them,would weigh about 3 kilograms (6.5 pounds).4 The dis-pensing mechanism for each balloon and the bottle thegas is stored in would add to this weight; if we assumethe dispenser and bottle are comparable in weight tothe balloon and gas, we get a total weight of 6 kilo-grams for each balloon deployed.

The NASA balloons were designed so that the gasused to inflate them would leak out over a period ofseveral days after their deployment; ground tests ofthese balloons indicated that without their pressurizinggas they would remain spherical down to an altitude ofabout 120 kilometers. Because these heavy balloon de-coys would be made of a material with the same thick-ness as the NASA balloons, they would also retain theirshape down to about 120 kilometers if the gas wasvented out after inflation.5

Considerably lighter balloons could be made byusing thinner balloon materials. In this way the weightof the balloon, the gas, and the gas bottle could be re-duced considerably. For our lightweight balloon model,we assume the material used to construct the balloonconsists of a 0.00025-inch-thick mylar layer and a0.0001-inch-thick aluminum layer. The thinner layerof aluminum will reduce the pressure required to takethe aluminum foil to its yield strength, so less gas willbe needed to inflate the balloon and a smaller and lighterbottle can be used to hold the gas. The total weight ofthe balloon, gas, and gas bottle would be about 500grams (roughly 1 pound).6 ,7 If we again assume the

2 Another means of inflating the balloons once they aredeployed would be using chemical gas generators, likethose that are used to inflate automobile airbags.3 The yield stress is defined to be the applied load at whichthe stress-strain relationship is no longer linear. In groundtests, the NASA researchers found that by subjecting theballoon to this level of stress, not only was its structuralstrength increased, but the folds in its surface weresmoothed out.4 The volume of this balloon would be 14.1 cubic metersand its surface area 28.3 square meters. Filling a balloonof this size with nitrogen at a pressure of 700 Pa (0.1 PSI)would require roughly 96 liters of nitrogen at standardtemperature and pressure (STP), or about 120 grams ofnitrogen.

5 The Ballistic Missile Defense Organization has a goal of130 kilometers for the minimum intercept altitude of theNMD kill vehicle, so the heavy balloon decoys would retaintheir shape below this altitude.6 For material of this thickness, a balloon with a diameter ofthree meters would weigh about 440 grams.

The thinner layer of aluminum would reduce thepressure required to take the aluminum foil to its yieldstrength by a factor of ten. The inflation pressure could thusbe reduced to roughly 70 Pa (0.01 PSI), so that only 12rather than 120 grams of nitrogen gas were needed toinflate the balloon. (The dynamic pressure on the balloondue to reentry would not exceed the inflating pressure of70 Pa until the balloon reaches an altitude of about 90 km.)

The gas bottle, made of steel, would weigh about70 grams. If we assume a moderate bottle pressure of1.4×10 7 Pa (2,000 PSI), the volume of the bottle wouldhave to be about 0.07 liters (or 70 cubic centimeters) tohold the nitrogen. If we assume the bottle is a cylinder oflength 8 centimeters, then its inner radius must be about 1.7centimeters. Assuming a fairly low value of yield strength of3.0×108 Pa (44,000 PSI), a steel bottle (with a density of 7.8grams per cubic centimeter) would have a wall thickness of0.085 centimeters and a mass of about 70 grams.

Note that we neglect the weight of the glue used tobond the balloon together.7 Even lighter balloons could be made. The aluminum layercould be made approximately a factor of ten thinner (only0.00001 inches thick) and still be a good reflector of radar(this thin aluminum layer could be vapor-deposited onto themylar). A mylar thickness of 0.00025 inches may be near


weight of the dispensing mechanism is roughly that ofthe balloon, this would give a total weight penalty foreach balloon of very roughly 1 kilogram (2 pounds).

If the inflating gas was vented from them, theselightweight balloons would not retain their shape aslow into the atmosphere as would the heavier ones.Their deformation would likely begin at an altitude ofperhaps 150 to 160 kilometers.8 However, they wouldretain their shape lower into the atmosphere if the gaswas not vented.

A simple nuclear weapon would weigh perhaps1000 kilograms, and it is reasonable to assume that theattacker could use about 10 percent of the payload, orroughly 100 kilograms, for countermeasures. Thus, ifthe attacker is satisfied with a relatively small numberof decoys (15 or less) per missile, then a weight of6 kilograms per decoy is acceptable, and the heavy bal-loon decoys could be used. However, if the attackerprefers to use a larger number per missile, then lighterdecoys would be needed. It is reasonable to expect thatan attacker could deploy as many as 100 of the light-weight (0.5 kilogram) balloon decoys we describe aboveon a missile along with a nuclear warhead. An emerg-ing missile state with tens of missiles might not haveenough nuclear warheads to arm each missile, in whichcase it could have several missiles whose entire pay-loads were devoted to balloons of various weights. (Theattacker would probably not want to use the entire pay-load to deploy light balloons because the defense mightwell conclude that the missile could not carry hundredsof decoys and a warhead. Instead, the attacker wouldlikely choose to deploy perhaps 25 to 50 heavy balloondecoys.)

We also note that the attacker could test the con-struction and deployment mechanism using clandes-tine ground tests. The attacker would likely not want to

test these balloon decoys by flight testing them sincethe United States could observe such tests.

How Anti-Simulation Balloon Decoys WouldPrevent Midcourse DiscriminationTo understand how the balloons would prevent theNMD system from discriminating the balloon contain-ing the warhead from the other balloons duringmidcourse, it is useful to first consider the “ideal” case:a metal-coated balloon travelling through the vacuumof space where the warhead suspended inside the bal-loon does not interact with the balloon in any physicalway. Compared with an empty but otherwise identicalballoon, the appearance of the balloon with the war-head would be exactly the same to radar, infrared, andvisible sensors. No sensor planned for use by the NMDsystem could determine which balloon had the war-head and which did not; discrimination would be im-possible.

However, the real world would differ from this idealcase in two ways, either of which could potentially beused by the NMD system to discriminate a balloon con-taining a warhead from an empty balloon:

(1) The warhead would interact thermally with theballoon, possibly causing changes in the balloontemperature (either over the whole balloon, or inhot or cold spots), including changing the rate atwhich the balloon changed temperature after itwas released.

(2) The warhead would interact mechanically withthe balloon, possibly causing changes in theshape or motion of the balloon.

Below we consider each of these issues in turn, show-ing how the attacker could mask these effects to pre-vent the defense from determining which balloon con-tains a warhead.

We also note that it would not be possible for theNMD sensors to discriminate a balloon with a warheadfrom the empty ones during their deployment becauseat this distance the sensors would be unable to resolveclosely-spaced objects, and would therefore not be ableto observe the deployment of countermeasures in anydetail. As discussed in Appendix B, the resolution ofthe SBIRS-low infrared sensors would be too poor toallow any imaging of a balloon or warhead-sized ob-ject; instead these sensors would see all midcourse ob-jects as point emitters. The early warning radars haveeven poorer resolution (see Appendix D). Even ifan X-band radar was in a position to observe the

the practical lower limit for the mylar thickness. Neglectingthe weight of glue, this would give a balloon mass of about280 grams, including the inflating gas, but not the gasbottle. (The designers of NASA’s 100-foot-diameter Echo Isatellite planned to use 0.00025-inch-thick mylar with a0.000009-inch-thick layer of vapor-deposited aluminum,but found that to get the required inflation reliability0.0005-inch-thick mylar was required. However, for themuch smaller balloons considered here the 0.000025 mylarthickness would likely be sufficient.) See G.T. SchjedahlCompany, “Design and Fabrication of Inflatable andRigidizable Passive Communications Satellites (Echo I andEcho II),” Conference on Aerospace Expandable Struc-tures, Dayton, Ohio, October 23–25, 1963, pp. 576–604.8 The reentry forces on the balloon would be a factor of tenlower at an altitude of 160 km than they would be at120 km.


deployment, its resolution would also be inadequateto distinguish between the different objects, whichwould be densely spaced when they are deployed.9

Discrimination by Infrared Sensors: The Ther-mal Effects of the Warhead on the Balloon. As weshow in this section, the attacker could completelyeliminate any ability the defense might have to discrimi-nate based on infrared data from either SBIRS-low orthe kill vehicle’s seeker. We consider daytime andnighttime attacks separately since the thermal behav-ior of objects in space is different in these two cases.The attacker can choose to fly its missiles on trajecto-ries that are either (entirely or mostly) sunlit or in theearth’s shadow. The attacker can thus choose to designits balloons to be effective against infrared sensors ineither regime. Alternatively, it could choose to build bal-loons that would be effective against IR sensors on bothdaytime and nighttime attacks, as we discuss below.

Daytime AttacksThe thermal behavior of empty balloons. As dis-

cussed in detail in Appendix A, the equilibrium tem-perature of an object in sunlit space is largely deter-mined by its surface coating. Thus, the attacker caneasily vary the equilibrium temperatures of its emptyballoons by applying various surface finishes, such aspaint. Table 8-1 lists the equilibrium temperature for asunlit spherical object with different surface coatings:seven paints, two metal finishes, and mylar. The equi-librium temperature varies by more than 300 K, from227 K for white titanium dioxide paint to 540 K forpolished gold plate.

The examples we use in this section are all spheri-cal balloons, because it is more straightforward to cal-culate the thermal properties of a spherical object thanfor a nonspherical object. However, we emphasize thatthe general results are applicable to a balloon of anyshape. (We discuss later in this chapter and in moredetail in Appendix A the effect of balloon shape onthermal behavior.) This discussion initially assumes thatthe entire surface of any given balloon will be at a uni-form temperature; we will consider temperature varia-tions over the surface of the balloons subsequently.

Table 8-1 shows that by painting all or part of thesurface of a balloon whose outer layer is aluminum foilwith one or several different paints, any equilibriumtemperature between 227 K and 454 K can be obtained.(For example, if the aluminum is entirely covered withwhite titanium dioxide paint, it will have an equilib-rium temperature of 227 K. If instead part of its sur-face is covered with black paint, it will have an equi-librium temperature between 314 K and 454 K, depend-ing on how much of its surface is painted.) Thus, theattacker can choose the equilibrium temperature of eachballoon. In fact, NASA used just this approach to con-trol the temperature of its Air Density ExplorerBalloons in order to keep the radio beacons inside theballoons within their operating temperature range. Thealuminum outer surface of these Air Density ExplorerBalloons was partly covered with small circles of whitepaint to reduce the balloon’s equilibrium temperature.10

(See Appendix G).

Table 8-1. Equilibrium temperature, for variouscoatings, of a sphere in sunlight.If an object in orbit is in sunlight, its surface coating willdetermine the equilibrium temperature. This equilibriumtemperature is listed for a sphere (or spherical shell)coated with each material; it is independent of the size ofthe sphere. Unless otherwise stated, all objects areassumed to be in low earth orbit, at an altitude of severalhundred kilometers. It is also assumed that the spheresare spinning and tumbling in such a way that all parts oftheir surface are equally exposed to sunlight, althoughclearly this can only be approximately true. (See AppendixA for details of calculation.)

9 Although the X-band radars would have high rangeresolution, they would have poor angular resolution.Because the objects will be densely spaced when they aredeployed, each radar range slice would contain multipleobjects, and the radar’s poor angular resolution would makeit unable to distinguish between different objects that wereat the same range. In addition, there would be screeningeffects because balloons between the radar and the deploy-ment mechanism would block the radar’s view.

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If the initial temperature of a lightweight balloonwhen it is released is significantly different from itsequilibrium temperature (which need not be the case),its temperature would change rapidly, since the heatcapacity of such a balloon would be very low. Howquickly a balloon comes to its equilibrium temperaturedepends on how different its initial and equilibrium tem-peratures are from one another and how great its heatcapacity is. As we show in Appendix A, using the light-weight balloon model described above (with a mass of0.5 kilograms), an empty balloon initially at room tem-perature (300 K) will reach its equilibrium tempera-ture within about a minute, while the 3-kilogram bal-loons could require several minutes, depending on theirsurface coatings.

The thermal behavior of a balloon containing awarhead. How would the presence of a warhead in-side a balloon affect its thermal behavior?

The temperature of the massive warhead wouldchange only slightly in the short time between when itis launched and when it reenters the atmosphere, butthe much lighter balloon enclosing the warhead wouldrapidly reach its equilibrium temperature. Because awarhead inside a balloon could be at a different tem-perature than the balloon, and because the warhead hasa much greater heat capacity than the balloon, its pres-ence inside a balloon could affect the thermal behaviorof the balloon. If the attacker did not take steps to pre-vent or mask these effects, there are several ways inwhich the defense might be able to determine whichballoon contained the warhead. These include: the di-rection of the temperature change of the balloon afterits release (i.e., whether the balloon heats or cools); therate of the temperature change of the balloon (i.e., howquickly it reaches its equilibrium temperature); and itsfinal equilibrium temperature. We show below that theattacker could prevent the NMD system from using anyof these thermal effects to discriminate an empty bal-loon from one containing a warhead.

Direction of temperature change. Assuming thatboth the warhead and the balloon are at the same tem-perature when deployed, the presence of a warhead in-side a balloon would not cause that balloon to warm upwhen, if empty, it would have cooled down (or viceversa). The warhead can only pull the temperatureof the balloon back towards the initial warhead

temperature. (For example, if the warhead and balloonsare initially at room temperature (300 K) when theyare released, the warhead will pull the balloon tempera-ture back towards room temperature.) Thus, a balloonwith a warhead inside may cool down or heat up moreslowly than a similar empty balloon. However, whethera balloon warms or cools after its release does not, byitself, indicate whether the balloon contains a warhead.

Rate of temperature change and equilibrium tem-perature of balloons. If the warhead and balloon are atdifferent temperatures, the warhead will transfer heatto (or from) the balloon in several ways: by radiation,by conduction through any spacers used to position thewarhead within the balloon, by conduction through thegas in the balloon, and by motion-driven convection ofthe gas. As discussed in more detail in Appendix H,radiation is likely to result in the largest heat transfer.Thermal conduction through any spacers could be madenegligible. Conduction through the gas will give riseto a smaller effect than radiation and could be avoidedby venting the gas. The effect of convection of the gascould also be avoided by venting the gas.

What is most important is that the rate of heat trans-fer between the warhead and the balloon will be smallcompared with the solar power incident on the balloon(which would be about 10,000 watts for a balloon witha diameter of three meters). This would permit the at-tacker to use balloons that have small differences inhow efficiently they absorb solar energy (and radiateinfrared energy) to completely obscure the thermal ef-fect of the warhead (see Appendix H).

Although it is possible to hide a warhead in a bal-loon with any equilibrium temperature, the attackercould essentially eliminate the effect of the warheadon the thermal behavior of the balloon by simply choos-ing a surface coating that produces a balloon equilib-rium temperature close to the initial temperature of thewarhead. In this way, there would be only a small tem-perature difference to drive the heat transfer, and thewarhead would produce only a negligible thermal ef-fect for the defense to detect. Thus, if the warhead isinitially near room temperature (300 K), the attackercould enclose it in a balloon with a surface coating thatproduced an equilibrium temperature near 300 K. Theattacker would construct the other balloons so that theywould have equilibrium temperatures within a narrowrange around this temperature. The defense would thensee numerous balloons at slightly different tempera-tures and would have no way of knowing which onecontained the warhead.

10 The first of these 3.7-meter-diameter balloon satellites,Explorer IX, had 17 percent of its surface covered with whitepaint, and the second, Explorer 19, had 25 percent of itssurface covered with white paint.


This approach is illustrated in Figure 8-2.The heavy lines show the temperature variationof several lightweight aluminum balloons (ourlightweight balloon model described above witha mass of 0.5 kilograms), covered with varyingamounts of white enamel paint. We assume thepaint is distributed over the surface of the bal-loon, perhaps using small circles as was donefor the NASA balloons. We also assume the bal-loons are at room temperature (300 K) whenreleased. As the figure shows, varying the frac-tion of the balloon surface that is covered bythe white paint from 21 to 26 percent wouldproduce a temperature spread slightly greaterthan 10 K around the initial temperature of300 K, with the balloons reaching their equilib-rium temperatures in less than one minute.

Figure 8-2 also shows the effect of addinga warhead to a balloon (specifically, to the bal-loons with 21 and 26 percent of their surfacecovered with white paint).11 We assume thewarhead has a low emissivity finish or surfacecoating, such as aluminum foil (with an emis-sivity, ε, of 0.036), and treat the inside of theballoon as a blackbody. For both balloons, theheat transfer is taken to be five times larger thanwould actually be produced by a warhead witha surface emissivity of 0.036.12

As this figure shows, the effect of the war-head would be to pull the equilibrium temperature ofits balloon back towards room temperature, but theshape of the curve is essentially unchanged. The ther-mal behavior of the balloons containing the warheadswould be indistinguishable from that of empty balloonscovered with slightly different amounts of white paint.Thus, by using balloons with surfaces designed to pro-duce a small span of temperatures around the war-head temperature, the attacker could easily hide anythermal effect of the warhead.

Alternatively, a spread of temperatures could beobtained by painting the same fraction of each balloon’ssurface, but using slightly different paints on each bal-loon. For example, consider using three different typesof white paint. A balloon with 25 percent of its surfacecovered with white enamel paint will have an equilib-

11 Although our calculations allow the temperature of thewarhead to vary, because the thermal mass of the warheadis large, its temperature would remain essentially un-changed during the trajectory.12 This is done to allow for the possibility that motion-drivengas convection could increase the heat transfer.

13 As discussed in Appendix A, for a balloon of a givensurface coating, its equilibrium temperature will beproportional to the fourth root of the ratio of its averagecross-sectional area to its surface area. For a sphere, thisratio is 0.25, and its fourth root is 0.707. For a cylinder thatis 3 meters long and has a base diameter of 3 meters, the

rium temperature of 297 K. However, the same cover-age with white titanium dioxide paint gives an equilib-rium temperature of 287 K and painting it with whiteepoxy paint gives an equilibrium temperature of 292K. (All of these assume a warhead initially at 300 Kwith an emissivity of 0.036 inside, and heat transfer byradiation only.)

The attacker can instead introduce a variation inthe balloons’ equilibrium temperature by varying theshapes of the balloons (see Appendix A). For example,if a sphere with a diameter of 3 meters had an equilib-rium temperature of 300 K, then a cylinder with thesame surface composition that was 3 meters long andhad a base diameter of 3 meters would have an equilib-rium temperature of roughly 284 K.13 Thus, by using avariety of balloon shapes the attacker could also get aspread of equilibrium temperatures.

Figure 8-2. Temperature as a function of time after deployment oflightweight (0.5 kg) aluminum balloons coated with varying amountsof white paint to give equilibrium temperatures near 300 K (thickcurves).Changing the fraction of the balloon covered by white paint from 21percent to 26 percent produces a temperature span of just over 10 Karound 300 K. In addition, two balloons containing warheads withemissivities of 0.036 are also shown. These calculations assume that heattransfer occurs only through radiation to or from the warhead, but for oneballoon the magnitude of the heat transfer is taken to be five times largerthan it would actually be. The calculations also assume that both theballoon and the warhead are spherical; balloons and warheads of othershapes will give qualitatively similar results.

22 percent white paint


21 percent white paint plus warhead

26 percent white paint plus warhead


23.55 percent white paint




0 2 4 6 8 10

Time after deployment (minutes)

Empty Balloons Partially Covered with White Paint

294

296

298

300

302

304

306

308

Tem

pera

ture

(K

)


For balloons designed to equilibrate around roomtemperature, the attacker could also introduce additionaluncertainty by slightly heating or cooling the warheadprior to launch, and placing it in a balloon with thesame equilibrium temperature. Then even if the defensecould exactly determine the balloons’ surface compo-sition, it could not tell which balloon had the warheadinside. However, such heating or cooling of the war-head would not be necessary, since the defense wouldnot be able to exactly determine the balloons’ surfacecompositions (particularly when, as discussed below,the variation of temperature over the balloons’ surfacesis taken into account).

Thus, we have shown above that neither the equi-librium temperature of a balloon nor the rate at whichit obtains this temperature is sufficient for discrimina-tion. In fact, it is clear that the attacker has many dif-ferent options for designing balloons to prevent dis-crimination based on the thermal behavior of the bal-loons. The only possible thermal effect that might al-low discrimination would be the very small drift inballoon temperature that could take place if the tem-perature of the warhead inside changed. However, thiswould only be a factor for balloons designed to havean equilibrium temperature considerably different fromthe warhead’s initial temperature, and even so wouldbe very small.14 Moreover, as we discuss next, the tem-perature of the balloons would not be uniform over theirsurfaces, and this variation would mask any small tem-perature drifts due to the warhead.

Nonuniform balloon temperatures. The abovediscussion assumes that the entire surface of each bal-loon would be at a uniform, albeit changing, tempera-ture. However, in actuality this would not be the casebecause different parts of the balloon would be exposedto and absorb different amounts of incident radiation.For example, for a balloon directly between the sunand the earth, the hottest area of the balloon surfacewould be facing directly towards the sun, while thecoldest area would be located about 90 degrees away.

The temperature variations due to this effect canbe significant. NASA calculations for their Air Den-sity Explorer balloon satellites indicated there wouldbe about a 50 K temperature difference between thehottest and coldest points on the satellite (for the casein which 17 percent of the balloon surface was coveredwith white paint).15 If the inside of the balloon satellitewere aluminum rather than mylar, this temperature dif-ference would have more than doubled. These calcula-tions assumed a balloon with a stable orientation rela-tive to the sun. For a balloon without such a stable ori-entation, these temperature differences would get av-eraged out to some degree. However, unless the bal-loon was spinning in such a way that all parts of itssurface received equal exposure to the sun, some tem-perature variation would remain.

There could also be temperature variations due tothe distribution of paint on the surface of a balloon. Infact, the attacker could deliberately create hot and coldspots on the surfaces of the balloons by using differenttypes of paints. (NASA used an area of white paint onthe surface of its balloons to create a cold spot over thelocation of the radio tracking beacon inside the bal-loon.) From the point of view of the attacker, such tem-perature variations over the surface of the balloonswould in fact be desirable, because if there were anypossibility that the presence of the warhead inside theballoon would create hot or cold spots (for examplevia conduction along spacers), such deliberately cre-ated temperature variations would mask any such war-head effects.

Thus, in general, it must be expected that the bal-loons would not be at uniform temperatures, but wouldhave significant and spatially complex temperaturevariations over the surface of each balloon. This wouldfurther complicate the already nearly impossible taskof thermal discrimination.16

Nighttime Attacks. At night the situation is con-siderably different because the only significant exter-nal source of heating is infrared radiation from the earth.

15 Coffee, et al., figure 17.16 In fact, even if the attacker designed the balloonenclosing the warhead to have an equilibrium temperaturevery different from the initial temperature of the warhead,and there was a very small drift in the balloon temperaturedue to the changing temperature of the warhead inside,such temperature variations over a balloon’s surfacewould make precise measurements of the averagetemperature of the balloon very difficult and would preventany potential warhead-related hot or cold spot from beingused for discrimination.

ratio of its cross-sectional area to its surface area can varybetween 0.167 and 0.212, depending on its orientation. Ifwe assume an average of 0.2, we get a balloon tempera-ture of 283.7 K for a balloon surface composition thatwould give a temperature of 300 K for a sphere. (SeeAppendix H.)14 For example, consider an aluminum balloon containing awarhead with an emissivity of 0.036 and an initial tempera-ture of 300 K. The temperature of the balloon wouldchange at a rate of 0.0007 K per minute due to thechanging temperature of the warhead.


At night all spheres at low earth-orbit altitudes wouldequilibrate at about 180 K, regardless of their surfacecomposition. Objects of different shapes would equili-brate at slightly different, but still low, temperatures(see Appendix A).

Given this low equilibrium temperature, the effectsof a room-temperature warhead inside a balloon couldbe quite significant. For example, the balloon discussedabove (with 25 percent of its surface covered with whiteenamel paint), which would equilibrate at 297 K in sun-light, would at night equilibrate to about 180 K if emptybut to about 187 K if it contained a warhead with asurface emissivity of 0.036 and the heat transfer wasdue only to radiation (and to 204 K if the heat transferfrom the warhead is five times that due to radiation).However, the attacker can take straightforward mea-sures to prevent discrimination based on the thermaleffects of the warhead inside the balloon.

One straightforward way for the attacker to pre-vent discrimination would be to put heaters in the emptyballoons and heat them to temperatures similar to thatof the balloon containing a warhead. Such a heaterwould not need to provide a large amount of power.For a balloon with a shiny aluminum outer surface, aheater that delivered 25 watts to the interior surface ofthe balloon (the actual power output of the heater mighthave to be somewhat higher) would raise the balloon’sequilibrium temperature from 180 K to 197 K. A heaterthat delivered 50 watts would increase the equilibriumtemperature to 210 K. Such heaters could be made bydepositing a resistive layer on the inner surface of bal-loon (or by using a resistive tape), similar to the waymany rear car windows are defrosted. A small batterycould be used to provide power.17 But there would beno reason to use even this big a heater if the attackerfirst reduced the heat transfer from the warhead to theballoon (in the ways we discuss below) so that the dif-ference in equilibrium temperatures between an emptyballoon and one containing a warhead would be only afew degrees K. In fact, keeping all the balloons at aslow a temperature as possible would be to the attacker’sadvantage since the colder the balloons were, the moredifficult it would be for an infrared sensor to detectthem.

To reduce the heat transfer from the warhead, theattacker could vent out the inflating gas and give thesurface of the warhead the lowest possible emissivity.18

By covering the warhead with shiny aluminum foil, theattacker could reduce the emissivity of this surface to0.036. However, the attacker could devote special at-tention to reducing the emissivity of the warhead andthus ought to be able to obtain an even lower emissiv-ity than that of aluminum foil. For example, the attackercould give the warhead a surface finish of polished sil-ver, with an emissivity of 0.01.

The attacker would also want to use a balloon whoseoutside surface had an emissivity as high as possible,so it would radiate heat away rapidly. For example, theattacker could cover the entire surface of the balloonwith white paint. In this case, and for a warhead with asurface of polished silver, the equilibrium temperatureof the balloon would be 0.5 degrees K higher with thewarhead than without it. Another option, which mightgive an even smaller temperature difference, would beto cover the warhead with a multilayer superinsulation,with the outer layer having a low emissivity.

Thus, the attacker could readily reduce the tem-perature difference to a few Kelvin or less. In this case,a heater of only a few watts power could be used toheat the empty balloons to the temperature of the bal-loon with the warhead. Of course, the attacker couldgive each balloon a slightly different temperature fromall the others to further complicate the task of thedefense.

Moreover, once the attacker reduced the heat trans-fer from the warhead to such low levels, the attackercould use entirely passive means instead of heaters tomask the presence of the warhead in one of the balloons.

One straightforward passive way for the attackerto mask the presence of the warhead would be to varythe shape of the balloons. As discussed in Appendix A,the equilibrium temperature of a balloon varies withits shape. By using balloons of different shapes, theattacker could introduce a range of equilibrium tem-peratures that vary by at least 10 K, more than sufficientto mask the presence of a low-emissivity warhead.19

17 For example, a Duracell DL245 lithium manganesedioxide battery is capable of putting out 4.5 watts for atleast 30 minutes. Each battery weighs 40 grams and theycan be operated in series for higher power levels. It wouldbe necessary to enclose them in superinsulation to keepthem warm, as their performance falls off rapidly as theirtemperature falls below room temperature. For datasheets, see www.duracell.com/oem/lithium/DL245pc.html.

18 Since the surface area of the balloon would be muchgreater than that of the warhead, reducing the emissivity ofits inner surface would have a relatively small effect, and sohere we will take the inside of balloon to be a blackbodywith an emissivity of 1.19 If, as discussed previously, we assume a cylinder with aratio of cross-sectional area to surface area of 0.2, we get atemperature of 170 K instead of 180 K for a sphere.


20 First, collisions between the warhead and the ballooncould slightly change the velocity of the balloon. Forexample, if there was on average 1 meter of “rattle room”and a collision took place every 10 seconds, then onaverage the balloon would change its velocity by about10 cm/second with each collision, the radial component ofwhich might be detectable by the radar. Thus the radarmight be able to detect a pattern of discontinuous radialspeed changes superimposed over a smooth variation ofradial velocity as the warhead travels through space.However, if the inflating gas is vented out of a balloon,there will be considerable “give” in the balloon wall, andthe relative motion of the warhead will be quickly dampedout.

Second, each collision between the warhead and theballoon might also change the spin characteristics of theballoon. Thus the defense might, by observing the balloonover a period of time, be able to observe that the way inwhich it is spinning is changing in a way inconsistent withthe balloon being empty.

Third, any tumbling motion of a non-spinning warheadmay get transferred to the enclosing balloon over a periodof time, resulting in changes in the tumbling motion of theballoon that may be detectable. The attacker can attempt tominimize such differential tumbling by inflating the balloonafter the warhead is deployed, so that the inflated balloontumbles in the same way as the warhead.

Fourth, collisions could “dent” the balloon, slightlychanging its shape. This could result in changes in theballoon’s radar cross section that the radar might be ableto detect superimposed on top of the changes taking placeas the balloon spins or tumbles through space.

Decoys Effective for Both Daytime and NighttimeAttacks. The attacker could also choose to use balloondecoys that would be effective for both daytime andnighttime attacks. As described above for nighttimeattacks, the attacker could insulate the nuclear warheadand give it a surface coating with a low emissivity toreduce the transfer of heat from the warhead to the bal-loon during nighttime attacks. Again as describedabove, the attacker could either use balloons withslightly different shapes so they would have slightlydifferent nighttime equilibrium temperatures or use asmall heater in the empty balloons. Then to ensure thatthe decoys would also work for daytime attacks, theattacker could give the balloons a surface coating sothey would have daylight equilibrium temperatures thatwere near room temperature but slightly different fromeach other. The surface coating would not significantlyaffect the nighttime equilibrium temperatures. If theattacker chose to use balloons of slightly differentshapes, then they could be given identical surface coat-ings because the variation in shape would result in dif-ferent daytime equilibrium temperatures. If the attackerchose to use balloons of the same shape with a smallheater, then they could be given slightly different sur-face coatings to give them slightly different daytimeequilibrium temperatures.

Discrimination by Radars: Mechanical Inter-actions Between the Warhead and BalloonWe have shown above that the attacker could takestraightforward steps—simply choosing the surfacecoating and shape of the balloons—to prevent the NMDinfrared sensors from being able to discriminate theballoon with the warhead inside it from the empty bal-loons by observing the thermal behavior of the balloons.We have also shown that these steps would work forboth daytime and nighttime attacks.

However, there remains the possibility that a me-chanical interaction between the warhead and ballooncould change the balloon’s behavior in a way that thedefense could observe and then use to determine whichballoon contained the warhead. For example, a spin-ning warhead nutating about its spin axis might causeits enclosing balloon to nutate as well.

The X-band radars can make very detailed mea-surements of the time variation of the balloons’ radarcross-section and radial velocity, and may even be ableto produce an rough image of the target. (See Appen-dix D for details.) In order for this capability to be use-ful, however, there needs to be a way for the defense torelate the observed signal to phenomena occurring

inside the balloon. As we discuss below, the attackercould take steps to prevent the defense from doing so.

The mechanical interaction between the warheadand balloon differs for a warhead that is not physicallycoupled to the balloon and one that is. In the case inwhich the warhead is not attached to the balloon, wecan think of the warhead as “rattling around” inside theballoon. The warhead would collide with the insidesurface of the balloon. Such collisions could have sev-eral effects on the balloon: they could change the ve-locity of the balloon, cause the balloon to spin or tumble,or change its shape.20 Whether the NMD radars wouldbe able to discriminate the balloon containing the war-head by observing such changes is difficult to assess.However, to avoid this possibility, the attacker couldsimply choose to physically attach the balloon to thewarhead. This could be done using strings; the lengthcould be such that the balloon is either tightly or looselyconstrained when it is inflated. Alternatively, severalspacer rods made of a low conductivity material couldbe used. The Explorer 9 satellite used a set of glassepoxy rods to stand off the transmitter unit from the


balloon’s inner surface. In either case, the balloon wouldmove with the warhead, eliminating the detection pos-sibilities discussed above for an unattached warhead.

The remaining concern of the attacker would bethat some characteristic of the balloon and warheadmotion would be measurably different from that of anempty balloon.

If the warhead is not spinning, but is tumbling, thenthe balloon will tumble in the same way as the war-head. However, empty balloons could also easily bemade to tumble when they are ejected from the missile(in fact, it may be difficult to make them not tumble).The defense would not know precisely what the under-lying tumbling behavior of the warhead is.

If the warhead is spinning, then the balloon willtake on the spin characteristics of the warhead (assum-ing the balloon skin is sufficiently rigid). However,empty balloons could also be made to spin—as indeedthe Air Density Explorer Satellites were. More com-plex motions of the warhead could occur, such as nu-tating about its spin axis. However, such motions couldalso be simulated by empty balloons using properlydistributed weights on the inner surface of the balloon.Indeed, even if the warhead is not spinning, the attackercould deploy several balloons specifically designed tohave spin and nutation characteristics similar to what aspinning warhead might have.

The attacker might also be concerned that the at-tachment points would distort the balloon, making theone containing the warhead look different from the oth-ers. In general, this would argue against making theballoons perfectly spherical, so that any such distor-tion would not stand out. Clearly the attacker could alsointroduce deliberate distortion into empty balloons inorder to mask any such effect. For example, if the at-tacker used strings to attach the warhead to the bal-loon, it could also use a simple structure of strings inthe empty balloons so that the surface of the balloonswould be exactly the same whether there was an inter-nal string structure or a warhead to which the balloonwas tethered.

More generally, the attacker could use a variety oftechniques to obscure any motion that would signal thepresence of a warhead, or as noted above, could evendesign one or more of the empty balloons to produceobservable effects of the type that the defense wouldbe trying to exploit in order to identify the balloon withthe warhead. For example, some or all of the balloonscould be equipped with a small vibrational device. Ifballoons that retain the inflating gas are used, one couldbe equipped with a small valve that would be opened

periodically for a short period, giving the balloon a small“kick.” The attacker could also tether a number of theballoons together, using either flexible or rigid tethers,to obscure any motions of the balloon containing thewarhead. The possibilities of this type are numerous.Thus, even if the defense saw signatures of the typethat it would associate with a warhead inside a balloon,until it actually saw an interceptor kill vehicle impact-ing a warhead, it could not be sure the balloon con-tained a warhead.

Discrimination and Intercept During EarlyReentryAs discussed above, by enclosing the warhead in a bal-loon and simultaneously releasing numerous emptyballoons, the attacker could prevent discrimination byboth infrared sensors and radars as the balloons travelthrough the vacuum of space in the midcourse part oftheir trajectory.

However, if the attacker took no other measures toprevent it, the X-band radars might be able to discrimi-nate the balloon with the warhead from the other bal-loons early in the reentry phase of their trajectories bymeasuring the velocities and positions of the balloons.21

Although for many purposes the atmosphere can betreated as negligible above altitudes of 100 kilometers,here we consider the possibility of discrimination atsignificantly higher altitudes. As the balloons descendthrough the atmosphere, the lighter-weight empty bal-loons would be slowed more by atmospheric drag thanwould the heavier balloon with the nuclear warhead.This effect is sometimes called “atmospheric filtering.”If the effects of atmospheric drag on the decoys rela-tive to the much heavier warhead became apparent at ahigh enough altitude, the defense might have enoughtime to attempt to intercept the warhead before it passedbelow the kill vehicle’s minimum intercept altitude.

In general, the heavier the balloons, the lower thealtitude at which they could first be discriminated.22

21 The ability of the upgraded early warning radars toaccurately measure small velocity or position changes dueto atmospheric drag will be far inferior to that of the X-bandradars (see Appendix D).22 The discrimination altitude would also be lower thesmaller the balloon was, but since the balloons must be ableto contain a warhead, they could not be made too small.The behavior of an object during reentry is largely deter-mined by its ballistic coefficient, β = W/ (CD A), where W isthe object’s weight, A is its cross-sectional area perpendicu-lar to its velocity, and CD is the drag coefficient. At altitudesabove where the mean free path of air molecules is largecompared with the size of the balloons, the drag coefficient


For example, Figure 8-3 shows the change invelocity due to atmospheric drag of three bal-loons with masses 0.5, 5, and 20 kilograms rela-tive to that of a balloon containing a 1,000 ki-logram warhead, for altitudes less than 500 ki-lometers. In this case, the balloons are sphereswith a diameter of three meters. For a light-weight 0.5-kilogram balloon, the velocitychange due to atmospheric drag would beroughly one meter per second at an altitude of250 kilometers. For a 5 kilogram balloon, thevelocity change due to atmospheric drag wouldbe roughly 0.1 meters per second at an altitudeof 250 kilometers.

Figure 8-4 shows the change in position(along the trajectory) due to atmospheric dragof balloons of various weights (again relativeto that of a balloon containing a 1,000-kilogramwarhead), for altitudes less than 500 kilome-ters. For example, for the 5-kilogram balloon,the displacement due to atmospheric dragwould be roughly 1 meter at an altitude of 275kilometers.

Nevertheless, statements by BMDO offi-cials make it clear that they are counting onbeing able to discriminate the warhead inmidcourse and are not planning to use atmo-spheric filtering to discriminate it during reen-try. For example, NMD Program Manager Maj.Gen. William Nance stated recently that thegreatest technical challenge in getting to the ob-jective system and being able to deal with“more complex countermeasures” was the stepfrom the C1 to the C2 system.23 Yet the transi-tion from the C1 to the C2 system does not placeany X-band radars in the lower-48 states or Ha-waii. Thus, the C2 system would have no X-band radars that could observe the reentry phaseof a trajectory aimed at the lower 48 states orHawaii, and therefore could not even attemptto use atmospheric filtering to discriminate thewarhead for such attacks.

of the balloons would have a value of CD = 2,independent of their shape. The mean free path ofmolecules in the atmosphere is several meters at120 kilometers altitude.23 Michael Sirak, “A C1 to C2 Move Is NMDSystem’s Most Stressing Upgrade, Says NMDHead,” Inside Missile Defense, 3 November 1999,p. 10.

Figure 8-3. Velocity change due to drag.This figure shows the change in speed due to atmospheric drag atvarious altitudes for three balloons with diameters of 3 meters anddifferent masses, relative to the case of no drag. Since at thesealtitudes, the drag would have negligible effect on a heavy object likea nuclear warhead, these are effectively speed changes of theballoons relative to a warhead. The calculations assume the balloonsare on a standard, 10,000-km range trajectory.

0.5 kg balloon

5 kg balloon20 kg balloon

Velocity Change due to Drag

10-4

10-3

10-2

10-1

100

101

102

Vel

ocity

cha

nge

(m/s

)

100 200 300 400 500

Altitude (km)

Figure 8-4. Position change due to drag.This figure shows the change in position along the trajectory dueto atmospheric drag for three balloons of different mass, relativeto the case of no drag. Since at these altitudes, the drag wouldhave negligible effect on a heavy object like a nuclear warhead,these are effectively changes in the positions of the balloonsrelative to a warhead. The calculations assume the balloons areon a standard, 10,000-km range trajectory.

Position Change Due to Drag

100 200 300 400 500

Altitude (km)

10-3

10-2

10-1

100

101

102

103

Posi

tion

cha

nge

(m)

0.5 kg balloon

20 kg balloon

5 kg balloon


from a balloon containing a warhead at such a highaltitude, so the defense would have to launch its inter-ceptors before it could distinguish the target from thelight decoys.

The fact that the defense would have to launch itsinterceptors well before atmospheric discriminationcould begin would present the defense with a signifi-cant problem. The defense would have to decide howmany interceptors to launch and at what intercept pointto launch them before it could know how many of theballoons the radars could filter out and at what altitudethis discrimination would take place. Although the de-fense could determine the size of the various balloonsin midcourse, it could not determine the weight of eachballoon decoy, and hence the altitude at which it couldpotentially be discriminated, until reentry. The defensewould also not know how much total weight the at-tacker could devote to decoys, since that would dependon what the payload capacity of the missile was andhow heavy the warhead was. Moreover, if North Koreahad a missile that could deliver a warhead to 11,000–12,000 kilometers, which would be required to reachtargets throughout the United States, it could carry con-siderably more weight to the shorter ranges needed toattack targets in Hawaii or the western United States.

The attacker could use a mix of balloon weights,either by making some balloons from thicker materialor by putting small weights inside lighter balloons. (Likethe nuclear warhead, these weights could be attachedto the balloon by several strings or spacers of the ap-propriate length.) The heavier balloons would reachlower altitudes before they could be discriminated thanwould the lighter balloons. The attacker would prob-ably want to have a relatively large total number ofdecoys to prevent the defense from trying to interceptall the balloons in midcourse. At the same time, theattacker would probably want to have a number of heavydecoys that would be difficult for the defense to dis-criminate in reentry.

If we assume that a missile carrying a nuclear war-head could devote 100 kilograms of payload to the bal-loons, then the attacker could easily deploy dozens ofballoons of various weights. For example, the attackercould deploy 3 dozen lightweight balloons each weigh-ing 0.5 kilograms and each using a deployment mecha-nism weighing another 0.5 kilograms. It could put thenuclear warhead in one balloon, and then use the re-maining payload to distribute roughly 60 kilograms ofweights throughout other balloons. For example, itcould use six 10-kilogram decoys or twelve 5-kilogramdecoys, or a mix of both.

Another indication that the NMD system is not in-tended to discriminate using atmospheric filtering isthat the X-band radars that are being built for the NMDsystem would have only a single face with a limitedfield of view (see below). A radar would therefore needto physically rotate to view balloons on widely sepa-rated trajectories as they neared the radar during reen-try, which is when the radar would need to track themfor atmospheric filtering. As a result, the radar wouldlikely be unable to track balloons deployed by missileson widely separated trajectories (such as those aimedat different cities). Thus, NMD X-band radars are notappropriate for a defense system designed to do atmo-spheric filtering.

It is not surprising that the United States is not plan-ning to discriminate the warhead during reentry. Forattacks against cities that are not located near one ofthe interceptor launch sites (which would include mostUS cities and the vast majority of the US population),the defense would have to launch its interceptors wellbefore it could use atmospheric filtering to discrimi-nate the balloon with the warhead, in order for the in-terceptor to have enough time to reach the interceptpoint. If the defense were able to discriminate the tar-get from the decoys at a high enough altitude, it wouldthen need to divert the interceptors in mid-flight oncediscrimination took place.

For example, for a North Korean attack on SanFrancisco, the interceptors closest to the reentry part ofthe trajectory would be those at Grand Forks, NorthDakota.24 It would take an interceptor at least 6.5 min-utes to reach a potential intercept point near SanFrancisco. The defense could wait as long as possibleto launch its interceptors by aiming at an intercept pointjust above the kill vehicle’s minimum intercept alti-tude, say 150 kilometers. For a standard trajectory fromNorth Korea to San Francisco, the balloons would beat an altitude of roughly 1,050 kilometers when the in-terceptors were launched (and at higher altitudes ifNorth Korea used a lofted trajectory). If the defensewanted to attempt intercepts before the last possiblesecond, the balloons would be at even higher altitudeswhen the interceptors would need to be launched. Itwould not be possible for the X-band radars to use at-mospheric filtering to discriminate lightweight decoys

24 Although the trajectory might carry the balloons relativelyclose to the interceptor site in Alaska, they would then be atvery high altitudes. Since the defense must rely on atmo-spheric filtering, the engagement must occur when thewarhead is near San Francisco and thus closer to the NorthDakota site.


Of course, the defense would not know how muchof the payload the attacker devoted to decoys or whatmix of balloon weights the attacker chose to use. But ifthe defense was planning to use atmospheric filteringfor discrimination and saw dozens of balloons deployedfrom each missile, it would need to assume that someof these balloons would be heavy enough to deny dis-crimination above the kill vehicle’s minimum interceptaltitude.

To achieve the high effectiveness and confidencelevels planned for it, the defense would need to err onthe side of launching too many interceptors. Since thedefense reportedly plans to fire up to four interceptorsper target, if it made the reasonable assumption that adozen or more balloons would remain viable decoys,the defense would have to launch several dozen inter-ceptors per missile to have high confidence that it couldprevent the warhead from getting through.

Thus, because the defense would need to launch itsinterceptors before it knew how many of the decoyswould remain viable down to its minimum interceptaltitude, it would need to fire a large number of inter-ceptors at the balloons deployed by each attacking mis-sile. Recall that the planned NMD system is intendedto defend against an attack of tens of missiles fromNorth Korea and other emerging missile states. Thus,the defense would still be in the position of choosingbetween letting the warhead penetrate unchallenged orrunning out of interceptors.25

For this reason, forcing the defense to abandon itspreferred midcourse intercept strategy to operate in this“last chance” mode would use up a large number ofinterceptors and would significantly degrade the confi-dence in the defense effectiveness. Moreover, as wediscuss below, the attacker has several ways in whichit can exploit some of the limitations of the X-bandradars to further complicate this defense tactic.

First, we consider what steps the attacker could taketo prevent discrimination at altitudes that would be highenough to permit an intercept. Second, assuming theX-band radar viewing the reentry part of the trajectorywas able to discriminate the balloon containing thewarhead from the others, we consider what steps the

attacker could take to prevent the radar from determin-ing the position of the discriminated balloon accuratelyenough that this information would be useful to the killvehicle, which must still home on the target using itsown sensors.

Measures the Attacker Could Take to PreventDiscrimination. There are several measures an attackercould take to lower the altitude at which an X-bandradar could discriminate the balloon containing thewarhead from a balloon decoy. We discuss some ofthese below. The attacker could use some of these mea-sures in combination with the others.

Denying High Precision Velocity and PositionMeasurements. As discussed in Appendix D, the X-band radars that would be part of the NMD systemshould be capable of making very precise measurementsof the radial velocities of the balloons. Specifically,these radars would be able to measure the Doppler shiftin the frequency of the radar return due to the radialcomponent of an object’s velocity. However, an X-bandradar could not use Doppler shifts to measure the com-ponent of velocity in the direction perpendicular to theradar line of sight (i.e., the cross-range direction). Theradar could measure the cross-range velocity of a bal-loon by plotting its angular position versus time, butbecause radars are limited in their ability to measurethe angular position of an object, this method is gener-ally less accurate.26 Thus, an X-band radar would notbe able to measure the cross-range velocity of a bal-loon as accurately as the radial velocity.

The X-band radars should also be capable of mea-suring the range to each balloon with high accuracy(see Appendix D). However, an X-band radar would

26 While the radar would not be able to measure the cross-range position of a given balloon with high accuracy, itcould possibly use ISAR (Inverse Synthetic Aperture Radar)techniques to image the collection of balloons and deter-mine the position of the one containing the warhead relativeto that of the other balloons (see Appendix D). However,since the collection of balloons—whether or not they weretethered together—would not be rigid, ISAR would be ofvery limited utility. Using the orbital motion to generate anISAR image would take time, perhaps tens of seconds. Asingle image is only accurate in two dimensions, not three,although full three-dimensional imaging is (again inprinciple) possible if the radar makes two separate imageswith the right separation in angular aspects of the twoimages. If the attacker considers ISAR techniques to be athreat, it could take measures to thwart ISAR, including ran-dom motion of surfaces or appendages on the warheadsand decoys, or random motion of the entire object (via

25 If the attacker had more missiles than nuclear warheadsand was trying to force the defense to run out of intercep-tors, it might choose to use a missile to deploy only decoyballoons. A missile not carrying a nuclear warhead (andwith a total payload of 1,000 kilograms) could deploy threedozen balloons with an average weight of 25 to 30 kilo-grams. Since the defense could not know that none of theseballoons contained a warhead, it would have to assumethat one did.


not be able to determine a balloon’s angular (or cross-range) position with as great an accuracy as its range,even after tracking it for most of its trajectory.

Although the C3 NMD system would deploy up tonine X-band radars around the world, only some of thesewould be in a position where they could view the reen-try phase of a missile trajectory that was targeted onthe United States. Moreover, for attacks on most UScities, only one X-band radar would be in a position toview the reentry phase of the trajectory below an alti-tude of 400 kilometers. (See Table 8-2). Thus, the de-fense would have to rely on the measurements of oneX-band radar to determine the velocity and position ofthe balloons during reentry.

The atmospheric drag would affect the total veloc-ity and displacement along the trajectory of an incom-ing balloon, not just the components of velocity anddisplacement in the direction towards the radar. There-fore, unless the balloon was on a trajectory directly to-wards the X-band radar, the defense would need to es-timate the total velocity of the balloon based on theradar’s very accurate measurements of the balloon’sradial velocity and its less accurate measurements ofthe balloon’s cross-range velocity. Similarly, the de-fense would need to estimate the displacement alongthe trajectory based on its very accurate range mea-surements and its less accurate measurements of theballoon’s cross-range position. The defense could at-tempt to use these velocity and position estimates fordiscrimination by comparing them with the values thatwould be expected for a balloon containing a heavywarhead and with those estimated for each of the otherballoons. By making repeated measurements of theballoons and attempting to fit them to trajectories, thedefense can reduce but not eliminate these uncertainties.

As discussed in more detail in Appendix D, theprecision with which a radar could measure the radialvelocity of an object depends in part on the character-istics of the X-band radar and how it was operated. Forexample, it would depend on the integration time cho-sen by the operator. Because some of these detailedcharacteristics about the X-band radars and how theywould be operated are not publicly available, we can-not determine precisely the accuracy with which anX-band radar could measure the radial velocity of anincoming balloon. However, even if the X-band radar

could measure the radial velocity perfectly, the limita-tions in how accurately it could measure the cross-rangevelocity would limit how accurately it would know thetotal velocity of a balloon. Similarly, even if the X-band radar could measure the range of a balloon per-fectly, the limitations in how accurately it could measurethe cross-range position would limit how accurately itcould know the position of a balloon along its trajectory.

From the defense perspective, these inherent mea-surement limitations would be least problematic formissiles on trajectories that approached the radar di-rectly during reentry and worst for missiles on trajec-tories with a reentry that was perpendicular to the ra-dar line of sight. Thus, the attacker could exploit theseradar limitations by targeting cities not located directlyin front of an X-band radar (of which there would bemany). Doing so might be enough to prevent the de-fense from discriminating even relatively lightweightballoons at a high enough altitude to allow an interceptattempt. (See box.)

Nevertheless, the attacker might still choose to takeother steps to lower the defense’s discrimination alti-tude, as we discuss below.

Table 8-2. The radar horizon for targets at differentaltitudes.Because the earth’s surface is curved, a radar would not beable to see a target at a given altitude if the target wasfurther away than the corresponding radar horizon for thataltitude. This table gives the radar horizon (the grounddistance at which a radar could observe a target) for targetsat different altitudes, assuming the radar can view an object3 degrees above the horizon. For example, a radar wouldnot be able to see an object at an altitude of 500 kilometersif that object was more than 2,250 kilometers from theradar (as measured on the ground.) Alternatively, the radarcould only see an object at a distance of 2,250 kilometersfrom the radar if that object was at an altitude of 500kilometers or greater.

cold-gas thrusters). Either technique would deny the fineDoppler discrimination necessary for ISAR images. Themotions of surfaces and appendages would not need to belarge; amplitudes of a radar wavelength (about 3 cm)would be sufficient.

)mk(tegratfoedutitlA )mk(nozirohradaR

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To what accuracy, in actual practice, an X-band radarcould measure the location and velocity of an object isnot publicly known. In this box we estimate limits onthis accuracy based on general principles.

The discussion here assumes that the accuracy withwhich a radar can measure an object’s angular posi-tion after tracking it for a period of time is roughly givenby the radar beamwidth divided by 100 (seeAppendix D, particularly equation (D-3), which for theNMD X-band radars would be roughly 2.4×10-5 radi-ans, or 0.0014 degrees. Thus, for the discussion in thisbox, we assume the cross-range measurement accu-racy would be approximately given by (2.4×10-5 radi-ans) R, where R is the range from the radar to the object(not the ground range). The cross-range position uncer-tainty would increase with the object’s range from theradar; for an object during reentry at a range of 500kilometers from the radar, it would be roughly 12 meters.If the actual angular position measurement accuracy isless than or greater than the beamwidth divided by 100,these figures should be scaled accordingly.

Consider an object travelling on a trajectory at anangle γ with respect to the line of sight of the radar. Atany given time, the radar could accurately measure therange to the object, but would measure its cross-rangeposition with an uncertainty R ∆θ . Thus, the uncer-tainty in the object’s position along its trajectory, ∆P,would be approximately ∆P = R sin γ ∆θ, where R isthe range from the radar to the object. (This is easilyseen when the object is travelling perpendicular to theline of sight of the radar, so that γ = 90 degrees, since∆P is then just the full cross-range uncertainty in theposition.) For a balloon reentering at an angle γ of 30degrees or greater, the position uncertainty along thetrajectory ∆P would be roughly 6 to 12 meters at arange R of 500 kilometers (when the balloon was atan altitude of 200 kilometers), and roughly 12 to 24meters at a range R of 1,000 kilometers.a

Next consider the same object travelling at a ve-locity V and an angle γ with respect to the line of sightof the radar. At any given time, the radar can accu-rately measure the radial component of velocity Vr .We assume it can measure Vr perfectly. The defensethen estimates the full velocity from this measurementby using V = Vr /cos γ. But the value γ is uncertainsince the defense does not know the object’s trajec-tory precisely. The uncertainty in V due to the uncer-tainty in γ is thus ∆V = V tan γ ∆γ . Since V is large,even a small ∆γ can lead to a significant uncertainty∆V.

The uncertainty in determining the object’s trajec-tory arises from the uncertainty in measuring its an-gular position. On the other hand, tracking the objectover time and attempting to fit the measurements to atrajectory allows the defense with repeated measure-ments to reduce the angular uncertainty ∆θ of theobject’s position to the value given above. Thus, theuncertainties ∆γ and ∆θ are related, and we expectthat they must be roughly the same size, or about2.4×10-5 radians. Using this value of ∆γ, a balloon re-entering at a speed V of 7 kilometers per second wouldhave a velocity uncertainty ∆V of roughly 0.1–0.2meters per second for trajectories having γ equal to30 degrees or greater.

Thus, the defense would have the most difficultyif the attacker targeted cities that were on trajectoriesthat did not travel directly toward an X-band radarduring reentry.

Moreover, these estimates suggest that the inac-curacy ∆P with which the defense could determinethe position of a balloon along its trajectory may begreat enough that the defense could not use positionalong the trajectory for discrimination. For example,the position uncertainty would be roughly the samesize as the position change due to drag on a 5-kilo-gram balloon at 150 kilometers altitude (see Figure 8-4).

The Velocity and Position Measurement Accuracy of an X-Band Radar

a This assumes the trajectory is a standard one with areentry angle of 23.5 degrees with respect to the earth.

Exploit the Defense Geometry. The attacker couldmake atmospheric filtering more difficult by exploit-ing other defense weaknesses. As noted above, theplanned NMD system may have a serious vulnerabil-ity since each X-band radar would have only a singleface with a limited electronically scanned field of view

of about 50 degrees in both azimuth and elevation. Theradar would therefore need to rotate to view balloonson widely separated trajectories (with azimuths thatvaried by more than 50 degrees). One offensive tacticwould be to launch two (or more) missiles, timed toarrive simultaneously, with one aimed at a target to one


side of the radar and the other at a target on the otherside of the radar. For example, North Korea couldlaunch missiles that were simultaneously targeted onSan Francisco and San Diego, and Iran or Iraq couldlaunch missiles that were simultaneously targeted onBoston and Washington. Depending on the speed withwhich the X-band radar (in California and Massachu-setts, respectively) was able to rotate, it would lose timeand might even be unable to observe the reentry por-tion of one of the two missile trajectories. For example,if the radar could rotate at a rate of 10 degrees per sec-ond, 15–20 seconds might be required to switch be-tween the two targets.27 In practical terms, this wouldlikely force the radar to choose to observe only one ofthe targets during the critical reentry phase.

Cold-Gas Thrusters. Another option for the at-tacker would be to use small thrusters to speed up theempty balloons or slow down the balloon containingthe warhead.28 To avoid having to equip each decoywith its own thruster (and orientation system), the at-tacker could equip the warhead with such a thrusterand then orient the warhead so that the thrust is alongits velocity axis. Such a drag-simulating thruster coulduse cold gas to avoid being detected by infrared sen-sors. While this measure may sound difficult, smallthrusters of the type that would be needed for this pur-pose would not be difficult to make or acquire and arecertainly simpler technology than that required to makea long-range missile, which the attacker is assumed tohave.

As noted in Appendix E, two decades ago Britainreportedly developed decoys with small liquid-fueledthrusters attached to compensate for the difference inatmospheric drag on light decoys and heavy warheadsduring reentry.29

Spinning or Oscillating Balloons. To create varia-tions in the velocity and position of the surface of theballoons and thus mask the effects due to atmosphericdrag, the attacker could spin the balloons as they werereleased. By using a balloon with an irregular shape,and/or attaching lightweight corner reflectors (which

could be made out of aluminum foil) at random posi-tions to the surface of a spherical balloon, the attackerwould ensure that strong radar reflections would begenerated by various parts of the balloon as it spun.The attacker could spin the balloons so that the surfacevelocities due to spinning would be large compared withthe velocity changes due to atmospheric drag.30 TheX-band radar would then see a set of irregular time-varying Doppler shifts from each balloon that wouldmask the velocity change due to atmospheric drag. Theirregularity and spinning would also mask the displace-ment of the balloons due to atmospheric drag. To en-hance this effect, the attacker could even make irregu-lar star-shaped balloons that had long points stickingout with corner reflectors attached to them. Construct-ing such balloons would not require high quality con-trol since variations between the balloons would onlyadd to the variation of the signals seen by the radar.Moreover, if the balloon was nutating or more gener-ally spinning in a complicated way, which one wouldexpect, this would tend to randomize the Doppler shiftsseen by the radar.

Rather than spinning the balloons, the attacker coulduse lightweight springs to cause variations in the ve-locity and position of the surface of the balloons. Forthe balloons containing small weights or a warhead,the attacker could attach one or more springs betweenthe weight (or warhead) and the balloon. The springswould remain compressed during most the balloon’sflight, so that these balloons could not be distinguishedfrom those without springs. A simple timer could thenrelease the springs early in reentry. The springs wouldcause the balloon to oscillate irregularly around thecenter of mass. Using two springs with different springconstants attached to different parts of the balloon couldproduce a very complicated motion. The attacker couldeven add a small battery-powered motor to drive thesprings if it was concerned about the oscillations damp-ing during reentry.

As a result of simple measures like these, the radarwould measure a time-varying spread of velocitiesand positions for each balloon. The irregularity in the

27 Ten degrees per second appears to be a typical rotationrate for such large radar structures.28 Richard L. Garwin. “The Future of Nuclear Weapons”presentation for the Second ISODARCO School, Beijing,China, April 1990.29 Robert S. Norris, Andrew S. Burrows, and Richard W.Fieldhouse, Nuclear Weapons Databook, Volume 5: British,French, and Chinese Nuclear Weapons (Boulder, Colo.:Westview Press, 1994), p. 113.

30 A balloon with a diameter of 3 meters spinning at onerevolution per second would have a maximum surfacevelocity due to the spinning of about 10 meters per second,whereas one spinning at 5 revolutions per second wouldhave a maximum surface velocity of about 50 meters persecond. In these cases, the difference in surface velocitybetween one edge of the balloon and the edge on theopposite side would be 20 and 100 meters per second,respectively.


Doppler shifts and the position measurements wouldkeep the radar from being able to time average to findthe small signal it was looking for. Thus, although thedefense would be able to average out these effects tosome degree, the attacker could deny the radar the ex-tremely precise measurements that would be requiredfor discrimination at high altitudes.

Tethered Clusters of Balloons. The attacker couldtether together the balloons deployed by a missile inone or more clusters so that during the early part ofreentry, the balloon containing the warhead would helpcompensate for the drag on the others in the cluster.Thus, tethering would reduce the change in velocityand position that the lighter weight balloons would ex-perience due to atmospheric drag relative to the bal-loon containing the warhead. Even at an altitude of200 km, the atmospheric force on a balloon with a di-ameter of 3 meters is less than 0.14 newtons (half anounce) so the tethers would not need to have much struc-tural strength. Moreover, if the decoys were spacedclose enough together, then even if the radar could dis-criminate the balloon containing the warhead, it wouldnot be able to spatially resolve the individual balloonsin order to provide information to the kill vehicle as towhich balloon was the target.

Exploit Defense Limitations in Determiningthe Target PositionEven if the X-band radars could discriminate the bal-loon with the warhead from the other balloons, the ra-dar would need to convey this information to the killvehicle in a form that would allow the kill vehicle toidentify and home on the correct balloon. Ideally, theradar would create a three-dimensional “map” of theballoons, and the defense would then use this informa-tion to create a two-dimensional map of the balloonsas seen by the kill vehicle.

However, the inability of radars to measure thecross-range position of an object with high accuracymeans that a map an X-band radar constructs could haveintrinsic ambiguities regarding the position of the ob-jects it sees. Depending on the situation, these ambigu-ities could prevent the radar from being able to constructa map that the kill vehicle could use to identify theproper target.

More specifically, as we discuss above, an X-bandradar viewing the reentry of a cluster of balloons wouldbe able to determine that a given balloon was locatedat a certain range, but in the cross-range directions couldonly tell that it was located within a circular area per-

pendicular to that range. We will refer to that circulararea within which the X-band radar can locate the ob-ject as the “uncertainty disk.” The size of the uncer-tainty disk grows as the distance from the radar. If theballoons were close enough together and far enoughaway from the radar, their uncertainty disks would over-lap so that the radar would not be able to physicallydistinguish the balloons in the cross-range directions.In this case, the radar could only distinguish differentballoons by their range, but could not create a three-dimensional map of where the balloons were relativeto one another.

The infrared sensor on the kill vehicle would notbe able to measure the range to an object, but only itsangular position. The ambiguity in the radar map wouldmean that in general, if the balloons were close enoughtogether, the kill vehicle could not determine the posi-tion of the target using the radar map. This would onlybe possible for some cases, depending on the interceptgeometry.

The optimal situation for the defense would be ifthe kill vehicle were approaching the object at roughly90 degrees to the radar line of sight, in which case therewould be no uncertainty in the cross-range positions ofthe two balloons as seen by the kill vehicle. The worstsituation for the defense would be if the kill vehicleline of sight were the same as that of the radar, then theradar would simply pass on the uncertainty in the cross-range positions of the two balloons to the kill vehicle.

Thus, the attacker could make the map confusionproblem worse for the defense by attacking cities forwhich the radar and kill vehicle lines of sight to thereentry part of the trajectory would not be close to 90degrees during the reentry phase. The attacker couldalso choose to attack cities far from the radar so thatthe distance from the radar to the object would be largeand the uncertainty in cross-range position would belarge.31

We illustrate this problem in Figure 8-5 for a casein which there are only three balloons. However, themap confusion for the kill vehicle would increase asthe number of balloons increased.

By considering a simple case with two balloonshaving their uncertainty disks centered on the radar lineof sight we derive an estimate of when the radar mapcould be inadequate to determine the position of twoballoons as seen by the kill vehicle. The condition is:

31 The attacker could even use cold-gas thrusters on some ofthe balloons, as described above, but oriented in randomdirections to create greater confusion during reentry.


Figure 8-5. Radar map confusion for three bal-loons. This figure shows a simple situation in which theradar observes three balloons, at ranges R1, R2, and R3from the radar. The balloons are distinguished by shadingin the figure, but would be indistinguishable to the killvehicle. The plane of the page is the plane containing boththe radar line of sight and the kill vehicle line of sight to thetarget cluster. In this plane, the kill vehicle line of sight isat an angle θ with respect to the radar line of sight.

The radar’s best guess at the cross-range positions ofthe balloons is shown in (a), along with the uncertainty diskfor each balloon. The uncertainty disks are shown hereedge-on as lines with a length equal to the cross-rangeuncertainty ∆X.

But the balloons may actually lie anywhere on theiruncertainty disks (and need not lie in the plane of the paper,since the disks are two dimensional). Figures (b) and (c)show other possible positions of the three balloons. Asshown, the kill vehicle would see very different relativelocations of the balloons in the three cases. Thus, even if theX-band radar could identify the balloon containing thewarhead, it could not in this case construct a map thatwould allow the kill vehicle to identify that balloon.

Radar

Kill vehicle sees:

∆X

R1 R R2 3

θ

(a)

Radar

Kill vehicle sees:(b)

Radar

Kill vehicle sees:(c)

32 For a North Korean attack on Seattle, the closest radarwould be the one at Beale Air Force Base in NorthernCalifornia (to be deployed as part of the C-3 system) and theinterceptors could be launched from either Alaska or NorthDakota. At an altitude of 150 kilometers, the slant rangefrom the balloons to the radar is roughly 1,200 kilometers.For interceptors launched from North Dakota, θ would beapproximately 70 degrees; for interceptors launched fromAlaska, θ would be closer to 180 degrees. Thus, it wouldbe to the advantage of the defense to use the interceptorslaunched from North Dakota. In this case, position confu-sion could occur if ∆R were roughly 10 meters or less.

∆R = (R2 – R

1) ≤ ∆X / tan θ = (2 .4 × 10-5 ) × R / tan θ (8-1)

where R2 and R

1 are the ranges from the radar to the

two balloons, θ is the angle between the kill vehicleand radar lines of sight, ∆X, is the uncertainty in theradar measurement of the cross-range position of theballoons, and R is the approximate range from the ra-dar to the balloons.

Equation (8-1) shows that the position confusioncould occur for any angleθ as long as ∆R/R was suffi-ciently small. The attacker could ensure that this ratiowas small by tethering the balloons together. Note that∆R is the component of the separation between bal-loons along the line of sight of the radar, and in generalwill be smaller than the physical spacing of the bal-loons. Assuming the cluster of balloons was slowlyrotating, the ranges from the radar to the various bal-loons would change, and the range differences ∆R be-tween pairs of balloons could get arbitrarily small asballoons rotated past each other. The potential for con-fusion would increase significantly as the number ofballoons in the cluster increased.

To understand the effect of this position confusionon the defense, we consider several specific attack sce-narios. We assume that all of the X-band radars plannedfor the full C3 system would be in place, and that inter-ceptors could be launched from either of the sites inAlaska and North Dakota. We further assume that theradar would attempt to identify the warhead by atmo-spheric filtering, so that the intercept attempt wouldoccur late in the trajectory. We look at the geometry ofthe intercept engagements, and determine what valueof ∆R would lead to the confusion described above,and could thus prevent the kill vehicle from attemptingto intercept the right balloon.

We find that for attacks from North Korea againstSeattle or Los Angeles, such position confusion couldoccur if the range differences ∆R seen by the radar wereless than about 10 meters.32 The attacker could easilyensure this would be the case by tethering the balloons


For a North Korean attack on Los Angeles, the closestradar would be the one at Beale Air Force Base in North-ern California (also to be deployed as part of the C-3system) and the interceptors could be launched fromeither Alaska or North Dakota. For interceptors launchedfrom North Dakota, θ would be approximately 25 degrees,whereas θ would be approximately 45 degrees for inter-ceptors launched from Alaska. Thus, it would be to theadvantage of the defense to use the interceptors launchedfrom Alaska. At the closest intercept point to the radar atBeale, the altitude of the balloon would be 250 kilometersand the range to the radar would be 390 kilometers. In thiscase, the kill vehicle would again be unable to distinguishbetween balloons if ∆R were roughly 10 meters or less.33 For attacks from Iran or Iraq against Los Angeles, theclosest X-band radar would be the one at Beale, California.For interceptors from Alaska or North Dakota, θ would beroughly 50 degrees. At the closest intercept point to theradar, the balloons would be at an altitude of 250 kilome-ters, and the range to the radar would be roughly 470 kilo-meters. In this case, position confusion could occur if ∆Rwere roughly 10 meters or less.34 For attacks from Iran or Iraq against Chicago, the closestX-band radar would be the one at Grand Forks, North

together. We find a similar result for attacks from Iranor Iraq against Los Angeles.33 For an attack from Iranor Iraq against Chicago, position confusion could occurif the range difference were less than about 40 meters.34

If the reentry of the balloons could be seen by morethan one X-band radar, it might be possible for the de-fense to combine the position information provided by

Dakota (to be deployed as part of the C-3 system). Forinterceptors from Alaska, θ would be roughly 30 degrees;and for interceptors from North Dakota, θ would be zerodegrees, so interceptors from Alaska would have a betterviewing angle. For an intercept at an altitude of 150 kilo-meters, the range to the radar would be roughly 1,000 kilo-meters. In this case, position confusion could occur if ∆Rwere roughly 40 meters or less.35 Adding a laser range-finder to the kill vehicle couldaddress the problem somewhat. (While there are currentlyno plans to do this, the possibility has been discussed. SeeSirak, “A C1 to C2 Move.”) Doing so would allow the killvehicle to make better use of the range information from theradar to reduce position ambiguities. For example, thiswould be the case if the kill vehicle and radar line-of-sightswere parallel or antiparallel (θ = 0 or 180 degrees).However, we find that potential ambiguities would still existfor a range of angles θ, and that even with the additionalinformation provided to the kill vehicle by a laser range-finder, it would be very difficult if not impossible for thedefense to pass an adequate map for a large cluster ofclosely spaced objects.

the multiple radars to construct a better map for the killvehicle and to eliminate or reduce the position confu-sion discussed above. However, as discussed above,since we are considering engagements below about 400kilometers altitude, for most target cities the balloonswould only be in the field of view of one X-band radar(see Table 8-2).35


Chapter 9

Emerging Missile State Countermeasure 3:A Nuclear Warhead with a Cooled Shroud

If a nuclear warhead were covered with a metal shroudcooled to a low temperature, then the range at whichthe infrared sensors on the kill vehicle could detect thewarhead would be reduced. If the warhead is cooled toa low enough temperature, then the detection range canbe reduced enough so that even if the kill vehicle isable to detect the warhead, it would not have enoughtime to maneuver to hit it.

As we discuss below, a thin metal shroud that iscooled by liquid nitrogen to a temperature of 77 K wouldbe straightforward to implement above the atmosphere.The level of technology required for such a cooledshroud is very low relative to that required to build along-range missile or a nuclear warhead. Such shroudedwarheads would prevent hit-to-kill homing by exoat-mospheric interceptors using infrared seekers and wouldthus defeat the planned US National Missile Defense(NMD) system.

The Design DetailsLiquid nitrogen boils at the low temperature of 77.4Kelvin (K) (–196 degrees Celsius). A metal shroud incontact with liquid nitrogen will thus remain at about77 K until all the nitrogen has boiled away. Liquid ni-trogen is widely used in research and engineering ap-plications to maintain materials at a low temperatureand is readily available (it can be produced by coolingair, which is about 78 percent nitrogen).

A warhead shroud that could be cooled to liquidnitrogen temperature could readily be made from alu-minum. A simple design would be a double-walledcone-shaped shroud containing liquid nitrogen coolantin the cavity between the inner and outer walls. Sincethe warhead would give off heat, the designer wouldthermally isolate the warhead from the shroud to mini-mize the heat transfer to the shroud. The shroud could

be attached to the warhead with pegs made of a mate-rial with low thermal conductivity, such as Teflon.

Multilayer insulation would be placed in the gapbetween the warhead and shroud to greatly reduce theheat transfer by radiation from the warhead to theshroud. Multilayer insulation, sometimes referred to as“superinsulation,” consists of many layers of metallizedplastic (such as thin sheets of mylar with aluminumevaporated onto the surface) with very thin spaces be-tween the layers. (To the human eye, it appears similarto aluminum foil.) Multilayer insulation is availablecommercially and is a very effective insulator.1

A first generation nuclear warhead deployed by anemerging missile state would likely be large. We as-sume here that such a warhead could be contained in acone with a base diameter of one meter and a height ofthree meters. The shroud would then be slightly largerthan the warhead, which would be inserted through theopen back end of the shroud.

Within this design concept, there would be manydesign choices available to the engineers building ashrouded warhead. For this discussion, we will assumethat a pressure release valve in the base of the shroudwould be used to control the gas pressure between thewalls of the shroud as liquid nitrogen boils off into gas.One side of the pressure release valve would be attachedto a tube that vents expended nitrogen gas through theshroud-base to space. To prevent this venting gas fromproducing a thrust, a simple T-shaped outlet nozzlecould be used. The net force from gas leaving one end

1 The highly reflective metallized surfaces reduce the heattransfer by radiation with an effectiveness that increasesgeometrically with the number of layers. Multilayer insula-tion is punctured with many small holes to permit air toescape quickly in a vacuum, and the vacuum betweenlayers greatly reduces heat transfer by conduction.


of the T opening would be offset by the force from gasleaving the opening that points in the opposite direc-tion. The other side of the valve is connected to a tubethat transfers expended gas-phase nitrogen coolant fromthe shroud nose area. A schematic of such a cooledshroud is shown in Figure 9-1.

For this design, when powered flight was com-pleted, standard techniques would be used to deploythe shrouded warhead so that it would be spin stabi-lized, rotating slowly around its axis of symmetry, andoriented in the desired direction. We discuss later inthis chapter what orientations the attacker could use tocontrol reflected infrared radiation from the Earth. The1999 NIE concludes that countermeasure technologiessuch as “separating RVs, spin-stabilized RVs, and RVreorientation” are “readily available” to countries suchas North Korea, Iran, and Iraq.

Since the shroud would be attached to a rotatingstabilized warhead, centrifugal forces would confinethe liquid-phase nitrogen coolant to the outer and lowerregions of the shroud (see Figure 9-2). There wouldonly be gas-phase nitrogen coolant in the tip of theshroud—where gas would be released through an alu-

minum tube connected to the pressure release valve inthe base of the shroud. This design would thereforeavoid the complicating problems of dealing with mixedphases of liquid nitrogen and gas in an environmentwith no gravity.

The shroud could be designed so that it could beremoved from the warhead prior to reentry by a spring-loaded device or small gas generator behind the shroud-nose, which would be activated by a timer. Such a sepa-ration process is shown in Figure 9-3.

As noted above, a reasonable estimate for the sizeof such a conical shroud would be a base diameter ofone meter and a height of three meters. Thus, the innerand outer walls of the shroud would each have a sur-face area of approximately 5 square meters—for a to-tal surface area of 10 square meters. If we assume thewalls of the shroud are a generous 1.5 millimeters thick(roughly 1/16 of an inch), then an aluminum shroud(with a density of roughly 2.7 grams per cubic centi-meter) will weigh some 40 kilograms.

Such a shroud would require at most a roughly equalweight of liquid nitrogen coolant (40 kilograms) to chillit from room temperature (300 K) to liquid nitrogen

If NMD Interceptor Acquisition RangeReduced from 300 Kilometers to≤ 0.1 kilometers.

Interceptor Homing Time Reducedfrom 30 seconds to ≤ 0.1 seconds

At X Band:

Radar CrossSection

m

dBsM

−

≈ ≅

−

0.1 0.0001

40

2 2λ

( )

If NMD Interceptor Acquisition RangeReduced from 300 Kilometers to≤ 0.1 kilometers.

Interceptor Homing Time Reducedfrom 30 seconds to ≤ 0.1 seconds

At X Band:

Radar CrossSection

m

dBsM

−

≈ ≅

−

0.1 0.0001

40

2 2λ

( )

Figure 9-1. Schematic diagram of liquid-nitrogen cooled shroud.


temperature of 77 K.2 As we calculate in Appendix I,about 200 grams of coolant per minute would then berequired to maintain this temperature while the shroudis exposed to direct sunlight, sunlight reflected fromthe Earth, infrared radiation radiated by the Earth(earthshine), and heat radiated from the warheadthrough the superinsulation and the Teflon supports.Thus, about 6 kilograms of liquid nitrogen would berequired to keep the shroud cool for 30 minutes, whichis the flight time of an intercontinental-range ballisticmissile. (If part or all of the warhead trajectory werenot in sunlight, then less coolant would be required tomaintain the shroud at a temperature of 77 K.) As we

discuss below, the attacker wouldlikely choose to use cooled shroudson trajectories that are partially orcompletely in the Earth’s shadow;for a trajectory completely in theEarth’s shadow the amount of ni-trogen required to maintain theshroud at 77 K would be about onekilogram.

For this discussion, we will as-sume that the attacker would beginto cool the shroud only after themissile was launched and wasabove the atmosphere in thevacuum of space. Although it wouldbe possible to cool the shroud priorto launch, by waiting until the war-

head is above the atmosphere, the attacker would avoidany potential problems associated with water freezingon the inside and outside of the shroud.3 Once the war-head is above the atmosphere, the liquid nitrogen couldbe pumped into the space between the two walls of theshroud using gas pressure or a small pump. Until then,the nitrogen could be stored in a flat, cylindrical tankattached to the bottom of the warhead. While it would

Figure 9-3. Prior to reentry, the shroud could beseparated from the warhead using a spring-loadeddevice.

3 If the attacker wanted to cool the shroud prior to launch,in order to prevent water from freezing on the shroud, theattacker would need to control the humidity in the warheadenvironment while the warhead remained in the atmo-sphere. One way to do so would be to house the warheadin an aerodynamic fairing flushed with or containing drynitrogen.

2 The specific heat of aluminum at room temperature isapproximately 900 J/kg-K (Ray E. Bolz and George L.Tuve, eds., Handbook for Tables of Applied EngineeringScience, Cleveland, Ohio: The Chemical Rubber Com-pany, 1970, p. 96). Thus, to cool a shroud weighing Mkilograms from a temperature of 300 K to 77 K wouldrequire (900)(223) M = 2×105 M joules. Since the heat ofvaporization of liquid nitrogen is approximately 2×105 J/kg(ibid., p. 74), then the amount of liquid nitrogen required tocool the shroud would be M. However, this calculationsomewhat overestimates the amount of nitrogen required,since it neglects the decrease in the specific heat of alumi-num as the temperature is decreased as well as any coolingeffect of the gas-phase nitrogen. For example, the specificheat of aluminum at 77 K is about 330 J/kg-K (Y. S.Touloukian and E. H. Buyco, Thermophysical Properties ofMatter, Volume 4: Specific Heat–Metallic Elements andAlloys (New York: IFI/Plenum, 1970), pp. 1–3.)

Figure 9-2. Spinning motion of warhead and shroudwill confine liquid-phase nitrogen to outer and lowerregions of the shroud.

WarheadWarhead


take some minutes for the shroud to cool down to 77 K,it would be fully cooled before an interceptor couldreach the shrouded warhead. (Although the SBIRS-lowsensors would be able to detect the shrouded warheaduntil it had cooled somewhat, this would not help thedefense.)

The total amount of liquid nitrogen needed to coolthe shroud down to 77 K and maintain it at that tem-perature would be less than 46 kilograms, or about58 liters of liquid nitrogen. This amount of liquid couldbe contained in a double-walled tank with a base diam-eter of 1 meter (to match that of the warhead) and aheight of 8 centimeters. If made from 1.5-millimeter-thick aluminum, such a cylinder would weigh about14 kilograms.4 Thus, the total weight of the shroud, theliquid nitrogen coolant, and the nitrogen storage con-tainer would be roughly 100 kilograms. This would addabout 10 percent to the payload for a first generationwarhead weighing 1,000 kilograms. Thus, an existingmissile could deliver a shrouded warhead of the sameweight to a somewhat shorter range or a somewhatsmaller and lighter warhead to the same range. For ex-ample, a missile that could carry an unshrouded war-head weighing 1,000 kilograms to a range of 12,000kilometers could instead deliver a shrouded warheadto a range of roughly 10,000 kilometers.

Reduced Infrared Detection RangeThe exact wavelength of the radiation that the infraredsensors on the kill vehicle will use is not publiclyknown. However, sensor arrays that detect infrared ra-diation at wavelengths of 3 to 5 microns (µm) that wouldbe suitable for use on a kill vehicle are currently avail-able, and sensor arrays that operate at a wavelength of10 microns may now be available or may become avail-able in the future. A shrouded warhead at liquid nitro-gen temperature would radiate a 5-micron infrared sig-nal roughly a trillion times (1012) less intense than thatof an unshrouded warhead (see Figure 9-4a). This meansthat if a kill vehicle’s 5 µm sensor allowed it to beginhoming on a room temperature warhead at a range of1,000 kilometers,5 it could only begin to home againsta warhead with a cooled shroud at a range of about onemeter! As Figure 9-4a shows, even if the NMD killvehicle uses a sensor that can operate at a wavelengthof 10 microns, the signal from the cooled shroud wouldbe roughly a million times (106) less intense than thatfrom an unshrouded warhead. In this case, the kill ve-hicle acquisition range would be reduced from 1,000kilometers to 1 kilometer.

Figure 9-4. (a) Relative emission of a blackbody at three different wavelengths as a function of itstemperature (b) The detection range as a function of warhead temperature, relative to the detec-tion range of a warhead at room temperature.

(a) Warhead Temperature (Degrees Kelvin)

Rel

ativ

e In

tens

ity o

f S

igna

l

(b) Warhead Temperature (Degrees Kelvin)

Rel

ativ

e In

terc

epto

r A

cqui

siti

on R

ange

(%

)

4 The surface area of a cylinder with a base diameter of1 meter and a height of 8 centimeters is roughly 18,000square centimeters. Aluminum has a density of 2.7 gramsper cubic centimeter.

5 This detection range may be generous to the defense. Forthe second sensor fly-by test, the Raytheon kill vehiclereportedly acquired the targets at a range of 700–800 km.(William B. Scott, “Data Boost Confidence in Kill VehiclePerformance,” Aviation Week and Space Technology,8 June 1998.)


The implications of such a reduction in kill vehicleacquisition range are dramatic, as shown in Table 9-1.We assume that the NMD system—using data fromthe ground-based radars and SBIRS-low—is able toguide the interceptor booster to its basket with nearperfect precision. Thus, we will assume that the lateralmiss distance the kill vehicle would have to correct foronce it acquires the target is only 20 meters. (SeeFigure 9-5).

An interceptor that begins to home on its target at arange of 1,000 kilometers will have roughly 100 sec-onds to maneuver laterally if the target and interceptorhave a closing speed of 10 kilometers per second. Un-der these conditions, a lateral movement of 20 meterswould require only a very small average lateral accel-eration of 0.0004 g (where g is the acceleration due togravity, which is approximately 10 m/sec2). This is eas-ily managed by the kill vehicle, which would likely

have a lateral acceleration capability of a few tens ofgs.6 However, if the kill vehicle instead detects the tar-get at a range of only 1 kilometer, the required averagelateral acceleration would be 400 g, well beyondthe capability of the kill vehicle, even assuming theinterceptor responds instantaneously after detecting thetarget.

If the shrouded warhead reduces the kill vehicledetection range to even several kilometers, for allpractical purposes the probability of an intercept willbe reduced to zero. With infrared sensors that detectradiation of 3–5 microns, the detection range would bereduced to about a meter; for 10-micron sensors, thedetection range will only be about a kilometer. In ei-ther case, it is clear that the kill vehicle will have nochance of intercepting the target.

Detection Using Reflected RadiationIn addition to an infrared signal radiated bythe shrouded warhead, there may also be asignal from infrared radiation or visible lightthat is reflected off the shroud. Such reflec-tions from the shroud could be due to visiblelight coming directly from the sun or fromsunlight that is reflected off the Earth. Sincethe Earth is an intense emitter of infrared ra-diation, the shroud could also reflect infraredradiation from the Earth. However, as we willshow, an attacker can take measures to

rotpecretnIegnaRnoitisiuqcA

emiTgnisolCrotpecretnilaretalegarevA

deriuqernoitareleccag( ≈≈≈≈≈ ces/m01 2)

sretemolik000,1 sdnoces001 g4000.0

sretemolik001 sdnoces01 g40.0

sretemolik01 sdnoces1 g4

retemolik1 sdnoces1.0 g004

Table 9-1. The average lateral kill vehicle acceleration requiredfor the kill vehicle to hit the target as a function of kill vehicledetection range (labeled “R” in Figure 9-5).We assume a closing speed of 10 km/sec, and a lateral miss distance of20 meters at kill vehicle acquisition.

Figure 9-5. Assumed intercept geometry. Here the kill vehicle is moving in the horizontal direction tothe right. When the kill vehicle first detects the warhead at a range R, we assume the lateral missdistance would be only 20 meters if the kill vehicle did not maneuver.

R

direction of motion of kill vehicle

20 meters

6 Based on the kill vehicle’s mass and fuel, weestimate its total ∆V to be about 1 km/sec. If weassume that it must have enough fuel for 10 sec-onds of thrust, the average acceleration would be10 g.)


θE10

o

Region of reflectedinfrared radiation

Region of no reflectedinfrared radiation

x

z

the Earth. For a warhead at 130 kilometers—the goalfor the minimum intercept altitude of the NMD killvehicle—the half-angle θ would be 82 degrees.

By tipping the shroud so that its axis is no longervertical, the attacker could shift the orientation of theregion of no reflected infrared radiation. In particular,the region could be rotated up so that it was no longersymmetric around the vertical and more of its volumefaced in directions from which an interceptor might ap-proach. Detailed calculations show that the shape ofthe region would distort somewhat from conical, butthe region would remain very broad for tip angles of afew tens of degrees. For example, Figure 9-6 showsthe boundary of the region of no reflection for a case inwhich the warhead is at an altitude of 1,000 kilometersand the axis of the warhead is rotated 10 degrees fromthe vertical. The warhead will reflect infrared radia-tion from the Earth only into directions lying in theregion above this surface. When releasing the warhead

essentially eliminate the possibility that the kill vehiclecould home on these signals.

Reflected Infrared Radiation. If the cooled shroudreflected all the Earth’s infrared radiation that impingedon it, then the reflected infrared radiation would becomparable in intensity to that emitted by a warhead atroom temperature.7 Since the infrared absorptivity ofaluminum is 0.03, the shroud would reflect almost allthe infrared radiation that strikes it. If some of this radia-tion were reflected toward the kill vehicle, it might beadequate to permit the kill vehicle to home on it. How-ever, a shroud made of polished aluminum would be aspecular reflector. Like a mirror, it would reflect radia-tion at the same angle relative to the surface of theshroud that the incident radiation strikes the shroud.8

As a result, for the type of shroud we consider here,there would be a broad range of directions into whichthe shroud will reflect no infrared radiation from theEarth.

To see this, consider a shroud with half-angle α atan altitude h above the Earth’s surface and pointedstraight down toward the Earth. In this case, it isstraightforward to show that this shroud would not re-flect radiation into a conical volume of half-angle θ,where

θ = 180 – 2α – arcsin[Re /(R

e+h)] degrees (9-1)

and Re is the radius of the Earth (6,370 kilometers).

This region lies below the warhead and is symmetricaround the vertical direction. For a shroud with a basediameter of 1 meter and a height of 3 meters, the half-angle α would be 9.5 degrees. The region of no reflec-tions would change with altitude since the angle atwhich radiation approaches the warhead from the Earthdepends on altitude. For example, at an altitude h of370 kilometers, the half angle θ would be 90 degreesand no infrared radiation would be reflected into thehalf space below the shroud, bounded by a horizontalflat plane. For a warhead at an altitude of 1,000 kilo-meters, the half angle θ would be 101 degrees (in thiscase, it is easier to think of the infrared radiation beingreflected into a cone of half-angle 180 – 101 = 79 de-grees, with the tip of the cone pointing down toward

7 The Earth infrared flux is about 240 W/m2, and a 300 Kblackbody emits about 280 W/m2 over the 3 to 16 µm band.8 The attacker could easily cover the shroud with a thin layerof another material, such as polished gold, if it was con-cerned that the surface of the aluminum might not besufficiently specular.

Figure 9-6. The region of no reflection for a tippedwarhead.This figure assumes the warhead is at an altitude of 1,000kilometers, and the axis of the warhead has been rotatedaround the y-axis by 10 degrees from the vertical. Thewarhead will reflect infrared radiation from the Earth intothose directions lying in the conical region above thesurface shown.

At this altitude, a kill vehicle looking down at thewarhead at angles less than θE = 59.8 degrees from thevertical would see the earth rather than space as a back-ground.

Along the positive x-axis, the lower boundary of theregion of no reflection lies at an angle that is less than5 degrees greater than θE. Thus, a kill vehicle approachingthe warhead from the right side of the figure would be ableto see reflected radiation against a space background only ifits direction of approach happened to fall within this narrowrange of angles. This range of angles could be furtherreduced by using a tipping angle greater than 10 degrees.


from the missile after boost phase, the attacker couldorient the warhead to point the region of no reflectiontoward the directions from which interceptors wouldbe approaching as they neared the warhead. The 1999National Intelligence Estimate noted that emergingmissile states must be expected to be able to spin stabi-lize warheads, which would allow such orienting.9

Since the region of no reflection is very broad, the at-tacker would not need to orient the shroud with highprecision.10

Reflected Visible Light. Although the kill vehiclewill have a visible sensor to aid in target detection, asthe system is currently configured, the final homing(during the last tens of kilometers) must be done usingthe infrared sensors.11 In this case, any visible lightreflected from the shroud could not be used to homeon the warhead. We do not know if the current designcan be modified to permit final homing using the vis-ible sensor, but to eliminate the chance that the killvehicle could home on visible light reflected from theshrouded warhead, the attacker can simply choose toattack at night (or more precisely, when the missile’strajectory would be in the Earth’s shadow), much asIraq chose to launch nearly all of its Scud missiles atnight during the Gulf War. Since the attacker wouldpresumably initiate the conflict with the United States,it would have considerable flexibility in choosing thetiming of the attack.

Since the Earth’s axis of rotation is inclined 23 de-grees relative to the Earth’s orbital plane, an emergingmissile state would be able to attack some cities usingtrajectories that are entirely in the Earth’s shadow atonly certain times of the year, whereas other cities couldbe attacked year round using such nighttime trajecto-ries. However, the entire trajectory would not need tobe in the Earth’s shadow, only the part where an inter-cept could occur.

For example, Figure 9-7 shows that in midwinter,North Korea could attack the entire United States us-ing trajectories that are never sunlit, although attack-ing the east coast would require it to use trajectoriesthat are depressed slightly below normal (about a17-degree, rather than a 23-degree, loft angle).

Flying missiles on the kinds of modestly depressedtrajectories considered here would not be difficult for acountry that had a missile capable of flying on stan-dard trajectories. Atmospheric forces during boost phaseare not a problem since the missile can be flown on astandard trajectory until it is high enough that the at-mospheric density is low and can then be turned onto adepressed trajectory.12 Indeed, in its 31 August 1998missile test, North Korea successfully launched its mis-sile onto a significantly depressed trajectory.

Since the reentry vehicle on a depressed trajectorytravels a longer path through the atmosphere duringreentry, there are two other potential concerns: that theaccuracy will degrade and that additional heating maybe a problem. However, missiles deployed by emerg-ing missile states would have very poor accuracy evenon a standard trajectory, and the additional loss ofaccuracy would not be significant. Moreover, detailedcalculations show that heating would also not be aproblem.13

9 National Intelligence Council, “National IntelligenceEstimate (NIE): Foreign Missile Development and theBallistic Missile Threat to the United States Through2015,” unclassified summary, September 1999, p. 16.10 Even if the kill vehicle approached the warhead so that itviewed the warhead against the background of the Earth,the warhead would not be detectable as a cold spot againstthe warm Earth background. Until the kill vehicle was closeto the warhead, the warhead would fill only a small fractionof a pixel on the kill vehicle seeker array, and thus wouldnot produce a detectable reduction in the Earth back-ground seen by that pixel. For example, if the seeker hada one-degree field of view and a 256 x 256 seeker array, awarhead would only fill about 2 percent of a pixel at arange of 100 kilometers and about 10 percent at a rangeof 50 kilometers (which would be about 5 seconds beforea possible intercept).

In addition, at all angles of approach over which thekill vehicle could view the warhead against the Earthbackground, the warhead would reflect earthshine towardthe kill vehicle. As Figure 9-6 shows, for a warhead at analtitude of 1,000 kilometers, a kill vehicle looking down atthe warhead at angles less than θE = 59.8 degrees fromthe vertical would see the Earth rather than space as abackground. Since the warhead reflects into those direc-

tions, this would further reduce the possibility that the killvehicle could detect the warhead as a cool spot against thewarm earth background.11 In the 18 January, 2000 intercept test, the kill vehiclefailed to hit its target because the infrared sensors were notfunctioning properly. In this test, the final homing began at6 seconds before the predicted impact time, and theclosing speed between the kill vehicle and the mockwarhead was 6.7 kilometers per second (Defense Depart-ment Background Briefing on Upcoming National MissileDefense System Test, 14 January 2000). Thus, the finalhoming—to be performed by the infrared sensors—began ata distance of roughly 40 kilometers from the target.12 This is discussed in detail in Lisbeth Gronlund and DavidWright, “Depressed Trajectory SLBMs,” Science and GlobalSecurity, Vol. 3, 1992, pp. 101–159.13 Calculations of reentry heating were conducted on10,000-kilometer-range trajectories with reentry angles of


Finally, we note that the modest depressions of tra-jectories we consider here would lead to minimal lossof range.14

Figure 9-8 shows that during midspring or midfall,the west coast of the United States, as well as Hawaiiand Alaska, could be attacked by North Korea ontrajectories that are never sunlit, although a slight de-pression (20-degree loft angle) would be needed forthe west coast.

20o and 15o, and were compared to similar calculations ona standard, minimum-energy trajectory of the same rangewith a reentry angle of 23o. (These calculations used themethod described in Appendix F.) The peak heating rate andtotal heat absorbed per area were calculated at the nose ofthe RV and on the wall of the RV at a point one meterbehind the nose. These calculations show that the peakheating rates are actually less on the depressed trajecto-ries than on the standard trajectory (by approximately 4%and 15% at the nose for the 20o and 15o cases, respec-tively, and by approximately 8% and 24% on the wall of theRV) since the RV’s speed on the depressed trajectories islower at the altitudes of peak heating. The total heat

Figure 9-9 shows that with some depression of tra-jectory (roughly a 19-degree loft angle), North Koreacould attack Hawaii on a trajectory that is not sunliteven in midsummer.

Thus North Korea could attack Hawaii onnonilluminated trajectories at any time of year. Withsome depression of the trajectories, North Korea couldattack San Francisco on nonilluminated trajectoriesfor more than six months a year, and North Korea couldeven attack Washington, D.C., on a nonilluminatedtrajectory by using a more depressed trajectory(15-degree loft angle) for about one month a year dur-ing midwinter.

23 degree Loft, 7.09 km/sec; 20 degree Loft, 7.12 km/sec; 20 degree Loft, 6.85 km/sec (NK to Hawaii)

Figure 9-8. Trajectories from North Korea to SanFrancisco and Hawaii during midspring and midfall(zero degree Earth inclination).Two trajectories are shown for San Francisco, one at astandard loft of 23 degrees (which will be partially sunlit),and one slightly depressed to a loft angle of 20 degrees(which would be entirely in the Earth’s shadow).

absorbed is somewhat higher on the depressed trajectories(by approximately 7% and 24% at the nose for the 20o and15o cases, respectively, and by approximately 4% and 9%on the wall of the RV), since the duration of heating issomewhat longer. An emerging missile state could easilyaccommodate these increases by a modest thickening ofthe heat shield.14 Flying a missile with a maximum range of 10,000kilometers on a depressed trajectory with a reentry angle of20o rather than a standard trajectory with a reentry angle of23o would only reduce the range by a few tens of kilome-ters, or by a few hundred kilometers for a trajectory with areentry angle of 15o.

degree Loft, 7.37 km/sec (NK to DC); 23 degree Loft, 7.3 km/sec (Chicago); 24 degree Loft, 7.09 km/sec (SF); 25 degree Loft, 6.75 km/sec (Hawai

Figure 9-7. Trajectories from North Korea to NewYork, Chicago, San Francisco and Hawaii duringmidwinter (23 degree Earth inclination).This shows the night side of the Earth (viewed from thesame distance as the sun). This figure demonstrates that atthis time of year all of these cities could be attacked byNorth Korea on trajectories that were entirely in the Earth’sshadow. Keeping the trajectory to New York entirely in theEarth’s shadow would require depressing its trajectory to aloft angle of 17 degrees. (The gray curves under thetrajectories show the ground tracks of the missile.)


Figure 9-10 shows that Iran or Iraq could attackWashington, D.C., on trajectories that are not sunlit inmidfall or midsummer, using standard trajectories(23-degree loft angle). With some depression of tra-jectories, these countries would be able to attack Wash-ington, D.C., on trajectories that are not sunlit at least8 months out of 12.

15 The full moon is about 1/400,000 as bright as the sun, soits flux is about 0.0034 W/m2. MSX’s visible sensor (Appen-dix B) is said to be able to detect targets with reflectivity-area products of 0.1–0.35 m2 viewed against a dark spacebackground at ranges of “several times” 6,000 km. Since thekill vehicle’s detection capability is likely to be at leastseveral times poorer, assume it would be 6,000 km againstsuch targets. If illuminated by full moonlight, this wouldcorrespond to a detection range of about 10 km. Thus wecan make a very rough estimate of the kill vehicle’s visualdetection range as ranging from about 10 km for a very highemissivity (low reflectivity) shroud to about 30–40 km for alow emissivity shroud.

Reflected moonlight could, in principle, also be asource of visible light, although it is unlikely that thissource is bright enough to be exploited by a homingkill vehicle.15 However, if an attacker is sufficientlyconcerned about this source of illumination, timing thelaunch so that the moon is also below the horizon wouldaddress this concern.

23 degree Loft, 7.2 km/sec

Figure 9-10. Trajectory from Iran to Washington,D.C., during midfall or midspring (zero degree Earthinclination).A standard (loft angle of 23 degrees) trajectory will never besunlit.

15 degree Loft, 6.95 km/sec; 20 degree Loft, 6.85 km/sec;

Figure 9-9. Trajectories from North Korea to Hawaiiduring midsummer (23 degree Earth inclination). Twotrajectories are shown.A slightly depressed trajectory with a 20-degree loft anglewill be briefly sunlit, while one with a smaller loft angle of15 degrees will never be sunlit.

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