the lincoln near-earth asteroid research (linear) program · • stokes, shelly, viggh, blythe, and...

• STOKES, SHELLY, VIGGH, BLYTHE, AND STUARTThe Lincoln Near-Earth Asteroid Research (LINEAR) Program

VOLUME 11, NUMBER 1, 1998 LINCOLN LABORATORY JOURNAL 27

A have crashed intoEarth since the formation of the solar system.In the late 1980s, scientists established that

the impact of a large asteroid or comet on the edge ofthe Yucatan Peninsula caused the extinction of the di-nosaurs [1]. The recent collision of the comet Shoe-maker-Levy 9 with Jupiter shows that such massivecollisions still occur in our solar system. While suchlarge-scale collisions occur only once in a millionyears, smaller impacts that cause severe regional de-struction happen about once in a hundred years [2].The presence of many asteroids in orbits that mayeventually collide with Earth has generated consider-

The Lincoln Near-Earth AsteroidResearch (LINEAR) ProgramGrant H. Stokes, Frank Shelly, Herbert E.M. Viggh, Matthew S. Blythe,and Joseph S. Stuart

■ Lincoln Laboratory has been developing electro-optical space-surveillancetechnology to detect, characterize, and catalog satellites for more than fortyyears. Recent advances in highly sensitive, large-format charge-coupled devices(CCDs) allow this technology to be applied to detecting and catalogingasteroids, including near-Earth objects (NEOs). When equipped with a newLincoln Laboratory focal-plane camera and signal processing technology, the1-m U.S. Air Force ground-based electro-optical deep-space surveillance(GEODSS) telescopes can conduct sensitive large-coverage searches for Earth-crossing and main-belt asteroids. Field measurements indicate that theseenhanced telescopes can achieve a limiting magnitude of 22 over a 2-deg2 fieldof view with less than 100 sec of integration. This sensitivity rivals that of muchlarger telescopes equipped with commercial cameras.

Working two years under U.S. Air Force sponsorship, we have developedtechnology for asteroid search operations at the Lincoln LaboratoryExperimental Test Site near Socorro, New Mexico. By using a new large-format2560 × 1960-pixel frame-transfer CCD camera, we have discovered over 10,000asteroids, including 53 NEOs and 4 comets as designated by the Minor PlanetCenter (MPC). In March 1998, the Lincoln Near-Earth Asteroid Research(LINEAR) program provided over 150,000 observations of asteroids—nearly90% of the world’s asteroid observations that month—to the MPC, whichresulted in the discovery of 13 NEOs and 1 comet. The MPC indicates that theLINEAR program outperforms all asteroid search programs operated to date.

able discussion among the press, scientists, and gov-ernment agencies about how to estimate the likeli-hood of such a collision and prevent it from happen-ing. To date, no consensus on the most effectivemethods for intercepting or otherwise diverting sucha collision has emerged; however, there is agreementthat the first step toward protecting Earth is to findand catalog all potentially threatening asteroids, andtrack new comets entering the solar system.

Figure 1 depicts the inner solar system, showingthe main asteroid belt between the orbits of Mars andJupiter. Most asteroids in the main belt have low-ec-centricity orbits that will not encounter Earth. Aster-


28 LINCOLN LABORATORY JOURNAL VOLUME 11, NUMBER 1, 1998

oids with orbits that cross or approach Earth’s orbitare one of three types of near-Earth objects (NEOs):Aten, Apollo, and Amor. Aten asteroids typicallyspend most of their time closer to the Sun than toEarth, crossing Earth’s orbit at aphelion, when theyare farthest from the Sun. The semimajor axis of anAten’s orbit is less than that of Earth’s orbit, which isone astronomical unit (150 million km). Apollo as-teroids spend most of their time farther away fromthe Sun than from Earth, crossing Earth’s orbit nearperihelion, when they are closest to the Sun. Thisperihelion distance averages less than the semimajoraxis of Earth’s orbit. The NEO in Figure 1, therefore,is an Apollo asteroid. Amor asteroids approach Earth’sorbit, but their orbits pass inside the semimajor axisof Mars’ orbit and do not cross Earth’s orbit. Amor as-teroids must be closely monitored because they comequite close to Earth’s orbit. Comets from the outer so-lar system can also cross Earth’s orbit, often with littlewarning and at high velocities.

The Spaceguard Survey, a NASA study on NEOs,estimates that 320,000 asteroids exist with diametersgreater than 100 m, and of these, 2100 asteroids have

diameters greater than 1 km, as shown in Figure 2 [2].Researchers have cataloged about 10% of asteroidswith diameters larger than 1 km, and they know of aneven smaller percentage of asteroids with diametersbelow 1 km. Larger asteroids could cause global dam-age in an Earth collision. Smaller-diameter asteroidscolliding with Earth could cause considerable re-gional damage, especially in coastal areas because ofimpact-generated tsunamis. The remaining asteroidpopulation must be detected and cataloged to assessthe near-term threat of such objects. As each asteroidis discovered, researchers can calculate its orbit to de-termine if the asteroid will pass close to Earth.

Historically, the task of discovering asteroids hasbeen expensive and time consuming. The traditionalsearch technique relied on photographic plates andlabor-intensive manual detection and measurementof objects. Newer electronic imaging techniques thatuse charge-coupled devices (CCDs) are faster thanphotographic surveys because the CCD images areprocessed with computers and the detection and mea-surement of objects is automated.

In the early 1980s, Tom Gehrels of the Universityof Arizona began the Spacewatch survey with a0.9-m telescope at Kitt Peak Observatory [3]. Thesurvey uses commercial CCDs operated to minimizethe data readout and processing load. During asearch, the telescope is stationary and the sky driftspast the focal plane of the telescope. The CCD out-put is read electronically at a rate that matches the star

FIGURE 1. Inner solar system showing the orbits of aster-oids and comets in the main asteroid belt between Mars andJupiter (relative orbits drawn to scale). Most asteroids in themain belt have nearly circular orbits and so do not pose athreat to Earth. Asteroids with highly elliptical orbits thatcross or approach Earth’s orbit are called near-Earth objects(NEOs).

FIGURE 2. Estimated population of Earth-crossing aster-oids with diameters above a given size. About 10% of aster-oids with diameters larger than 1 km are known.

Mars

Sun

Earth

NEO

Jupiter

Main asteroidbelt

Comet

10

8

6

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0

10 100 1000 10,000

Diameter (m)

Log

10 (n

umbe

r of a

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oids

) 150 million

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92002100

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motion. Multiple passes of the data are then pro-cessed on a workstation to find moving objects. Thisapproach was chosen to accommodate the slow datareadout rate of the CCD and the computer process-ing capabilities available at the time. The Spacewatchprogram has discovered more asteroids than any othersearch program, finding 164 NEOs, of which 37 are1 km or larger in size.

Since November 1995, the National Aeronauticsand Space Administration (NASA) and the Jet Pro-pulsion Laboratory (JPL) have operated a survey witha telescope from the ground-based electro-opticaldeep-space surveillance (GEODSS) site on the islandof Maui in Hawaii [4]. This survey, called Near-EarthAsteroid Tracking (NEAT), uses a front-illuminatedcommercial 4096 × 4096-pixel CCD manufacturedby Lockheed-Martin-Fairchild. NEAT operates in astep-stare mode that images a portion of the sky whilemoving the telescope to track the stars. Three imagesof the same portion of sky are collected over severalhours and processed by a workstation to identifymoving objects. The NEAT program has discovered29 NEOs, of which 12 are 1 km or larger in size.

The commercial CCDs and computers typicallyused by astronomers have limitations that compro-mise the productivity of an NEO search. First, manyof the commercial CCDs are front illuminated,meaning that the incoming photons must penetratethe wiring of the CCD device before being detected.The fraction of incident photons detected, known asthe solar-spectrum-weighted quantum efficiency, islimited to about 30% or less. Second, large-formatcommercial CCDs must be read out slowly to main-tain low-noise performance, which reduces search ef-ficiency. Because a single image can take tens of sec-onds to read out, the CCD integrates photons duringonly a fraction of its operation time.

The solution to these problems can be found in theCCDs that Lincoln Laboratory designed to trackEarth-orbiting satellites. These CCDs are back illu-minated for higher quantum efficiencies and have aframe-transfer design that eliminates slow readout.For asteroid search and detection, we estimate that anetwork of two or three 1-m telescopes equippedwith such CCDs could complete a twenty-five-yearSpaceguard-type survey in a decade.

Space-Surveillance Technology

Space surveillance involves detecting and trackingEarth-orbiting satellites and space debris, as well asmaintaining a catalog of such objects. Lincoln Labo-ratory has been developing technology for the U.S.Air Force Space-Surveillance Network (SSN) for overforty years. In 1957, for example, the Laboratorytracked Sputnik, the first artificial satellite, by usingthe Millstone Hill Radar in Westford, Massachusetts.The Laboratory continues to use the Millstone Hillfacility to develop and transfer radar and softwaretechnology for the SSN.

At the Experimental Test Site (ETS) at WhiteSands Missile Range near Socorro, New Mexico, Lin-coln Laboratory is testing its new generation of sensi-tive large-format frame-transfer CCD focal planes toupgrade GEODSS. Figure 3 shows the space-surveil-lance facilities at White Sands Missile Range.

A number of GEODSS sites are deployed world-wide, each currently equipped with 1-m-class tele-scopes and ebsicon (electron-bombarded silicon) de-tector systems based on 1970s television technology.The new Lincoln Laboratory focal-plane CCD andcamera system provides three benefits: significantlyimproved sensitivity, which reduces integration timesand allows tracking of fainter objects; fast frame-transfer readout, which allows the integration of thenext image to be started while the previous image isread out; and stringent blemish specifications, whichminimize the loss of detections attributed to focal-plane defects. The improved focal-plane CCDs havebeen installed in a new generation of camera systemsand are currently undergoing testing at the ETS. Fig-ure 4 shows the telescope used for the CCD cameratests and asteroid searches.

The new focal-plane and camera technology givesthe 1-m GEODSS telescopes considerable capabilityto conduct sensitive, large-coverage searches forspace-surveillance applications. With some modifica-tions, these capabilities can be extended to searchesfor Earth-crossing asteroids. Field measurements atETS with a GEODSS telescope equipped with Lin-coln Laboratory’s new-generation CCD and cameraindicate that the CCD-equipped GEODSS telescopecan achieve a limiting magnitude of 22 at a signal-to-



noise ratio of 4 over a 2-deg2 field of view in less thana 100-sec integration. This sensitivity compares tothat of much larger telescopes equipped with existingcameras. In addition to the high sensitivity, theframe-transfer capability enables the high coveragerates of the sky needed for asteroid search operations.

Detector Technology

The latest-generation Lincoln Laboratory CCD chipfeatures a focal-plane array of 2560 × 1960 pixels andan intrinsic readout noise of only a few electrons perpixel (Figure 5). The CCD chips are constructed byusing a back-illumination process, which providespeak quantum efficiency exceeding 90% and solar-spectrum-weighted quantum efficiency of 65%. Afteran integration is finished, the resulting image isquickly transferred to the frame-store buffers, allow-ing the active imaging area to conduct the next inte-gration while the image is read from the frame-storebuffers. This feature eliminates the need for a me-chanical shutter to define the exposure, since the im-age transfer time from the imaging area into theframe buffer is only several milliseconds. The focalplane is equipped with eight parallel readout ports toallow the five million pixel values to be read out of theframe-store buffers in about 0.2 sec. The LincolnLaboratory CCDs described above have been con-structed specifically to allow large portions of the skyto be searched for faint moving targets. Consequently,they have the best combination of large format anddetection performance among current CCDs.

Initial Field Tests

We conducted initial field tests of the new CCD andcamera system at ETS in August 1995 and July 1996.We wanted to determine the system’s ability to detectasteroids and to meet design specifications for satellitesurveillance. The Lincoln Laboratory CCD allows a1-m GEODSS telescope to achieve impressive limit-ing-magnitude performance in short integrationtimes. Figure 6 indicates the limiting magnitudeachieved as a function of integration time for the Lin-

FIGURE 3. Space-surveillance facilities at the White Sands Missile Range near Socorro, New Mexico. The four tele-scopes near the middle and the far-left telescope with open dome represent Lincoln Laboratory’s Experimental TestSite (ETS). The three telescopes on the right represent an operational ground-based electro-optical deep-spacesurveillance (GEODSS) site.

FIGURE 4. ETS telescope used to test the Lincoln Labora-tory large-format CCD camera and search for asteroids. Thetelescope is identical to an operational GEODSS telescope.



coln Laboratory CCD compared with the same infor-mation estimated for the JPL NEAT system, whichemploys a commercial CCD [5]. The difference inperformance is significant because the observing sitefor NEAT on Maui generally has better seeing thanthe ETS site near Socorro.

During the initial tests, a small amount of observ-ing time was dedicated to searching for asteroids.That effort yielded a total of 177 asteroid observa-tions, which were sent to the Minor Planet Center(MPC) in Cambridge, Massachusetts. From these ob-servations, 49 new objects received designations fromthe MPC, including a confirmed NEO, designated1996 MQ.

The LINEAR Program

Our initial results were modest in terms of the por-tion of sky covered and the numbers of asteroids dis-covered because they were made with a preliminarycamera and data system that provided a fraction ofthe possible discovery rate of a complete operationalsystem with the same CCD technology. Even so, theresults convinced the U.S. Air Force that this ap-

proach had considerable merit and potential. Conse-quently, the Lincoln Near-Earth Asteroid Research(LINEAR) program began with funding in early1997. We designed a new system to boost the search,processing, and discovery capabilities by integratingreal-time hard-disk storage, improved signal process-ing, and automation of data management tasks.

Figure 7 displays the process flow of the LINEARsystem at ETS. The input data consist of three to fiveCCD image frames of the same location of the skycollected at intervals of about thirty minutes. The de-tection algorithm includes four major steps: imageregistration, background suppression and normaliza-tion, binary quantization, and clustering and veloc-ity-matched filtering. First, image registration cor-rects any pointing errors between the images byshifting the second through last frames to line uptheir stellar backgrounds with that of the first image.Next, the LINEAR system normalizes the registeredimages to remove background noise in the back-ground suppression and normalization block. Esti-mates of background mean and standard deviationare computed at each pixel, averaging over all theframes. Data are normalized pixel by pixel by usingthe local background mean and standard deviation.The normalized data are then binary quantized with asimple threshold (currently 99.9%).

FIGURE 5. Five-million-pixel Lincoln Laboratory CCD chip.This chip is constructed by using a back-illumination pro-cess and features a 2560 × 1960-pixel focal-plane array. Fourframe-store buffers hold an image after integration, allowingthe active imaging array to conduct the next integrationwhile the current image is read from the frame-store buffers.

FIGURE 6. Limiting magnitude for a signal-to-noise (SNR)ratio of 6 as a function of integration time for the LincolnLaboratory CCD and the Near-Earth Asteroid Tracking(NEAT) system. For a specified integration time, the LincolnLaboratory CCD system detects fainter asteroids havinghigher magnitudes.

100-mm wafer

Frame-store buffers

Imaging array

Frame-store buffers

17

18

19

20

21Lincoln Laboratory CCD modelLincoln Laboratory CCD measurementNEAT (Lincoln Laboratory estimate)

1 2 3 4 5 6 7 8 9

Integration time (sec)10

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The binary-quantized data are clustered on aframe-by-frame basis to group adjacent pixels. Thecentroids and extents of these pixel groups, or clus-ters, are computed. Each cluster in the first imageframe is paired with each cluster in the last imageframe that falls within a specified radius, selected asan upper limit on asteroid rates of motion. Thesepairs form the list of candidate detections, or streaks.Each candidate streak is assigned a velocity by divid-ing the displacement from the beginning to the endof the streak by the time interval that it spans. Foreach candidate streak, the LINEAR detection algo-rithm searches intermediate frames for clusters withthe appropriate displacement to match the streak’s ve-locity. These matching clusters are added to the can-didate streak. Once all of the candidate streaks havebeen filled out, the LINEAR algorithm rejects thosestreaks which have too few clusters. The streaks re-maining are considered detections. Plate solutions aregenerated by using a star-matching algorithm and astar catalog, and then the precise locations of the de-tections in each frame are calculated. Detection loca-

tions are converted to final observations formatted inright ascension and declination sky coordinates. De-tections of fast-moving objects, which are potentialNEOs, are manually reviewed to identify and elimi-nate any false positives that have leaked through thesystem.

Figure 8 shows an example of the detection of anNEO asteroid. Figure 8(a) is a composite of five im-age frames, each separated by twenty-eight minutes.Figure 8(b) shows output of the LINEAR detectionsystem in which asteroid 1998 KD3 is highlightedwith a sequence of four red circles and an arrow thatindicates the direction of motion. Detections ofslower-moving main-belt asteroids are in green. Theimages in Figure 8 were cropped from a much largerCCD image.

LINEAR searches are typically repeated to search agiven area of the sky twice within a seven-day period.Occasionally, an area of the sky is searched three timesif one of the previous two nights of searching hadmarginal observing conditions. In addition, the out-put of the detection system depicted in Figure 7 for

FIGURE 7. Process flow at the Experimental Test Site (ETS) for the LINEAR detection system, which acquires and processes aseries of CCD images of the sky to detect moving objects. The detection list is matched against a star catalog to produce a setof final NEO observations, which are then sent to Lincoln Laboratory.

CCD images

Image registration

GEODSS telescope with LINEAR CCD camera

Observations

Detection algorithm

LINEAR detection system

Detectionlist

Backgroundsuppression

and normalization

Binaryquantization

Clustering andvelocity-matchedfiltering

Starcatalog match

Generation offinal

observations

Review andtransmission

to Lincoln Laboratory



transmission to Lincoln Laboratory in Lexington,Massachusetts, requires additional processing beforesubmittal to the MPC.

Interactions with Minor Planet Center

The MPC, which is part of the Harvard SmithsonianAstrophysical Observatory, maintains a catalog of allknown asteroids, comets, and other minor planets.

The role of the MPC for asteroids is analogous to therole of Cheyenne Mountain in Colorado Springs,Colorado, for satellite tracking and cataloging. Tosupport their cataloging efforts, the MPC receives as-teroid and comet observations from professional andamateur observers around the world. These observa-tions are checked against the catalog to identify thoseobservations which are of known objects, whose or-bits are updated by using the new data.

The MPC assigns designations to new objects withsufficient observations and credits discoveries. Gener-ally, a main-belt asteroid discovery is credited to thefirst observers who submitted their observations tothe MPC for processing and were able to observe theobject on two nights within a ten-day period. Fast-moving new objects that are possible NEOs are im-mediately reported and placed on the MPC’s NEOConfirmation Page on the World Wide Web. Profes-sional and amateur astronomers who specialize in fol-low-up observation can download the predicted posi-tion of NEO candidates, observe them, and send theresulting observations to the MPC. Once confirmed,a new NEO receives a designation and a Minor PlanetElectronic Circular announcing its discovery. Discov-ery of the NEO is credited to the first observer sup-plying the data leading to placement of the object onthe confirmation page.

At Lincoln Laboratory, we process LINEAR obser-vations received from ETS to extract fast-moving ob-jects with apparent motion greater than or equal to0.4 deg/day, medium-speed objects with motions be-tween 0.4 and 0.3 deg/day, and slow-moving objectswith speeds less than 0.3 deg/day. Because NEOstend to have faster apparent motion than main-beltasteroids and move faster as they leave the main beltand approach the sun, the fast movers are definiteNEO candidates. Figure 9 illustrates the process forcataloging observations. The fast movers are con-verted to MPC observation format, assigned tempo-rary LINEAR designations, and electronically mailedto the MPC for priority processing. Medium moversare possible NEO candidates, but typically turn outto be unusual, though not threatening, asteroids.These are processed and sent to the MPC in the samemanner as the fast movers, but are processed at alower priority.

FIGURE 8. Processing a series of image frames to detectmoving objects. (a) This composite of five image frames,each separated by twenty-eight minutes, shows the clutterfrom which faint yet moving asteroids must be extracted.The vertical streaks, associated with brighter stars, are asaturation effect in the CCD readout. (b) The bottom imageshows the output of the LINEAR detection system, with thenewly discovered NEO—asteroid 1998 KD3—highlighted inred. Detections of slow apparent-motion main-belt aster-oids are colored green.

(a)

(b)



Additional data processing tasks involve analyzingthe performance of the LINEAR system. Raw obser-vations are processed to extract the sky coverage for aparticular night. A plotting program displays the skycoverage, color coded for the number of nights eachregion is revisited. Software automatically downloadsMPC lists of NEOs, extracts LINEAR discoveries,

To ease the processing burden on the MPC, weprocess and link the slow-moving observations fromtwo or three nights on the same area of the sky. Eachset of linked observations is assigned a temporary des-ignation and converted to the MPC observation re-porting format. The resultant large data files are elec-tronically transferred to the MPC via the Internet.

(a)

(b)

FIGURE 9. (a) Processing flow of Lincoln Laboratory ETS observations, and (b) interactions be-tween the LINEAR program and the Minor Planet Center (MPC). Fast movers (definite NEO candi-dates) and medium movers are all converted to MPC format and electronically mailed to the MPC.Medium movers are processed at a lower priority. The slow movers identified from two or threenights of observations are linked. This set of linked observations is assigned a temporary designa-tion, converted to MPC format, and electronically transferred to the MPC.

Fastand medium

movers

Receiveobservations

from ETS

Separateobservations

by velocity

Link night-to-night

observations

Generatesky-coverage

plots

Convertto MPCformat

Flag aspossible

NEOs

Sendto

MPC

Analyze,search, and

updatedatabases

Slow movers

Previousnight's slow

movers

LINEAR submitsfast- and

medium-moving observations

MPC correlatesagainstcatalog

MPC places newcandidates on

NEO confirmationpage

MPC confirmsNEOs by usingmeasurementsfrom LINEAR

and otherobservers

LINEAR submitsslow-movingobservations

MPC correlatesagainstcatalog

MPC assignsdesignations tonew discoveries



and superimposes them on sky-coverage plots. Withthese tools, we can analyze the search pattern and ob-serving strategy of a given lunar dark period (duringnew moon) and adjust our observation strategies forthe next lunar dark period. Finally, the files of desig-nations received from the MPC are processed, and adatabase of raw observations, temporary LINEARdesignations, and MPC designations is updated andmaintained.

In addition to providing provisional designationsbased on two nights of observations of newly discov-ered objects, the MPC also coordinates the process ofnumbering and naming of new discoveries for the In-ternational Astronomical Union. Asteroids may benumbered and formally included in the catalog afterthere is a sufficiently good orbit on the object to guar-antee that it will not be lost. Observations must becollected over three or four oppositions, which occurabout every sixteen months, to generate an orbit ofsufficient quality to number the object. The credit fornumbering the object and the right to name the aster-oid are conferred to the observer who provides dataover a span sufficient to guarantee that the asteroidcan be recovered and correlated over the interval be-tween oppositions. This is called a principal designa-tion, which usually takes data over an interval ofthirty days to achieve. LINEAR currently has namingrights to approximately 450 asteroids when they areeventually numbered, and is acquiring these rights ata rate of approximately 200 per month. The firstLINEAR asteroid, numbered 7904, has been namedMorrow in honor of Walter Morrow, the recently re-tired director of Lincoln Laboratory.

LINEAR Search Results

The initial LINEAR system was tested during Marchthrough July 1997 to determine its search effective-ness. Because of limited equipment availability at thattime, a 1024 × 1024-pixel CCD was used rather thanthe full-scale GEODSS chip. Despite the smallerCCD chip, LINEAR productivity was high, and per-formance was comparable to search programs such asNEAT and Spacewatch. During this five-month trialperiod 3 new NEOs and 1367 main-belt asteroidswere discovered and assigned MPC designations.

LINEAR began search trials with the large-format

FIGURE 10. Equatorial plot of the sky covered by LINEARduring opposition in March, April, and May of 1998. Theplots are color coded to indicate the number of times the skyarea was covered during the month.

May

April

Nights revisited

1 2 3 4 5 6

March



1960 × 2560-pixel CCD chip in October 1997. LIN-EAR search operations were conducted during thehalf of the month when the moon was less illumi-nated to reduce the background noise and increasethe sensitivity of the search. In the initial 10 nights ofoperations LINEAR generated 52,542 observationsand detected 11 NEO candidates, of which 9 wereconfirmed and received new designations from theMPC. Two of the NEO candidates were lost in subse-quent follow-up attempts by the worldwide networkof astronomers who track new objects.

LINEAR resumed observations with the largeCCD during the lunar dark period in March 1998.During that time, LINEAR produced over 150,000observations and detected 22 NEO candidates, ofwhich only 13 were confirmed and received designa-tions. We believe one reason for the loss rate of NEOdiscoveries during this interval is that we overloadedthe capacity of follow-up observers. During subse-quent operations in April and May 1998, we investedmore effort in following up our NEO discoveries toavoid this problem, and have had no losses for thosetwo months. During the March 1998 lunar dark pe-riod, the MPC indicates that LINEAR provided over90% of the worldwide total of asteroid observations.Figure 10 contains a plot of the sky coverage achievedduring the March, April, and May 1998 observingintervals. Many of the areas near the ecliptic were cov-ered multiple times. During the three-month intervaldisplayed, the search planning methods that LINEARused matured significantly, leading to coverage of alarger extent of the sky with less overlap beyond thetwo visits required to achieve MPC designations onthe discovered objects.

Table 1 summarizes the LINEAR search observa-

Table 1. Monthly Performance of LINEAR Program

Period Observations Search Area (deg2) NEOs Comets

October ’97 52,575 3060 9 0

March ’98 151,035 9906 13 0

April ’98 91,495 7124 8 1

May ’98 51,068 12,124 14 4

FIGURE 11. Histogram of the size distribution of NEOs dis-covered by LINEAR through March 1998.

tions with the large-format CCD, demonstrating thatLINEAR has the capability to discover long-periodcomets. The comets are detected in the same processas asteroids and included on the MPC confirmationpage. Figure 11 shows a histogram of the estimatedsizes of the NEOs discovered. LINEAR is finding nu-merous asteroids larger than 1 km in diameter, thetheoretical threshold diameter for worldwide climaticeffects in a collision. Another metric of LINEAR per-formance comes from the MPC’s list of potentiallyhazardous asteroids that can potentially approachEarth to within 5 million km and are at least 200 min diameter [6]. As of 16 June 1998, this list has 123entries of which LINEAR has discovered 12.

Figure 12 shows a histogram of all LINEAR aster-oid detections, both known and newly designated,through the end of 1997. The histogram demon-strates that LINEAR can see beyond what is docu-mented in the current catalog of asteroids. The x-axis

0250 750 1250 1750 2250 2750 3250

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of the figure shows both absolute magnitude (on thebottom) and the estimated diameter of asteroids de-tected (on the top). Diameter estimates are based onabsolute magnitude by assuming an average surfacereflectance. The peak of the population discovered byLINEAR is approximately two visual magnitudesfainter than the peak for the known population, rep-resenting a major advance in detection capability. Im-proved detection sensitivity allows LINEAR to char-acterize more quickly the population curve shown inFigure 2. This capability is required to catalog allNEOs down to 1 km.

Another metric for determining the contributionmade by LINEAR is to compare its productivity withother major asteroid search programs. Table 2 com-pares three recent months of LINEAR results withthe recent results of the Spacewatch and NEAT pro-

grams. Note that LINEAR’s performance exceedsthat of the other programs.

Further insight into the relative performance of thesearch programs may be derived from sky plots pro-vided by the MPC. The MPC does not generallyknow what portion of the sky each search programhas covered; however, it does track the distribution ofthe asteroid detections provided by each search pro-gram. Figure 13 contains the sky plots of the detec-tions reported by major search programs, includingSpacewatch, NEAT, ODAS (a joint effort betweenthe Observatoire de la Côte D’Azur in Nice, France,and the Institute of Planetary Exploration in Berlin-Adlershof, Germany), and LINEAR for March, April,and May 1998 [7]. The LINEAR program domi-nated the sky over each of these months in both skycoverage and number of observations generated.

Table 2. Performance of Asteroid Search Programs

Program Period Detections Detections/Month Discoveries/Month NEOs/Month

LINEAR 3/98–5/98 58,719 9573 3027 12

Spacewatch 1/95–12/96 69,308 2888 459 2

NEAT 10/95–3/98 23,061 824 53 1

FIGURE 12. Histogram of asteroids detected by LINEAR through 1997, including new discoveriesand known objects. The peak of the population discovered by LINEAR is approximately two visualmagnitudes fainter than the peak for the known population.

024 22 20 18

1 km 10 km 100 km

16 14

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Estimated diameters:

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Summary

The LINEAR program has successfully demonstratedthe application of U.S. Air Force space-surveillancetechnology to the search for NEOs, asteroids, andcomets. Operational experience shows that the large-

format 1960 × 2560-pixel CCD used by LINEAR forwide-area asteroid searches performs more efficientlythan traditional CCDs used by other asteroid searchprograms. During three months of operations(March, April, and May 1998), LINEAR searched atotal of 29,154 square degrees of sky to a limiting vi-sual magnitude exceeding 19, and submitted over293,000 observations of asteroids to the MPC. Thesedata resulted in the discovery and designation of 53new NEOs, 4 new comets, and over 10,000 main-belt asteroids.

Acknowledgments

The authors would like to acknowledge and thankthe following contributors to the work presented inthis article: Eric Pearce, manager of the Lincoln Labo-ratory Experimental Test Site, where the observing isconducted; Mike Bezpalko, who helps with the ob-serving; Peter Trujillo and Robert Duncan, who keepthe site hardware and cameras in good working con-dition; and Gene Rork, who calculated the LINEARdetectability curves. The operations and developmentof the LINEAR program were funded by the U.S. AirForce.

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2. D. Morrison, R.P. Binzel, E. Bowell, C. Chapman, L. Fried-man, T. Gehrels, E. Helin, B. Marsden, A. Maury, T. Morgan,K. Muinonen, S. Ostro, J. Pike, J. Rahe, R. Rajamohan, J.Rather, K. Russell, E. Shoemaker, A. Sokolsky, D. Steel, D.Tholen, J. Veverka, F. Vilas, and D. Yeomans, “The SpaceguardSurvey: Report of the NASA International Near-Earth-ObjectDetection Workshop,” NASA, Washington, 25 Jan. 1992.

3. T. Gehrels, “New Research by CCD Scanning for Comets andAsteroids,” NASA Technical Report, 22 Sept. 1997, NASA-CR-205475.

4. E.F. Helin, S.H. Pravdo, D.L. Rabinowitz, and K.J. Lawrence,“Near-Earth Asteroid Tracking (NEAT) Program,” Ann. NYAcad. Sci. 822, 30 May 1997, pp. 6–25.

5. G. Stokes, R. Weber, F. Shelly, D. Beatty, H. Viggh, E. Rork,and B. Hayes, “Air Force Planetary Defense System: InitialField Test Results,” Proc. Fifth Int. Conf. on Space ’96 1, June1996, pp. 46–53.

6. http://cfa-www.harvard.edu/iau/lists/Dangerous.html7. Private communications, G. Williams.

FIGURE 13. MPC sky plots showing detections of asteroidsreported by the major search programs Spacewatch,LINEAR, NEAT, and ODAS (a joint effort between theObservatoire de la Côte D’Azur in Nice, France, and the In-stitute of Planetary Exploration in Berlin-Adlershof, Ger-many). The LINEAR program outperformed other searchprograms during the three-month period.

May 1998

April 1998

March 1998

SpacewatchLINEAR ODAS

NEAT



. is the associate leader of theSurveillance Techniques group,where he specializes in analy-sis, design, and operations ofspace-surveillance systems,including the Space-BasedVisible (SBV) and LINEARprograms. The SBV systemprovides the first space-basedspace-surveillance capability toSpace Command in ColoradoSprings, Colorado. The SBVwas initially operated as atechnology demonstration foreighteen months and has nowbecome a contributing sensorfor Space Command. Beforecoming to Lincoln Laboratoryin 1989, Grant worked as asenior scientist and operationsmanager at Geo-Centers Inc.,a small contracting companyspecializing in fiber-opticsensors. Previously, he per-formed nondestructive testingof laser fusion targets at LosAlamos National Laboratoryin New Mexico, and devel-oped fiber-optic data acquisi-tion systems and providedfield support for undergroundnuclear tests in Nevada. Heholds a B.A. degree in physicsfrom Colorado College inColorado Springs, and M.A.and Ph.D. degrees in physicsfrom Princeton University,Princeton, New Jersey.

is an associate staff member ofthe Surveillance Techniquesgroup, based at the Experi-mental Test Site near Socorro,New Mexico. He developsimage-processing software thatis used to automatically detectasteroids, comets, and satel-lites, as well as real-time con-trol software for positioningtelescope mounts. He alsogenerates system software forinterfaces to custom deviceslike weather systems, mountencoders, digital-to-analogconverters, CCD cameras, andfilter wheels. Frank helpeddevelop the TransportableOptical System (TOS), whichthe U.S. Air Force operates inSpain. He joined LincolnLaboratory in 1986 afterstudying computer science atthe New Mexico Institute ofMining and Technology inSocorro.

.. is a staff member of the Sur-veillance Techniques group,and is responsible for thesoftware development, com-puter system, and data analysisof the LINEAR program inLexington, Massachusetts.Herb acts as liaison with theMinor Planet Center in Cam-bridge, Massachusetts, submit-ting observations and receivingasteroid designations. Herbjoined Lincoln Laboratory in1992 after working on spacerobotics, mission planning forautonomous vehicles,multisensor data fusion, andairborne lidar-turbulencedetection systems for BoeingAircraft Corporation in Se-attle, Washington. As a flightengineer, he tested flightmanagement computers andairplane navigation systems onBoeing transports. He holds aB.S. degree in aeronautics andastronautics with an aviationoption, and M.S. degrees inelectrical engineering andcomputer science, and aero-nautics and astronautics fromMIT.



. is an assistant staff member inthe Surveillance Techniquesgroup, writing software tosupport asteroid detection andEarth imaging. He joinedLincoln Laboratory in 1993after earning a B.S. degree incomputer science at the Uni-versity of Pennsylvania, inPhiladelphia.

. works under contract for theExperimental Test Site nearSocorro, New Mexico,through Manpower Inc. ofAlbuquerque, New Mexico.He plans and conducts observ-ing sessions, then analyzes theobservations. He also providesfeedback about the softwareperformance of the LINEARsystem. Matt started workingat ETS in 1989 after graduat-ing from the New MexicoInstitute of Mining and Tech-nology in Socorro with a B.S.degree in physics with anelectronics option.

the lincoln near-earth asteroid research (linear) program · • stokes, shelly, viggh, blythe, and...

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