asteroid deflection via standoff nuclear explosions'the impact hazard', morrison, chapman...
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Lawrence Livermore National Laboratory
Aaron R. Miles
LLNL-PRES-408112
Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551This work performed under the auspices of the U.S. Department of Energy byLawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
Asteroid Deflection via Standoff NuclearExplosions
Asteroid Deflection Research Symposium 2008October 23-24, 2008
Arlington, VA
Acknowledgements: J.C. Sanders, Paul Amala, Red Cullen, Dave Dearborn, Bob Tipton
2Lawrence Livermore National Laboratory
Qualifications and disclaimers
• Subject of this presentation is using nuclear explosives for asteroiddeflection
• Most of the work was performed together with a summer student in2005 (JC Sanders - then a senior in the Oregon State U. PhysicsDepartment, now a graduate student at UT Austin)
• This is a technical talk and does not reflect policy of DOE, NNSA, oranyone else
• LLNL does not have an asteroid deflection program
• I am not advocating nuclear explosives development or deploymentin anticipation of possible future deflection requirements
• I am not advocating deflection capability as a motivation formaintaining or expanding the US nuclear weapons stockpile
3Lawrence Livermore National Laboratory
The Threat: The Earth has and will suffer collisions withlarge Near-Earth Objects (NEO’s)
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oid2
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• Estimated NEO population 960±120 objects of d>1km and24500±3000 objects of d > 100m
• 723 classified by Near-Earth object Program as “PotentiallyHazardous Asteroids” (Earth Minimum Orbit Intersection DistanceMOID < 0.05AU and d > ~150m) Y ~ 105 GT
4Lawrence Livermore National Laboratory
ImpactorDiameter Yield(m) (MT) Interval (yr) Consequences
<50 <10 < 1 Meteors in upper atmosphere most don't reach surface
75 10-102 1000 Irons make craters like Meteor Crater; stones produce airbursts likeTunguska; land impacts destroy area size of city
160 102-103 5000 Irons,stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo)
350 103-104 15,000 Land impacts destroy area size of small state; ocean impact produces mild tsunamis
700 104-105 63,000 Land impacts destroy area size of moderate state (Virginia); ocean impact makes big tsunamis
1700 105-106 250,000 Land impact raises dust with global implication; destroys area size of large state (California, France)
http://seds.lpl.arizona.edu/nineplanets/nineplanets/meteorites.html
Impact energies can be huge: Expect blast, fires,earthquakes, tsunamis, solar light extinction, etc…
Impact energy:
!
YMT = 50"g / ccR100m3v10km / s
2
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Y ~10 Mt
'The Impact Hazard', Morrison, Chapman and Slovic, Hazards due to Comets and Asteroids
5Lawrence Livermore National Laboratory
Past NEO-Earth impacts have resulted in devastatinglocal and global effects
http://en.wikipedia.org/wiki/Tunguska_event
• Tunguska event: 1908• Stony d ~ 60m• Airburst @ 6-10km• Y ~ 10-15MT,• Leveled 2150 km2 of forest
• Barringer crater: 50,000BC• Nickel-Iron d ~ 50m• Y ~ 20-40 MT• Fireball to ~ 10 km• Hurricane-force winds to ~ 40 km
• KT impactor• d ~ 10 km• Y ~ 105 GT• 145-180km crater• Mass extinctions?
http://www.meteorite.com/meteor_crater/index.htm
6Lawrence Livermore National Laboratory
Several mitigation schemes have been proposed
Continuous momentum transfer
• Chemical rockets
• Ion propulsion
• Mass drivers
• Mirrors
• White paint
Impulsive momentum transfer
• Conventional explosives
• Kinetic impact
• Pulsed laser or particle beam
• Nuclear explosives
7Lawrence Livermore National Laboratory
Most deflection schemes require very long lead times forglobal-catastrophe-scale NEO’s
For lead times on the orderof years or decades, onlynuclear explosives canrealistically avert collisionswith kilometer-scale NEO’s
*Chemicalpropellant/explosive:
*Ion drive:
Kinetic:
*NIF(2r=1km,ΔV=1cm/s):
Standoff nuclearexplosive device (NED):
!
"Vcm / s
~1
104
mtons
(2Rkm)3
!
"Vcm / s
~1
102
mtons
(2Rkm)3
!
"Vcm / s
~1
103
mtons
(2Rkm)3
!
"Vcm / s
~YMT
Rkm
2
~ 103 yrs @ 4 shots/day or~ month @ 1Hz
*Dave Dearborn, UCRL-PROC-202922
Required deflection velocity vs. leadtime with impulsive momentum transfer
Time to impact Required ΔV(cm/s) ~ R /t month ~ 100year ~ 10decade ~ 1century ~ 0.1millennium ~ 0.01
8Lawrence Livermore National Laboratory
The nuclear option: Deflection vs. destruction
Destruction• Standoff, surface, and subsurface options• Deeply buried detonation gives maximum sourceyield coupling efficiency (~1)• Must ensure that no large (> ~10 meters)fragments survive• Likely requires detailed knowledge of materialproperties or very conservative yield estimates• Effective for smaller targets (~ 102 meters)• For details, see T.J. Ahrens & A.W. Harris,Nature 360, 429 (1992).
Deflection• Requires standoff detonation• Requires knowledge of output energycoupling• Less sensitive to material properties?• Best option for larger targets (> a fewx 102 meters)
NASA Deep Impact Missionhttp://deepimpact.jpl.nasa.gov/gallery/images-flyby.html
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9Lawrence Livermore National Laboratory
Deflection of idealized NEO by standoffnuclear detonation: Parameters and method
Material properties:Density (ρ)Equation of state (EOS)Strength modelFailure model
Energy deposition coupling ηYobtained from Monte Carlotransport code TART
Kinetic energy coupling ηKobtained from radiationhydrodynamics code Cale
Source yield Yin neutron and x-rays
Standoffdistance d
θsθt
Imparted kineticenergy Ek=Edep*ηK
Deposited energyEdep=Yint*ηY
Radius RDeflectionvelocity vz
Intercepted YieldYint =Y*Ω(θS)/4π
10Lawrence Livermore National Laboratory
Simple analytic model Momentum and energy conservation:
MdV = (dm)vβdEdep = MdV2 + (dm)v2
Consider only the z-component of momentumdue to axial symmetry:
dpz = dpcosθ
Assume a solution for the deposited energydensity:
The solution for the momentum thus becomes:
!!= """"#$%& drdrrp depz cossin),(2 2
!
"dep (r,#) = "0
$R$r
%effe cos&
2
#
#t
'
( )
*
+ ,
dm
v
vcos(θt)
θt
dEdep
M
dV
!
pz = R "#eff$Edep % f (&t ) = R "#eff$'YY % g(&t )
11Lawrence Livermore National Laboratory
g(θt)/gmax
!
"K =#$effR
f (%t )
f&
'
( )
*
+ ,
2
!
vz(cm / s) =9.88
R2(km )
"eff (cm )#$YYMT /%g / ccg(&t )
gmax
Predicted KE coupling parameter
Predicted deflection velocity
⇒ Optimal standoff distance d/R = 0.33
f(θt)2/fmax2
22
2
)/4(1
)2sin()4/(1
)cos()/(21
)/(1)(
!"
""!
"!"
!"!"
t
tt
tt
ttf
#
#
##
#=
The function f(θt) has the form:
Simple analytic model
12Lawrence Livermore National Laboratory
Relationship between the modelparameters ηK and β
β tells how much of the deposited energy goes into kinetic energy ineither direction
βEdep = 2(EK + eK)
ηK tells how much of the deposited energy goes into deflectionkinetic energy
ηKEdep = EK
The two are related by the ratio of directional kinetic energies
� β/2η = 1 + eK / EK ~ 1 + M/m ~ M/m ~ R/λeff
Geometry determines the proportionality constant:
!
"K =#$effR
f (%t )
f&
'
( )
*
+ ,
2
13Lawrence Livermore National Laboratory
Parameter β can be calculated within ashock tube model
Ideal gas approximation
Shock tube model predicts β = 0.0395 α/γ
• γ is the adiabatic index
• α ≡ 1-(Q-P0)/εdep is accounts for energy expended in melting,vaporizing, ionizing material in the deposition region
Initial target density
Shocked targetPre-decompressiondeposition region
Rarefaction
Rarefaction
Ablated targetmaterial
14Lawrence Livermore National Laboratory
The yield coupling ηY is obtained from TARTsimulations
1UCRL-ID-127776, 2UCRL-ID-50400
Neutron mean freepath in 2.5 g/cc SiO2
Photon mean free pathin in 2.5 g/cc SiO2
• LLNL Monte Carlo Radiation Transport Code
• Uses 1ENDF/B-VI Release 8 neutron cross-sections and 2ENDL97 photon cross-sections
• Used spherical target geometry with 1, 300, and 72000 zones; typically used target radiusR=500m and a standoff distance of d=0.002R-5R
• Used 5 different target materials: SiO2, Fe, FeNi, Granite, and H2O
• Typically used 2.5e5 particles x 40 batches = 1e7 neutrons or 1e6 particles x 100 batches= 1e8 photons
15Lawrence Livermore National Laboratory
Several different sources were used inTART simulations
Neutron source spectra
• Fission
• 14.1 MeV monoenergetic
• Fission weapon*
• Fusion weapon*
Photon source spectra
• 1 keV blackbody
• 10 keV blackbody
*E.A. Straker and M.L. Gritzner, “Neutron and Secondary Gamma-Ray Transport in Infinite Homogeneous Air”, ORNL4464, 1969.
Weapon neutron sources
16Lawrence Livermore National Laboratory
Time-dependence of the energy deposition
Energy deposition in SiO2 for various sources
• Photon deposition time is significantly faster than neutron deposition times
• Neutron deposition includes early time KE deposition and late-time captureenergy deposition
17Lawrence Livermore National Laboratory
Yield coupling parameter ηY
Coupling ηY of various spectra of photons and neutrons forfive materials commonly found in asteroids and comets
Standoff-distance-averaged yield coupling for various materials and sources for an r = 500m target
0.4
0.5
0.6
0.7
0.8
0.9
1
Fusion Weapon
Neutrons
Fission Weapon
Neutrons
14.1 MeV
Neutrons
1keV Photons 10keV Photons Fission
Neutrons
Source
Yie
ld C
ou
plin
g
Fe
FeNi
Granite
H2O
SiO2
Standoff-distance-averaged yield coupling for various materials and sources for an r = 500m target
0.4
0.6
0.8
1
1.2
1.4
Fusion Weapon
Neutrons
Fission Weapon
Neutrons
14.1 MeV
Neutrons
1keV Photons 10keV Photons Fission Neutrons
Source
Yie
ld c
ou
plin
g
Fe
FeNi
Granite
H2O
SiO2
Neutron capture energy not included Neutron capture energy included
• High energy neutrons couple better than low energy neutrons when capture energy isneglected due to increased penetration
• Low energy neutrons couple better than high energy neutrons when capture energy isincluded because capture energy is proportionally greater at low initial energies
• X-ray yield coupling is nearly unity for all materials at 1 keV and greater than 0.8 at 10 keV
18Lawrence Livermore National Laboratory
TART results are input into the model to givecomparative predictions of deflection efficiency
• Higher energy photons are more efficient than lower energy photons
• Neutrons are only ~10-20 times more effective than the 10 keV blackbody
• For neutrons, higher energy particles are more efficient if capture energy isneglected. The reverse is true if capture energy is included.
• It is important to determine if late-time capture energy contributes to target deflection
!
vz(cm / s) =9.88
R2(km )
"#eff (cm )$YY(MT ) /%(g / cc )g(&t )
gmax
1.330.671.430.786.614.1 MeV Neutrons
1.691.260.8130.295.72 MeV Neutrons
1.621.320.6780.235.01.5 MeV Neutrons
0.08140.920.08140.921.80E-0210 keV Photons
0.006290.990.006290.991.00E-041keV Photons
(ηY λmfp / ρ)1/2ηY(ηY0 λmfp / ρ)1/2ηY0λmfp(cm)Source
The model predicts that the product λeffηYdetermines the kinetic energy coupling:
Capture energynot included
Capture energyincluded2.5 g/cc SiO2 at r = 500 m, d = r
19Lawrence Livermore National Laboratory
Energy deposition as a functionof solid angle for 14.1MeVmonoenergetic neutrons
Energy deposition as a function ofpenetration depth for 14.1MeVmonoenergetic neutrons
θt=0o
θt=60o
Δθt=5o
r=0
Δr=20cmr=4m
Deposition region depth chosen so that energy deposition falls off by 4 orders ofmagnitude through the deposition layer; typically 2-4 m for neutrons, 10 cm for 10 keVphotons, and 0.6 mm for the 1 keV photons.
TART provides spatially dependent energydeposition for interpolation onto hydro mesh
20Lawrence Livermore National Laboratory
The kinetic energy coupling ηK is obtainedfrom Cale simulations
LLNL radiation hydrodynamics Arbitrary Langrangian-Eulerian (ALE) code Uses finite differencing to solve the Euler equations TART energy deposition output is interpolated and sourced into the Cale mesh
vz (cm/s)ηK x 107
Δr (cm)
Resolution studyt = 1 sec
Momentum pz
Time (µs)
vz (cm/µs)
ηK
• Zoning study shows slow logarithmic divergence of vz withdecreasing radial resolution Δr in the deposition region.• At Δr = 0.5 cm, there are 13.2 zones per 14.1 MeVneutron mean free path.
• 2.5 g/cc SiO2 asteroid• 14.1 MeV neutron source• Y = 5 MT• Stand-off distance d = r = 500m
21Lawrence Livermore National Laboratory
Kinetic energy coupling vs. source and material
14.1 MeV neutron source @ Y = 5 MT
Material EOS Strength model Failure model ηK β vz(cm/s)
H2O Two-phase Const. strength+melt None 2.7E-05 8.8E-02 25SiO2 QEOS Tabular U vs P None 5.9E-06 1.5E-02 9.2SiO2 QEOS Tabular U vs P Brittle 3.4E-07 8.7E-04 2.2Fe QEOS Steinberg-Guinan Ductile 1.2E-06 9.3E-03 2.4
Source @ Y = 5 MT λeff(cm) (1.8ηY λmfp / ρ)1/2 vz(cm/s) ηK(e-7)Fission weapon (FW) neutrons 33 5.3 4.5 10.0Thermonuclear weapon (TNW) neutrons 30 5.1 4.0 8.5 14.1 MeV neutrons 25 3.5 3.5 9.0 10 keV blackbody x-rays 3.0e-2 0.14 1.1 0.631 keV blackbody x-ray 4.0e-4 0.018 0.25 0.029 @ 5µs
• ηK is sensitive to the presence of a failure model• Neutron KE coupling depends weakly on source spectrum• X-ray KE coupling depends strongly on photon temperature • Cale predicts that vz from x-rays at 10 keV is of order the neutron result
2.5 g/cc SiO2 target
22Lawrence Livermore National Laboratory
Kinetic energy coupling vs. yield for TNweapon neutron source
2 cm radial zoning for time-dependent source10 cm radial zoning for ts=30us source
• Cale results agree with the model-predicted deflection velocity with β = 5.0e-3, a yieldoffset of 1 MT, and a 1/√ 5 multiplicative factor• At high yield (Y ≥ 50 MT), Cale results agree with the model-predicted ηK with β = 5.0e-3• At low yield, the model under-predicts ηK Cale relative to the simulations• Cale predicts no significant difference between the time-dependent source and the 30 µslinear rise yield source• Cale predicts that the neutron capture energy contributes equally throughout the yieldrange considered
Time-dependent source30 us linear yield rise
Model with β = 5.0e-3
!
1"1
1+ YMt/6( )
1.3
#
$
% %
&
'
( ( )K
Model with β = 5.0e-3and 1MT offset
!
vz(cm / s) =1.17 Y
(MT ) "1( )R2(km )
g(#t )
gmax
23Lawrence Livermore National Laboratory
Maximum blow-off Mach numberMaximum T (eV)Pressure at 100 µs (Mb)
Tmelt = 0.11 eV
Deflection velocity goes to zero when the targetasteroid is not heated above the melt temperature
With the TN weapon source at d = r = 500 m, yields below1 MT do not heat the target surface enough to cause melting
24Lawrence Livermore National Laboratory
Kinetic energy coupling vs. standoffdistance for 14.1 MeV neutron source
10 cm radial zoning
d/r Y Yint ηK(e-7) vz(cm/s)
0.2 1.50 0.335 1.8 1.60.3 1.97 0.335 2.8 2.10.4 2.29 0.335 2.9 2.10.5 2.63 0.335 4.0 2.51.0 5.00 0.335 3.3 2.2
0.2 5.00 1.12 2.3 3.40.3 5.00 0.852 3.9 3.90.4 5.00 0.750 3.6 3.50.5 5.00 0.637 3.7 3.31.0 5.00 0.335 3.3 2.2
!
vz "g(#t )
gmax
!
"K #f ($ t)
f%
&
' ( (
)
* + +
2
Cale simulations atconstant Y = 5 MT
g(θt)/gmax
f(θt)2/fmax2
Constant Yint = 335 KTConstant Y = 5 MT
For a given source and target, the model predicts:
• Cale simulations agree with the model-predictedposition of the g(θt) peak but find a narrower width• Cale simulations agree qualitatively with the model-predicted f(θt) except at large stand-off distances
25Lawrence Livermore National Laboratory
Combined neutron and photon sources
2 cm zoning in 4 mneutron deposition layer
2.5 mm zoning in 10 cmx-ray deposition layer
7.4 m radial zoning out to r = 296 m
• Distinct neutron and x-ray deposition regions
• Neutron source: Monoenergetic 14.1 MeV
• Photon source: 10 keV blackbody
• Should repeat with 1 kev photon source
Mesh plots at 1/2 the actual radial resolution
Time (ms)
fn fx-ray Yt=5MT Yt=20MT Yt=100MT
1.0 0.0
0.5 0.5
0.1 0.9
0.01 0.99
0.0 1.0
vz (cm/s)16
0
7
0
3.5
03.5
03.5
03.5
03.5
0
7
07
07
07
0
16
016
016
016
01 1 1
1 1 1
1 1 1
1 1 1
1 1 1
26Lawrence Livermore National Laboratory
Combined neutron and photon sources
t = 1000 µs
Y = 5 MTY = 20 MTY = 100 MT
Y = 5 MTY = 20 MTY = 100 MT
Cale predictions:
• High energy neutrons are only 1.6 -2.6 times more effective than 10 keV x-rays despite model-predicted vz ratio of 25 at constant β (shallower deposition ⇒ higher T ⇒ higher β)
• Presence of x-ray blow-off increases effectiveness of neutron energy deposition
• Neutron fractions of 10% and 100% are equally effective over the range of total yields considered
Should repeat calculation series with 1 kev photon source
27Lawrence Livermore National Laboratory
Simulations• Resolution of the resolution problem• Direct (dynamic) sourcing of x-ray energy into Cale• Consider neutron propagation through x-ray blow-off• Improved material modeling in hydro calculations• Consider realistic target geometries (including 3D)• Allow internal structure (“Rubble pile” targets)• Consider multiple sources
• For fixed Y divided into n pulses, Δv ~ ΣΔvi ~ Σ(Y/n)1/2 ~ (nY)1/2~ n1/2Δvn=1
Analytic model• Include EOS modeling• Better description of x-ray energy deposition. A better fit to the photon TART runsis Exp{-[(R-r)/λeff]^0.4}
Experiments?• Can we measure the kinetic energy coupling efficiency?
Needed improvements and future directions
28Lawrence Livermore National Laboratory
Asteroid deflection in the laboratory?
790
29
22
Yint/M(J/g)
12.82.7e-7102e-43 µm16015.4kJ1.034kJ1.5cm003
10 µm
30cm
λeff
15.4kJ
100MT
Yeff
10002.2e-6336e-41206.71MT0.5km
890
Pmax(kBar)
2e-3
λeff/R
2.506.4e-7881.034kJ0.5cm001
Tstop(us)
ηKvdef (cm/s)YintR
Granite targetR ~ 1 cmρ=2.5
M=1.31g
1.0ns laserpulse at d = R
P (Mb) v (cm/us)
29Lawrence Livermore National Laboratory
Conclusions• Without mitigation, Earth will experience future collisions with large NEO’s
• Earth-asteroid collisions can cause (and have caused) devastating local andglobal effects
• Kilometer-scale NEO’s can be deflected by megaton-class nuclear explosiveswith lead times of order one decade and by 100 megaton-class nuclearexplosives with lead times of order one year
• Both x-ray and neutron energy deposition can be effective in deflection
• Cale simulations predict that neutron capture energy can make a significantcontribution to the deflection velocity
• Material properties are important, but uncertainties in kilometer-scale targetscan likely be compensated for by conservative required yield estimates
• Laser-driven laboratory experiments might be used to measure kinetic energycoupling efficiencies and validate models
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