LLNL-PRES-XXXXXX This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and partially funded by the Laboratory Directed Research and Development Program at LLNL under tracking code 12-ERD-005. Lawrence Livermore National Security, LLC

2015 Planetary Defense Conference, Frascati, Roma, Italy 14 April 2015

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 2

Apparent Fracture

Image from J. Veverka, et al, “NEAR at Eros: Imaging and Spectral Results”, Science 289, 2088 (2000)

Measurements Mass = 6.81 1015 kg Density = 2.67 g/cc Porosity = 10-30% Measurements from J. Veverka, et al, “NEAR at Eros: Imaging and Spectral Results”, Science 289, 2088 (2000)

Fractured Consolidated Body Example: Eros

Gravitational Aggregates Example: Asteroid 25143 Itokawa

Image from J. Saito, et al, “Detailed Images of Asteroid 25143 Itokawa from Hayabusa”, Science 312, 1341 (2006)

Measurements Mass = 3.58 1010 kg Density = 1.95 g/cc Porosity = 40% Measurements from S. Abe, et al, “Mass and Local Topography Measurements of Itokawa by Hayabusa”, Science 312, 1344 (2006)

*Other examples are: Mathilde [AF Cheng, Adv. Sp. Res, 33, 1558 (2004)] Castalia [Asphaug, et al, Nature, 393, 437 (1998)]

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 3

0

20

40

60

80

100

120

140

160

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

σxx

[GPa]

ρ [g/cm3]

(1)(2)(3)(4)(5)(6)(7)(8)

Solid EOS Fit: lower-ρ chondrites 50% porous EOS

Bruderheim (Anderson et al., 1998)

Lizardite (Tyburczy et al., 1991)

H6 chondrite Kernouve (Schmitt, 2000)

Murchison (Anderson et al., 1998)

Serpentine (Mader et al., 1980)

Antigorite (Tyburczy et al., 1991)

�  Asteroid regolith: developing new thermomechanical constitutive models

�  Strength coupled to EOS for chondritic materials

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 4

�  4 simulated materials with the same overall density (different levels of porosity).

�  1D planar shock results from 1-20 km/s shown for each material.

�  Blowoff momentum from a standoff explosion or hypervelocity impact

NEA population mostly LL chondrites [Vernazza, Nature (2008)]

0

50

100

150

200

250

1 1.5 2 2.5 3 3.5 4

σxx

[GPa]

ρ [g/cc]

ρs = 1.26, φ = 0.0ρs = 1.58, φ = 0.2ρs = 2.10, φ = 0.4ρs = 3.15, φ = 0.6

(1) (2)

(3)

(1)+(2)

(4) Saito, et. al., Science (2006) (3) Tomeoka, et. al., Geochimica et Cosmochimica Acta (1999)

(1) Binzel, et. al., Icarus (2009) (2) Consolmagno, et. Al. Chemi der Erde (2008)

Example Chondrite Morphologies

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 5

�  Stronger shocks observed in low porosity materials

�  Melt depth increases for porosities from 0-30% for our materials with ρ0=1.26 g/cc.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

mel

tdep

th[c

m]

porosity φ

melt dataspline fit

�  Shock wave not generated for 40% or 60% porous material with

0

0.5

1

1.5

2

-100 -80 -60 -40 -20 0 20

mel

tdep

th[c

m]

porosity φ

melt vapor region

solid region

0% porous20% porous30% porous40% porous

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 6

Image from J. Saito, et al, “Detailed Images of Asteroid 25143 Itokawa from Hayabusa”, Science 312, 1341 (2006)

Image from J.E. Richardson Jr, H.J. Melosh, R.J. Greenberg, D.P. Obrien, Icarus, 179, 325 (2005).

0 10 20 30 40 50 60 700

50

100

150

200

250

object size [m]

num

ber

of o

bjec

ts

Object composed of 2380 pieces

Image from D.E. Grady, “Length scales and size distributions in dynamic fragmentation”, Int J Frac 163, 85 (2010)

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 7

Gravitational Aggregate

Fractured Body

Initial conditions: Energy deposited into regolith “cap”

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 8

���� ���� � ��� ����

��

���

���

���

Average Aggregate Velocity [cm/s]

Bin

Cou

nt

microporosity ��� � ���

��

���

���

���

Average Aggregate Velocity [cm/s]

Bin

Cou

nt

macroporosity

Radial speed Radial speed

axial speed ~7cm/s

axial speed ~20cm/s

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 9

Impulse follows force chains

Asymmetric crater

Time: 7 seconds

Lawrence Livermore National Laboratory LLNL-PRES-xxxxxx 10

�  For impacts and stand-off explosions alike, high porosity (micro, macro or combined) may prohibit shock wave generation.

�  Comparing 4 materials with different porosities, a shock was not produced for porosities above 40%. This affects the melt depth by over an order of magnitude in 1d simulations.

�  The internal structure of asteroid objects affects the dispersion of fragments meaning that if external symmetry is observed one should not necessarily expect a linear response (i.e. may generate rotation due to internal structure too)