A second line of evidence is provided by numerical simulations of asteroid
collisions, which suggest that most collisionally-evolved bodies larger than
km are highly fractured (i.e., Asphaug and Melosh 1993; Greenberg et al.\
1996; Love and Ahrens 1996; Melosh and Ryan 1997). To demonstrate this more
clearly, I show results from an impact simulation provided by E. Asphaug, who
examined what happens when a km-sized solid homogenous asteroid is hit by a
much smaller object (Asphaug et al. 1998).
The target body used in this simulation was shaped to resemble the exterior of
Earth-crossing asteroid 4769 Castalia, with dimensions of 1.6 km by 0.8 km
(Hudson and Ostro 1995). Its density was chosen to be 2.1 g cm 
. The
interior of the target body was assumed to be homogeneous and solid, with an
elastic moduli and flaw distribution (i.e., ``strength'') the same as those
derived from laboratory experiments using basalt. The material equation of
state used in the code was that of lunar gabbro anorthosite, a crude but
adequate analogue for asteroid rock. The code itself, SPH3D treats shock
propagation in elastic solids with a plastic yield criterion, and models
explicit fracture and dynamic fractionation under principal tension.
The projectile used was a 16 m basaltic sphere. It was fired into the target body at 5 km s-1, yielding an impact energy of 17 kilotons, the same energy produced by the nuclear blast at Hiroshima. Note that this impact scenario is not equivalent to detonating a bomb on the surface of an asteroid, though the underpinning physics is related.
Fig. 2 shows the damage done to the target body less than a second after the collision. The colors correspond to material pulverized into strengthless ``dirt''. Crater excavation at the impact site is preceded by an advancing shock front that shatters the material as it passes through. Though some of this material has been given a velocity component, the crater will take longer than the simulation itself to form (on order of a few seconds). Vaporization and melting take place near the impact site, while fragmentation and crack growth occur throughout the target. Shock-created fissures break the target body into disconnected smaller pieces, but relatively little of this material is ejected.
This simulation shows that even a single impact can dramatically change an initially undamaged asteroid into a highly fractured one consisting of large blocks of material. Thus, since images of small bodies show highly cratered surfaces, we infer that asteroid interiors must be made of shattered debris.
Figure 2: Outcome of a asteroid impact simulation, which begins when 16 meter
projectile strikes a undamaged 1.6 km target body at 5 km/s. The target was
shaped to look like asteroid 4769 Castalia. The impact speed, squared, times
the mass of the impactor equals the impact energy: 17 kilotons, the same as
the Hiroshima explosion. The image shows impact fragmentation expressed on the
surface and in the interior, where the colors represent levels of damage. The
impact shatters the target into a number of smaller pieces without dispersing
them, leaving behind a rubble pile asteroid bound together by gravity. (Figure
from Asphaug et al. 1998).