12. Asteroids and meteorites
Rubble pile or solid rock
There are two hypotheses for the internal structure of asteroids. According to the rubble pile hypothesis, an asteroid is the reassembled debris of previous impacts, with a porous interior that is literally filled with holes. In this interpretation, an asteroid consists of smaller pieces loosely held together by their mutual gravitational attraction. As mentioned earlier in this chapter, investigations of asteroid rotation rates suggest that many of them are not solid, and that there is nothing stronger than gravity holding them together. An alternative scenario proposes that asteroids are solid inside, and held together by their own material strength. It turns out that both ideas may be correct, depending on the asteroid. Over billions of years, asteroids on intersecting orbits have collided with enough force to shatter and break them into pieces. Instead of dispersing, the fragments might have re-accumulated into a rubble pile. Asteroid 253 Mathilde, a dark, carbon-rich, C-type asteroid is an example of such an asteroid. On the other hand, the collisional energy may have been enough to throw the fragments far into space, leaving a solid, chipped rock behind. The bright, silicate, S-type asteroid 433 Eros is this kind of object.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University