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Rubble pile asteroids

As a child I learned about asteroids and have pictured them as solid pieces of rock of different sizes. Apparently, though, there have been suggestions since the 1970s and recent empirical data that some asteroids are actually collections of smaller rocks held loosely together by mutual gravity.

I don't know what proportion of asteroids are not monolithic, whether the majority of asteroids are such piles of rubble, but this was a piece of information that startled me, because it is from a field I was very interested in as a child, and as a child I had taken the astronomical statements as fact, rather than as interpretations of incomplete data.

The statement that asteroids are not monolithic blocks of stone, but are rather collections of small rocks is more interesting to me than some stupid political discussion about whether we should call a celestial body a 'planet' or a 'dwarf planet' or a salami hamburger.

From an article in Nature, this page and wikipedia it seems that the evidence for asteroids being rubble piles is:
  1. Many asteroids have very low desities, indicating a lot of empty space within them
  2. Clark R. Chapman and Donald R. Davis theoretically showed from energy considerations that, after a collision, pieces of an asteroid were more likely to fall back together under mutual gravity, than to fly apart.
  3. Many asteroids have large craters on them. If the asteroids were solid, the collision that generated such a large crater would have shattered the asteroid. If the asteroid was a rubble pile, the loose collection of rocks would absorb and dissipate the energy remaining together as a pile.
  4. No large asteroid has been found that spins faster than the limit at which a collection of rocks of that size would fly apart due to centrifugal force (bite me).
  5. When near earth asteroids pass us, their shape is altered due to tidal forces to an extent only possible if they are collections of small rocks.

I had a dream once, of sending robot spaceships to mine the asteroids and bring back valuable materials. I need to revise the design a little bit...
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