OG 2018New RC
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- When asteroids collide, some collisions cause
an asteroid to spin faster; others slow it down. If
asteroids are all monoliths—single rocks—undergoing
random collisions, a graph of their rotation rates
- should show a bell-shaped distribution with statistical
“tails” of very fast and very slow rotators. If asteroids
are rubble piles, however, the tail representing the
very fast rotators would be missing, because any
loose aggregate spinning faster than once every few
- hours (depending on the asteroid’s bulk density)
would fly apart. Researchers have discovered that
all but five observed asteroids obey a strict limit on
rate of rotation. The exceptions are all smaller than
200 meters in diameter, with an abrupt cutoff for
- asteroids larger than that.
The evident conclusion—that asteroids larger than
200 meters across are multicomponent structures
or rubble piles—agrees with recent computer modeling
of collisions, which also finds a transition at that
- diameter. A collision can blast a large asteroid to bits,
but after the collision those bits will usually move
slower than their mutual escape velocity. Over several
hours, gravity will reassemble all but the fastest
pieces into a rubble pile. Because collisions among
- asteroids are relatively frequent, most large bodies
have already suffered this fate. Conversely, most
small asteroids should be monolithic, because impact
fragments easily escape their feeble gravity.
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