Rubble pile
An asteroid made of many loose rocks and boulders held together only by their own weak gravity, rather than one solid lump.
Many asteroids are not single solid rocks. Repeated collisions can shatter an asteroid, and if the pieces don't fly apart completely, they fall back together into a loosely packed heap. Such a rubble pile has lots of empty space between its boulders, so its overall density is low. Bennu, for example, has a bulk density only a little over 1 gram per cubic centimetre.
Rubble piles help explain several puzzles. Few asteroids larger than a few hundred metres spin faster than once every two hours or so, which fits loose rubble that would fly apart if spun faster. Rubble piles also absorb impacts like a beanbag and can reshape themselves. When the DART spacecraft struck Dimorphos in 2022, the debris it threw off added to the push.
Japan's Hayabusa spacecraft showed in 2005 that Itokawa is a boulder-strewn rubble pile with no large craters, and its returned grains match LL chondrites. Many meteorites are breccias, rocks made of broken fragments cemented together, which records the same rough history. Rubble piles are identified from spacecraft images and density measurements.
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