Some collisions cause an asteroid to spin faster others slow it down. One indication is the observed pattern of asteroidal rotation rates. The size of an asteroid should determine which force dominates. We are just beginning to fathom the subtle interplay of these minuscule forces. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, etc. And anybody who builds sandcastles knows that even loose debris can cohere. Paltry though it may be, gravity binds a rubble pile together. The truth is neither strength nor gravity can be ignored. The material strength of an asteroid is nearly zero, and the gravity is so low one is tempted to neglect that too. At first, the rubble hypothesis may appear conceptually troublesome. It will absorb and dissipate a large fraction of the energy of an impact the far side might hardly feel a thing. The rubble hypothesis, proposed decades ago by scientists, lacked evidence, until the planetologist Schumaker realized that the huge craters on the asteroid Mathilde and its very low density could only make sense together: a porous body such as a rubble pile can withstand a battering much better than an integral object. In short, asteroids larger than a kilometer across may look like nuggets of hard rock but are more likely to be aggregate assemblages-or even piles of loose rubble so pervasively fragmented that no solid bedrock is left. Such colossal impacts would not just gouge out a crater-they would break any monolithic body into pieces. Most of them have one or more extraordinarily large craters, some of which are wider than the mean radius of the whole body. Those imaged at high-resolution show evidence for copious regolith despite the weak gravity. Most asteroids larger than a kilometer are now believed to be composites of smaller pieces. But observations and computer modeling are proving otherwise. Aggregate small bodies were not thought to exist, because the slightest sustained relative motion would cause them to separate. Originally, scientists predicted small asteroids to be hard and rocky, as any loose surface material (called regolith) generated by impacts was expected to escape their weak gravity.

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