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. Aggregate small bodies were not thought to exist, because the slightest sustained relative motion would cause them to separate. But observations and computer modeling are proving otherwise. Most asteroids larger than a kilometer are now believed to be composites of smaller pieces. Those imaged at high-resolution show evidence for copious regolith despite the weak gravity. Most of them have one or more extraordinarily large craters, some of which are wider than the mean radius of the whole body. Such colossal impacts would not just gouge out a crater—they would break any monolithic body into pieces. 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.

The rubble hypothesis, proposed decades ago by scientists, lacked evidence, until the planetologist Shoemaker 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. It will absorb and dissipate a large fraction of the energy of an impact; the far side might hardly feel a thing. At first, the rubble hypothesis may appear conceptually troublesome. The material strength of an asteroid is nearly zero, and the gravity is so low one is tempted to neglect that too. The truth is neither strength nor gravity can be ignored. Paltry though it may be, gravity binds a rubble pile together. And anybody who builds sandcastles knows that even loose debris can cohere. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, etc. We are just beginning to fathom the subtle interplay of these minuscule forces.

The size of an asteroid should determine which force dominates. One indication is the observed pattern of asteroidal rotation rates. Some collisions cause an asteroid to spin faster; others slow it down. If asteroids are monolithic rocks undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators. If nearly all asteroids are rubble piles, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours would fly apart. Recently, several astronomers discovered that all but five observed asteroids obey a strict rotation limit. The exceptions are all smaller than about 150 meters in diameter, with an abrupt cutoff for asteroids larger than 200 meters. The evident conclusion—that asteroids larger than 200 meters across are rubble piles—agrees with recent computer modeling of collisions. A collision can blast a large asteroid to bits, but those bits will usually be moving slower than their mutual escape velocity (the lowest velocity that a body must have in order to escape the orbit of a planet). Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile.

1. According to the rubble-pile hypothesis, an advantage conferred on a low-density asteroid is that it is

A. unlikely to fall apart over a long period of time
B. more amenable to computer modeling
C. less susceptible to powerful impacts
D. not likely to collide with another object
E. more readily observed by astronomers



2. How would the author of the passage most likely respond to the assertion of another scientist claiming that a crater greater than the radius of an asteroid is a result of an impact?

A. Asteroids actually contain a significant amount of regolith despite the force of weak gravity.
B. Because of a great degree of fragmentation, such an asteroid would have to have a solid bedrock.
C. Such a crater would most probably result from a series of small impacts over a period of time.
D. Most asteroids are held together by a series of forces that are often unstable.
E. This claim would constitute evidence that the asteroid is not monolithic.



3. The primary purpose of the passage is to

A. refute an unconventional theory regarding asteroid collisions
B. express doubt regarding the validity of evidence offered up by several notable astronomers
C. explain how earlier evidence used to describe an aspect of asteroids was misleading
D. explore common features of an asteroid in order to provide support for a theory
E. discuss how one explanation of an astronomical phenomenon is most likely correct



4. The example of the sandcastle (in the second paragraph) serves to

A. invalidate Schumaker's initial observation
B. offer an alternative hypothesis for an observed phenomenon
C. describe a condition in which the typical laws of the universe do not obtain
D. provide support for the rubble-pile hypothesis
E. present as an instance in which gravity has little effect



5. The reason that graphs of asteroid rotation rates lack the expected statistical tail associated with high rotational rates is that

A.the greater the speed in which an asteroid spins the more likely it is to cohere
B. the weak forces in asteroids displaying such a high rotational rate would not be able to prevent the asteroid from falling apart
C. asteroids are not being subjected to a uniform distribution of random collisions
D. most monolithic asteroids, upon colliding with other asteroids, are able to sustain such a high rate of rotation
E. for the most part, the asteroids surveyed were less than 150 meters in diameter and thus far less likely to be rubble-piles, which are better able to sustain the impact from collisions



6. Schumaker originally conceived of the rubble hypothesis because he surmised that

A. a solid body is able to withstand impacts if it has a diameter greater than 1 km
B. an object with low density can reassemble more easily after a major impact
C. an asteroid that is held loosely together is better able to withstand substantial impacts
D. the asteroid Matilda lacked the regolith common to asteroids of a similar size
E. forces holding together large meteors were too weak to deal with major collisions



7. Scientists originally believed that asteroids lacked regolith because

A. a sizeable enough impact would cause all accumulated surface material to become dislodged
B. the gravitational forces of asteroids were too weak to hold any aggregation of matter together
C. computer models had shown that loose pieces of rock tend to come dislodged from even the slightest impact
D. regolith was absent from smaller planets lacking an atmosphere
E. the velocity of asteroids was so great as to cause any loose matter to easily float off into space