"Imagine a package of rock and ice," David Levy challenges the reader, a small world lost in the depths of space far be- yond Pluto. No larger than a village, this world, which we call a comet, moves lazily around the Sun. . . . Suddenly, something-maybe a passing star-causes it to shift. Instead of moving around the Sun, the ball of rock and ice now moves in a new orbit that takes it closer to the solar system's central star.23

In this brief description of a comet floating in the distant Oort Cloud, Levy makes reference to the object's composition, its size, and its orbit, three of the four major vital statistics, so to speak, of comets. The other is the shape of such bodies.

Asteroids also possess vital statistics. Like comets, they can vary greatly in size, shape, and...