If a person goes outside on a clear, moonless night in the countryside, far from city lights, he or she will be treated to a breathtaking sight. Above stretches a velvet black canopy studded with thousands of pin- points of light, some bright and lustrous, others so faint they are barely visible. Most of these, of course, are stars like the Sun, except that they lie much far- ther away than the huge gaseous ball whose light and heat make life on Earth possible. A few of the brighter points of light in the dark canopy are planets in our own solar system, the Sun's family. But these few easily visible solid bodies, each thousands of miles in diameter, are only the tip of the iceberg, so to speak, of the material making up the solar system. Lurking in the...