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Where does the solar system end? The question is harder than it sounds.

Close in are the Sun, the eight planets and their moons, the asteroid belt and many comets. Beyond Neptune, the farthest planet, lies the Kuiper Belt, an almost empty ring around the Sun in which icy bodies, nearly all smaller than Pluto, travel slowly on their orbits. Pluto is the best known of them.

Beyond even that, scientists believe, lies the Oort Cloud. Most of the solar system is flat, like a disk: the planets and the Kuiper Belt all circle the Sun in roughly the same plane. The Oort Cloud is different. It is thought to be a giant round shell surrounding everything else, like a huge bubble with thick walls. It is made of icy pieces of space rubble the size of mountains, and sometimes larger, and it is where some comets are believed to come from.

The distances are almost impossible to imagine. The Voyager 1 spacecraft, one of the fastest objects ever built, took about thirty-five years just to leave the region where the Sun's influence is strongest. Reaching the Oort Cloud would take it many centuries more.