The Nature of Comets
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At one astronomers believed that the Solar System was comprised of two radically different classes of objects (actually three, if we count the single star at the center of it all). On the one hand, there were the planets and asteroids – also conveniently called minor planets, to place them in their proper planetary perspective. Although there were clear differences between the massive Jupiter and the rocky “terrestrial” planets, such as our own Earth, the similarities were great enough to ensure their inclusion in the one cosmic family. All these objects were solid and stable. The orbits they followed around the Sun were sedate, almost circular, and widely separated from one another. Asteroids, with a few errant exceptions such as Earth-crossing Apollos, shared this clockwork regularity and did not, therefore, present any great danger of rocking the astronomical boat of the Sun’s planetary family.
By contrast with this well-behaved planetary family, the second component of...