One of the most famous comets, Halley's comet has been appearing periodically in the inner solar system for two thousand years. Scientists believe that is may have first entered the inner solar system as early as 239 b.c. The comet's appearance in the 1066 is documented in the Bayeux Tapestry, and its 1301 appearance is recorded in a nativity painting by the Italian painter Giotto. Though it had been seen many times before Edmond Halley observed it in 1682, very little was known about the comet. Prior to Halley's studies, many scientists believed that comets appeared randomly and were not governed by the law of gravity. Though Isaac Newton had shown that comets, like the planets, moved through space in orbits that were conic sections. But no one could agree as to whether the section was parabolic, circular, elliptical, or hyperbolic. Because a comet is seen best when it...