comets

            Before the seventeenth century, comets were considered portents-warning shots fired at a sinful Earth from the right hand of an avenging God. However, in the post-Newtonian era, when their paths were understood to intersect that of the Earth, they were considered actual agents of destruction. Experts have described comets as the carriers of both life-seeds to the early Earth and horrific missiles that will one day snuff out life as we know it. At one time or another, people have blamed comets for war and held responsible for the deaths of men, the birth of good wine, the London fire of 1666, severely cold weather, etc . . . If one central theme runs throughout history of comets, it must be the public concern they have commanded. Comets are ancient objects, formed in the outer reaches of the Solar System from the ice of gases such as methane, water vapor, and ammonia, combined with dust from primitive rock compounds. Sometimes comets are described as "dirty snowballs" because they are icy lumps, or wandering icebergs. Comets are relatively tiny- just a few miles across on average. Their nuclei are very different from glowing balls of light, with multimillion-mile-long tails. This is one reason comets occasionally visit the inner Solar System. Astronomers divide comets into long-period types with orbits of more than 200 years and short-period types with orbits of less than 200 years (as cited in Branley 1988 p. 43). All comets begin their journey as long- period types. Gravitational fields of planets then capture long- period comets. Comets can have orbits at any angle because they can come from any region. Once comets are captured, they fall into line with the movement of planets, staying close to the ecliptic, orbiting the sun in the same direction as the planets. One exception is Halley's Comet. It is a short-period comet with an orbital period of about seventy-six years- known as retrograde orbits (as cited in Branley 1988 p.