Comets rarely come within a few million miles of Earth and, thus, have a slow apparent motion across our sky. Typical comets remain visible for periods of several weeks up to several months. Comets can suddenly brighten or fade from view in a matter of hours. They can lose their tail or develop multiple tails. Sometimes they can even split into two or more pieces, so that, through a telescope, several comets can be seen moving together across the sky.
Comets represent some of the oldest, basically untouched objects in the solar system.
Their very composition seems to represent the original makeup of the vast nebula that ultimately condensed to form the sun and planets. Recent years have revealed comets have shaped the progress of life as we know it here on Earth. Many astronomers are convinced that early collisions between Earth and comets brought the vast amounts of water that now make up Earth's oceans. These oceans enabled life to get a foothold. On the flip side, the dinosaurs are certainly an example of how collisions between Earth and comets can also bring extinction to different life forms.