long-period comet

A long-period comet is a comet with a period of more than 200 years and as much as several million years. Long-period comets, together with Halley-type comets, are now believed to come from the Oort Cloud, an enormous reservoir of frozen cometary nuclei orbiting the Sun at a distance of tens of thousands of astronomical units. The gravitational influences of passing stars, giant molecular clouds, and the central regions of the Galaxy, are thought to be instrumental in occasionally perturbing the orbits of some of the Oort objects and causing them to plunge toward the inner solar system on highly elliptical paths characteristic of long-period comets.