All comets probably have an elongated eccentric orbit. The period required for one orbit around the Sun has been calculated for about 200 comets. Comets have been divided into three groups. There are non-periodical comets, whose orbits are so elongated that they require several million years to circle the Sun. The long-period comets take over 200 years for one orbit.
The comet Donati, which was last seen in 1858 and is expected again in 2000 years, belongs to this group. Short-period comets reappear in a relatively short period. A comet of this group is Encke, which takes only 3.3 years for one orbit. About sixty comets with short periods are thought to be influenced in their path by Jupiter.
These return every three to nine years. Some comets may never return, because the gravity of the planets has changed their path to such a great degree. If several comets move in approximately the same path, they are classed as a group regardless of when they return. An example of this is Ikeya-Seki, which grazed the Sun in 1965. Another seven comets, with periods of about 1000 years, are thought to belong to it.
The paths of comets are associated with the paths of meteorites. As shown by the Italian G.V. Schiaparelli, the Perseids, a meteor stream that returns every year in August, follow the same path as the comet 1862III. The Leonids, which return every November, also follow the path of a comet.
The danger of a comet core ever striking a populated area on Earth is considered to be minimal. However, some scientists are convinced that just such an event caused the climate to change when the dinosaurs ruled the Earth, resulting in their extinction 65 million years ago.
