Origin

Long period comets are thought to originate in a spherical cloud of debris that surrounds the Sun at distances between 20,000 and 100,000 AU (almost two light years - half way to the nearest star). The gravitational perturbation that causes such bodies to leave the cloud and approach the Sun is thought to be provided by the relative movement of nearby stars, massive dust clouds or the shock wave from an explosive event such as a supernova. Whatever the mechanism, five to ten significant cometary bodies approach the Sun each year, while an unknown number of smaller bodies pass undetected.

Composition

The composition of long period comets is very similar to that of their short period companions, but they tend to contain more volatiles. Short period comets outgas huge quantities of their volatiles at each approach to the Sun, so they eventually run out, becoming asteroid-like bodies. (There is still some doubt over the relationship between dead comets and some classes of asteroids). Long Period comets visit the inner solar system only rarely, so retain much of their icy material (and mass).

Orbital Characteristics

Unlike short period comets, whose orbits are relatively close to the ecliptic, long period comets have orbits that are randomly orientated on the celestial sphere. Their orbital periods are longer than 200 years, and they will often only return after thousands or even millions of years (or not at all). Consequently, most long period comets will be new to science when they reappear in the inner solar system, and have yet to be catalogued.

Thanks to SpaceGuardUK for much of the content of this page.

Redesigned and hosted by Marc Chamberlin.