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Periods and Orbits of Comets

Comets have elliptical orbits around the Sun that take periods of time ranging from a few years to tens of thousands of years or longer. Astronomers classify comets according to their orbital periods: Short-period comets have periods shorter than 200 years, and long-period comets have periods 200 years or longer.

Short-period comets are subdivided into Jupiter-family comets and Halley-type comets. Jupiter-family comets are influenced by the gravitation of the giant planet Jupiter and have periods shorter than 20 years (about 8 years being average), and orbits that mainly lie closer in than the orbit of Jupiter. Halley-type comets (sometimes called intermediate-period comets) have periods from 20 up to 200 years, and have orbits that may reach beyond the planet Neptune. Halley’s Comet itself has a period that averages 76 years.

Most short-period comets and some long-period comets have orbits that lie close to the plane of the solar system in which the eight major planets orbit the Sun. By contrast, a significant number of long-period comets (more than 30 percent) have orbits that are strongly tilted relative to the plane of the solar system. Short-period comets, particularly in the Jupiter family, usually orbit the Sun in the same counterclockwise direction as planets and other solar system bodies. Some comets such as Halley’s Comet, however, may orbit the Sun in a clockwise direction, opposite the planets.

Scientists think short-period comets typically originate in the Kuiper Belt, a disk of icy objects that lies beyond the orbit of Neptune at 35 to 55 astronomical units (AU) from the Sun. (An astronomical unit is the average distance from Earth to the Sun, just under 150 million km, or 93 million mi.) In addition to the main disk, the Kuiper Belt also includes a larger region of scattered objects—icy bodies such as the dwarf planet Eris that have more inclined and very out-of-round elliptical orbits. In general, comets from the Kuiper Belt retain the same orbital direction and orientation around the Sun as the rest of the solar system.

Some of these comets may spend time as objects called centaurs between the orbits of Neptune and Jupiter, and then move into the inner solar system to become Jupiter-family comets. Comets from the scattered-disk region of the Kuiper Belt can have more variable orbits.

Long-period comets and some Halley-type comets likely come from the Oort Cloud, a much more distant sphere of icy objects thought to surround the solar system at a distance of 20,000 to 200,000 astronomical units (AU) from the Sun. Comets from the Oort Cloud can come from random directions. The gravitational pull of distant stars may nudge the orbits of objects in the Oort Cloud, causing the icy bodies to move inward and become comets.

The orbits of some long-period comets are so vast that they are indistinguishable from parabolas—open curves that would take the comets out of the solar system. But from technical analyses, astronomers assume that the orbits of these comets are ellipses of great eccentricity, with periods as long as 40,000 years or possibly much longer. The bright Comet Hyakutake, which was visible from Earth in 1996, has an estimated period of 10,000 years; Comet McNaught, which was visible in late 2006 and early 2007, may have a period of more than 100,000 years. No comets have been known to approach Earth on a hyperbolic orbit—such an orbit would have meant an origin outside the solar system.

When several comets with different periods travel in nearly the same orbit, they are said to be members of a comet group. The most famous group includes the spectacular Sun-grazing comet, Ikeya-Seki, of 1965, and seven other comets having periods of nearly 1,000 years. The American astronomer Brian G. Marsden concluded that Ikeya-Seki and the even brighter comet of 1882 split from a parent comet, possibly the one of 1106 ad. This comet and others of the group probably split away from a truly giant comet thousands of years ago. The European Space Agency’s Solar and Heliospheric Observatory (SOHO) spacecraft, designed to observe the Sun, has detected more than 500 comets from another group, the Kreutz Sun-grazers.

A close relationship also exists between the orbits of comets and the orbits of meteor showers in Earth’s atmosphere. The Italian astronomer Giovanni Schiaparelli proved that the Perseid meteors, which appear each year in August, move in the same orbit as Comet Swift-Tuttle. Similarly, the Leonid meteors, which appear in November, were found to follow the same orbit as Comet Tempel-Tuttle. Several other showers have been related with known cometary orbits, and are explained by the dust and other solid debris left behind by a comet along its orbit. Some of the dust collected in Earth’s upper atmosphere by scientific instruments on high-altitude aircraft and balloons is thought to come from comets.