When several comets 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 others having periods of nearly a thousand years. The astronomer Brian G. Marsden has concluded that the 1965 comet and the even brighter comet of 1882 split from a parent comet, possibly that of 1106. This comet and others of the group probably split away from a truly gigantic comet thousands of years ago.

A close relationship also exists between the orbits of comets and those of the particles (meteoroids) that produce meteor showers. The Italian astronomer Giovanni Virginio Schiaparelli proved that the Perseid meteors, which appear annually around August 12, are associated with Comet Swift-Tuttle. Similarly, the Leonid meteors, which appear annually in November, are closely associated with Comet Tempel-Tuttle. Several other showers have been related to known cometary orbits, and are explained as the stream of debris scattered by a comet along its orbit.

Comets were once believed to come from interstellar space. Although no detailed theory of origin is generally accepted, many astronomers now believe that comets originated in the outer, colder part of the solar system from residual planetary matter in the early days of the solar system. The Dutch astronomer Jan Hendrik Oort has proposed that a “storage cloud” of comet material has accumulated far beyond the orbit of Pluto, and that the gravitational effects of passing stars may send some of the material towards the Sun, where it becomes visible as comets.

Comets have long been regarded by the superstitious as portents of calamity or important events. The appearance of a comet has also given rise to the fear of collision between the comet and Earth. Earth has in fact passed through the tails of occasional comets without measurable effect. The collision of the nucleus of a comet with Earth would probably have catastrophic worldwide effects, but the probability of such an event occurring is exceedingly small. Some scientists suggest, however, that collisions have taken place in the astronomical past and may even, for example, have had a climatic role in the extinction of the dinosaurs.

In 1992 the progenitor of Comet Shoemaker-Levy 9 produced 21 large fragments following its exceptionally close approach to the planet Jupiter. During a week-long bombardment in July 1994, the fragments crashed into Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the dissipation of their tremendous kinetic energy into heat led to massive explosions, some resulting in fireballs larger than Earth.

Shoemaker-Levy 9 Bombarding Jupiter

Fragments of Comet Shoemaker-Levy 9 collided with Jupiter between July 16 and July 22, 1994, stirring up the planet’s atmosphere and enabling scientists on the earth to gain a rare glimpse of the planet’s interior. The comet had broken into 21 large fragments on July 8, 1992, when it ventured too close to Jupiter. Trapped by Jupiter’s strong gravitational pull, these fragments bombarded the planet at speeds of about 210,000 km/h (130,000 mph). This image, taken by the National Aeronautics and Space Administration’s Hubble Space Telescope, reveals the impact sites (dark spots near the centre of the image) created by two of the comet fragments.

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Comet Groups