Updated August 5, 2020 | Infoplease Staff
SWAN comet, 2006

The SWAN comet, taken in 2006. Photo: NASA

Comets, according to the noted astronomer Fred L. Whipple (1906–2004), are enormous “snowballs” of frozen gases (mostly carbon dioxide, methane, and water vapor) and contain very little solid material. The whole behavior of comets can then be explained as that of frozen gas being heated by the Sun. When the comet Kohoutek made its first appearance to human observers in 1973, its behavior seemed to confirm this theory, and later the international study by five spacecraft that encountered Halley's comet in March 1986 seemed to confirm Whipple's idea of the makeup of comets.

In Jan. 2006, NASA's Stardust mission returned with samples collected from Comet Wild 2. Examination of the dust found materials formed near the Sun or other stars that was unexpected in a comet born on the outer edges of the solar system. Scientists will have to reexamine how comets are formed and of what materials they are composed. (For more on Stardust, see p. 534.)See also Stardust (U.S.)

Since comets appear in the sky without any warning, people in classical times and especially during the Middle Ages believed that they had a special meaning, which, of course, was bad. Since a natural catastrophe of some sort or a military conflict occurs every year, it was quite simple to blame the comet that happened to be visible. But even in the past, there were some people who used logical reasoning. When, in Roman times, a comet was blamed for the loss of a battle and hence was called a “bad omen,” a Roman writer observed that the victors in the battle probably did not think so.

Up until the middle of the 16th century, comets were believed to be phenomena of the upper atmosphere; they were usually explained as “burning vapors” that had risen from “distant swamps.” That nobody had ever actually seen burning vapors rise from a swamp did not matter.

But a large comet that appeared in 1577 was carefully observed by Tycho Brahe (1546–1601), a Danish astronomer who is often, and with the best of reasons, called eccentric, but who insisted on precise measurements for everything. It was Tycho Brahe's accumulation of literally thousands of precise measurements that later enabled his younger collaborator, Johannes Kepler (1571–1630), to discover the laws of planetary motion. Measuring the motion of the comet of 1577, Brahe could show that it had been far beyond the atmosphere, even though he could not give figures for the distance. Brahe's work proved that comets were astronomical and not meteorological phenomena.

In 1682, the second Astronomer Royal of Great Britain, Dr. Edmond Halley (1656–1742), checked the orbit of a bright comet that was in the sky and then compared it with earlier comet orbits that were known in part. Halley found that the comet of 1682 was the third to move through what appeared to be the same orbit, and that the three appearances were roughly 76 years apart. Halley concluded that this was the same comet, moving around the Sun in a closed orbit, like the planets. He predicted that it would reappear in 1758 or 1759. Halley himself died in 1742, but a large comet appeared 16 years after his death as predicted and was immediately referred to as “Halley's comet.”

Halley's comet appeared again in 1986, sparking a worldwide effort to study it up close. Five satellites in all took readings from the comet at various distances. Two Soviet craft, Vega 1 and Vega 2, provided detailed pictures of the comet, including the first of the comet's core. The European Space Agency's craft, Giotto, entered the comet itself, coming to within 450 mi of the comet's center and successfully passing through its tail. In addition, two Japanese craft, the Suisei and the Sakigake, passed at a farther distance and analyzed the cloud and tail of the comet and the effect of solar radiation upon it.

Astronomers refer to comets as periodic or nonperiodic, but the latter term does not mean that these comets have no period; it merely means that their period is not known. The actual periods of comets run from 3.3 years (the shortest known) to many thousands of years. Their orbits are elliptical, like those of the planets, but they are very eccentric, long, and narrow ellipses. Only comet Schwassmann-Wachmann has an orbit that has such a low eccentricity (for a cometary orbit) that it could be the orbit of a minor planet.

When a comet coming from deep space approaches the Sun, it is at first indistinguishable from a minor planet. Somewhere between the orbits of Mars and Jupiter, its outline becomes fuzzy; it is said to develop a coma (the word used here is the Latin word coma, which means “hair,” not the phonetically identical Greek word that means “deep sleep”). Then, near the orbit of Mars, the comet develops its tail, which at first trails behind. This grows steadily as the comet comes closer and closer to the Sun. As it rounds the Sun (as first noticed by Girolamo Fracastoro, 1483–1553), the tail always points away from the Sun so that the comet, when moving away from the Sun, points its tail ahead like the landing lights of an airplane.

The reason for this behavior is that the tail is pushed in these directions by the radiation pressure of the Sun. It sometimes happens that a comet loses its tail at perihelion; it then grows another one. Although the tail is clearly visible against the black of the sky, it is very tenuous. It has been said that if the tail of Halley's comet could be compressed to the density of iron, it would fit into a small suitcase.

Although very low in mass, comets are among the largest members of the solar system. The nucleus of a comet may be up to 10,000 mi in diameter; its coma between 10,000 and 50,000 mi in diameter; and its tail as long as 28 million miles.

Comet Shoemaker-Levy 9 broke up into 21 fragments in July 1992 and crashed into the surface of Jupiter, July 16–22, 1994, in the most violent event in the recorded history of the solar system.

In 1951, Dutch astronomer Gerard Kuiper first suggested the existence of a disk-shaped swarm of short-period comets that begin beyond the orbit of Neptune and extend past Pluto. In 1995, the Hubble Space Telescope detected the long-sought Kuiper Belt and an estimated 200 million comets were discovered orbiting it.

On July 4, 2005, NASA's Deep Impact spacecraft traveled 268 million miles over a period of six months to blast a hole into Comet Tempel 1. The crash emitted the energy of 4.5 tons of TNT, releasing a tremendous plume of dust and gases. Comets are believed to have been formed out of the remaining dust and gases after our solar system formed 4.6 billion years ago. By blasting a crater into the surface of Tempel 1 and exposing its pristine interior, scientists hope to examine the primordial remnants of our solar system to gain insight into its formation. Early examination of data found unexpected water ice on the body surface of Tempel 1, the first ever detected on a comet.

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