Great balls of fire from the heavens


Comets have always exerted a powerful effect on people's imagination. For most of recorded history the appearance of a comet was thought to announce an important earthly event. The astrologers of the Italian Prince John Galeas Visconti predicted that his death would be announced by a comet. In 1402 a comet appeared. When the prince was told he dropped dead with fright.

We now know that a huge cloud of comets, the Oort cloud, exists on the frontiers of our solar system. Study of this cloud helps to explain the formation of the solar system and mass extinctions on earth. Paul Weissman describes the Oort cloud in the September edition of Scientific American.

Aristotle believed a comet (in Latin stella cometa - "hairy star") was a cloud of gas in the atmosphere. Later, Seneca proposed that comets travel along their own paths through the heavens. Tycho Brahe confirmed Seneca's hypothesis by comparing observations on the 1577 comet made in several different locations in Europe. If the comet were close by it would be seen in a slightly different position against the stars from each observation location. All observations were the same, and Brahe concluded that the comet was farther away than the moon.

Isaac Newton showed that comets are subject to the same laws that control the orbits of planets. Edmond Halley showed that comet orbits are ellipses with the sun at one focus. He noted a striking similarity in the orbits of three comets observed in 1531, 1607 and 1682. He concluded that these were the same comet with a period of 75/76 years. Halley's comet reappeared this century in 1910 and in 1985-86.

Astronomers divide comets into three groups depending on the time it takes them to orbit the sun (their period). Long-period comets like Hale-Bopp, have periods greater than 200 years. Intermediate-period comets like Halley have periods between 20 and 200 years, and short-period comets like Encke have periods less than 20 years.

Intermediate and long-period comets enter the planetary region of our solar system randomly from all directions whereas short-period comets have orbits with planes that are inclined no more than 40 degrees from the ecliptic plane (plane of the earth's orbit). Short-period comets are thought to come from the Kuiper Belt, beyond the orbit of Neptune. Intermediate and long-period comets come from the Oort cloud.

The planets in our solar system orbit the sun, held there by its gravitational pull. Most people would identify Pluto as the outermost object in our solar system. But a significant gravitational pull of the sun extends 3,000 times farther, as much as half-way to the nearest stars. This space swarms with comets that orbit the sun - the Oort cloud, named after Jan H. Oort, who proposed in 1950 that a "storage cloud" of comet material had accumulated about 50,000 astronomical units (AU) from the sun (an AU is the distance from the sun to the earth).

If you stood on a comet in the Oort cloud you would see the sun as the brightest star in the sky, but only as bright as Venus in the Earth's evening sky. The comets in the cloud are weakly bound to the sun and their orbits are readily affected by random passing stars. About 12 stars pass within one parsec (206,000 AU) of the sun every one million years. These close encounters throw a sprinkle of comets into the inner solar system on long elliptical orbits.

Oort characterised the cloud as "a garden gently raked by stellar erturbations". Occasionally a star approaches so close that it violently disturbs cometary orbits in the Oort cloud. A star passes within 10,000 AU of the sun every 36 million years and within 3,000 AU every 400 million years. Such close encounters could send showers of comets into the solar system, colliding with the planets and possibly causing biological mass extinctions on earth. The dinosaurs may have vanished in this manner. Fortunately, the next close passage of a star isn't expected for 1.4 million years.

Comet collision with a planet was illustrated dramatically in 1994. Comet Shoemaker-Levy 9 broke into fragments in the gravitational field of Jupiter. The fragments crashed into Jupiter's dense atmosphere at 130,000 miles an hour. The energy of the comets was dissipated in massive explosions, some producing fireballs larger than the earth.

A COMET consists of a small nucleus embedded in a nebulous disc called the coma. The astronomer Fred Whipple proposed in 1949 that the nucleus is a "dirty snowball" made of ices and dust, containing most of the mass of the comet. Gases and particles are ejected from the nucleus and make up the coma and tail of the comet. The gases are made of bits of molecules broken away from molecules such as CH 4, NH 3, and H 20, which exist as ices in the nucleus. As a comet approaches the sun, solar heat evaporates the ices and the comet brightens greatly. It may develop a bright tail sometimes millions of kilometres long. The tail is directed away from the sun, blown by the solar wind, a stream of gases continuously ejected from the sun. As the comet recedes from the sun, emission of gas and dust decreases and eventually the tail disappears.

Astronomers believe comets originated in the outer, colder part of the early solar system from residual planetary matter. The material in the comets may be little altered from the era of condensation. Conditions probably suited the condensation of comets at solar distances around the orbits of Uranus and Neptune. Gravitational perturbations by the major planets dispersed the ring of comets, some outwards to form the Oort cloud and some inwards to become short-period comets.

William Reville is a senior lecturer in biochemistry at UCC