(5 of 10)
Comets, in fact, are nowhere near as large as planets. Their central structure, or nucleus, is usually no more than a few miles in diameter; it is believed to consist largely of frozen gases—mainly water vapor, methane, carbon dioxide and ammonia, and perhaps some hydrocarbons—and dust particles. That, at least, is the commonly accepted "dirty snowball" theory, originally proposed by Harvard's Whipple in 1950. But there are those who take exception to Whipple. British Astronomer Raymond A. Lyttleton prefers his own "gravel-bank" theory, which holds that the cometary nucleus is really a loose mass of dust particles with little or no ice. By training their instruments on Kohoutek, astronomers may at last be able to settle that argument.
There is less debate about where comets originate. The most widely accepted explanation is that of Dutch Astronomer Jan Oort, who says that comets exist by the billions in a vast swarm of debris beyond Pluto that stretches halfway to the nearest star, Proxima Centauri. The debris, called Oort's Cloud, coalesced from the swirling dust and gases in the original solar nebula, from which the sun, earth and other planets and moons were formed. Thus comets are primordial matter, largely unchanged since the solar system's birth. (Lyttleton ascribes a different origin to the comets: he thinks that they are swept up by solar gravity as the sun wheels around the galaxy through clouds of interstellar matter.)
Wispy Hydrogen. Tugged by the gravity of a passing star, chunks of the Oort debris are occasionally pulled into orbits closer to the sun. Then perturbed further by the gravity of a massive planet, probably Saturn or Jupiter, they often enter a highly elliptical orbit that swings them close to the sun and then so far out again that they do not return to the vicinity of the sun for years. Some, like Encke's comet, which makes a pass around the sun every 3.3 years, have relatively small orbits. Others loop out billions of miles from the sun, and millions of years elapse before they return.
As a comet enters the inner part of the solar system, the sun's heat begins to liberate dust and gases from the nucleus, forming a large cloud called the coma. Such clouds may become Jovian in proportions, with a diameter of more than 100,000 miles, though they are very thinly dispersed. In 1969 and 1970, NASA'S Orbiting Astronomical Observatory (OAO-2) discovered that the coma of comets is surrounded by a still larger ball of wispy hydrogen that may far exceed the sun's diameter of 860,000 miles.
The hydrogen cloud is believed to be formed from the dissociation of water molecules in the nucleus. As the comet nears the sun, it acquires its most characteristic feature. Bombarded steadily by the charged particles of the solar wind and by the slight but measurable pressure of sunlight itself, the cometary gases and dust are swept back to form one or more glowing tails. These may reach lengths of 60 million miles or more, roughly two-thirds the distance between earth and sun. Regardless of the direction of the comet's travels, its tail is always directed away from the sun. Thus, when Kohoutek reappears in the evening sky after swinging around the sun, its tail —which now trails the comet—will precede it as Kohoutek races away toward the outer reaches of the solar system.