Comet Coming

Astronomers all over the world were readying their telescopes this week for Comet Arend-Roland. which is about to make an appearance from behind the glare of the sun. This week it should become visible just after sunset, low in the northwest. It will soon climb higher in the sky, but since it will be moving away from the sun, it will gradually lose brilliance, become invisible to the naked eye about June 1.

There is a chance that Comet Arend-Roland will be the first really bright comet since 1910 (Halley's, not due to be seen again by earthlings until about 1984), but astronomers hate to make predictions about comets. Far from behaving like respectable members of the solar system, they are skittish and unpredictable. They wax and wane capriciously. Some of them grow magnificent tails; others do not.

No one knows where comets originate. One respected theory is that they are loose aggregations of matter distantly associated with the solar system. They may have been left over from the dust cloud that went to form the sun, or they may have originated in a Saturn-like ring that once surrounded the sun. Most of them are believed to stay far beyond the outermost planets, moving on orbits so distant that they are invisible. A few have been affected by some passing star and deflected into lopsided orbits that carry them periodically down toward the sun. These are the comets that become visible to man's eye and telescopes.

Dirty Snowballs. Some astronomers think that comets are swarms of dustlike particles, with a few larger chunks of matter at their centers. Another theory, developed by Astronomer Fred L. Whipple of Harvard, is that they are made mostly of "ices." Out in cold, dark outer space, says Whipple, beyond the last of the planets, wandering molecules of methane, water or ammonia tend to stick together as solids. Gradually snowflakes of a sort form. Attracting one another feebly over millions or billions of years, they gather into sizable bodies of solidified gas peppered with grains of sand or dust. They may get to be several miles in diameter.

As long as these "dirty snowballs" stay far enough from the sun, as most of them do, they lead peaceful lives, but a plunge toward the center of the solar system is a wild adventure. As a comet approaches the sun, its surface is warmed by the strengthening sunlight. Layer after layer, the ices turn into gas. Soon the nucleus is surrounded by a rapidly growing cloud, of gas and dust boiled out of the solid nucleus. This cloud, the comet's head, may be many thousands of miles in diameter. It is so transparent that stars show through it plainly.

As the head grows bigger, some of the fine material is blown out of it by the pressure of sunlight, which has more effect than gravitation on particles of proper size. This fine material forms the tail, which always points away from the sun no matter how the head is moving. It may become many millions of miles long. The light from the head and tail is partly reflected sunlight; the rest of it comes from atoms or molecules made to fluoresce by solar radiation.

Comet Arend-Roland was discovered last Nov. 8 by S. Arend and G. Roland of the Royal Observatory at Uccle. Belgium. At that time it was a faint, hazy object, much too dim to be seen without a telescope. Astronomers studied its motion and decided that it would pass within 30 million miles of the sun. Heading for outer space again, it will come within about 52 million miles of the earth on April 20.

Radical Tail. During its plunge toward the sun, Comet Arend-Roland developed a respectable head and tail, and there is good reason to hope that it will come through its solar ordeal without too much loss of substance. Astronomers have plenty of questions to ask it; their instruments and understanding have improved enormously since 1910.

One important new technique will be to observe the comet's tail with radio telescopes. If it is really full of peculiar chemical fragments (free radicals), as astronomers suspect, the fragments should be excited by sunlight and made to broadcast on characteristic wave lengths. The Naval Research Laboratory in Washington has turned its 50-ft. radio disk on the comet in the hope of detecting waves from hydroxl (OH) radicals. If astronomers find this odd stuff in comets, they may be able to trace it back into interstellar space. This may lead them, in turn, to new knowledge about what the universe is made of.

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