SOMEWHERE out there, in the direction of the constellation Aquarius, a ball of primordial ice and dust known as Halley's Comet is gathering speed, falling toward our sun, racing toward a rendezvous with earth early in 1986. More than likely it has been making this round trip every 76 years since the formation of the solar system; certainly its passage has been recorded regularly since the time of Christ. But this time, in addition to the army of soothsayers and astrologers who heretofore have interpreted it for us, Halley is preceded by a first-rate PR man. At T minus five years, Nigel Calder, veteran science-writer-turned-advance-man, has put together a potpourri of fact and fancy, a fascinating compendium of all we would ask about comets, if we only knew the right questions.

You'll enjoy this disjointed but lively book. The only problem is that after reading it you may find the comet itself something of a bore. It will not have changed significantly since its last visit, but consider where we have been and what we have seen since 1910. We have peered into the depths of our oceans, walked the maria of the moon, peeked out at the surface of Mars, and zipped vicariously through the rings of Saturn. Visually jaded, we may yawn at a small bright spot in the night sky (even if it does sport a tail) and demand something more spectacular from this periodic interloper.

Calder is ready to counter this ennui: a comet can make you sick! To be more precise, Halley may bring with it a new epidemic of Asian flu. Calder doesn't really believe this, mind you ("I try to moderate my ridiculeof the theory of diseases from comets") but he nonetheless devotes a chapter to it, and it may be the best chapter in the book.

The basic idea is that life originated in outer space inside a brew of organic molecules in a primordial dust cloud, and was then transported to earth. For the past century, eminent scientists have taken this notion seriously and now a pair of astrophysicists in Britain, Sir Fred Hoyle and Chandra Wickramasinghe, have narrowed in on the comet as the ideal intergalactic nursery. Comets, they say, are "snowballs coated with interstellar food and then partly melted." As Calder puts it, Charles Darwin "visualized life beginnin;g in 'some warm little pond' containing the necessary chemicals. Hoyle and Wickramasinghe offer enclosed ponds larger than cathedrals, in a billion comets, each of them insulated from the outside universe by hundreds of metres of frozen crust. There, viruses and bacteria would evolve spontaneously, they say. In time the igloo-ponds would freeze, preserving the viruses and bacteria in a state of suspended animation." Asian flu and smallpox are the most likely candidates to emerge from this process, according to Hoyle and Wickramasinghe. Nor do they let the matter drop there. "The human nose, they say, has evolved its shape, with nostrils opening downward, because of the protection it gives against viruses falling out of the sky." Yes, of course. And don't sleep on your back. (I've also found that wearing a tin hat to protect against meteorites falling out of the sky is 100 percent effective.)

A book about Halley's Comet must of course include a word or two about Halley, the alcoholic Royal Astronomer who died in 1742 (the comet was named after him when it next appeared in 1758; the modern practice is to name a comet after whoever spots it first). Recognizing that Halley was no heavyweight, the author has skillfully woven in glimpses of Aristotle, Sir Isaac Newton, Galileo, Tycho Brahe, and even two Chinese astronomers, Hi and Ho, "who were too drunk even to notice an eclipse of the sun . . . the Emperor had their heads cut off." Not only were all of these luminaries interested in comets, but so also have been some less informed folks who, over the centuries, have used the appearance of a comet to scratch whatever itched them. At the 1910 passage, for example, in Oklahoma "the sheriffs arrived just in time to prevent the sacrifice of a virgin . . ." Noah's flood is commonly blamed on a comet. When a comet appeared around A.D. 60, the historian Tacitus wrote "as if Nero were already dethroned, men began to ask who might be his successor."

What are we doing to prepare for Halley's next visit? The Japanese plan a peek at it with a space-borne ultraviolet telescope. The European Space Agency has a more ambitous plan, involving a "kamikaze mission" which will shoot a camera-equipped probe at high speed through the head of the comet. We Americans, who are usually in the forefront of such ventures, are in something of a muddle this time. With NASA's buying power shrinking, we may have to watch Halley through Japanese and European eyes. Perhaps we can blame the flu on them. But in the meantime, invest in this slim volume, salt it away for five years, and you'll be the comet expert on your block. Just don't forget to say that it's going to miss the earth by nearly 40 million miles.