Halley's Comet is back, and once again the human race is starting to skitter around like a drop of water on a hot griddle.

Entrepreneurs are popping out of the ground, spieling as they come. Already 16 different Halley's T-shirts are being sold. An ad man named Owen Ryan has founded a General Comet Industries. At least a dozen new comet books will come out this year and next, not to mention comet cocktails, comet stamps, comet medals, hats, posters, gag pills, stock certificates and bumper stickers ("Halley's Comet Watchers Do It Every 75 Years").

Waiting in a New Jersey warehouse are stocks of "Halleyscopes," relatively simple refractors retailing for about $200 and "especially designed for first-time telescope users." Cruise lines are booking passages for trips to South America, Australia and other points in the Southern Hemisphere, where the comet will be brightest. Somebody is campaigning to get American cities to turn out their lights after midnight so people can see the thing better.

And Halley's is still on the far side of Jupiter, almost a billion miles away, a 6-billion-ton dirty snowball three miles wide, idling along at 20,000 miles an hour.

The 200-inch Palomar telescope claims the first sighting during the current visit, Oct. 15, 1982, of what was then a speck of light in Canis Minor. But by late November, shortly after it begins its eight-month rush around the sun at 140,000 mph, it should be visible to anyone who looks hard enough in the right place at the right time.

By then the front end, the coma, blossoming brightly in the sun as it sublimates from solid to gas, will be 100,000 miles in diameter, followed by a 50-million-mile tail and a history that goes back 3,000 years. Who Saw It First?

Halley's comes to us every 76 years, and by figuring back through the anniversaries, a Chinese scientist has estimated that a comet his ancestors spotted in 1057 B.C. must have been Halley's. But a Halley's historian, Donald Yeomans of the Jet Propulsion Laboratory in Pasadena, Calif., doubts those dates because subsequent close passes in 374, 607 and 837 A.D. would have changed its time schedule.

The conventional best bet for the first sighting seems to be a Chinese report of 240 B.C., making this the 29th recorded appearance, though Yeomans feels happier with a Han dynasty record, "a comet appears in the east," in the fall of 87 B.C. An Antique Streaker

In any case, the comet has had plenty of publicity over the years. It is woven into the Bayeux Tapestry because it appeared in 1066, the year of the Norman Conquest. It shines above a Giotto Nativity fresco disguised as the Star of Bethlehem, painted after Giotto saw it in 1301. (Was it indeed the Star of Bethlehem? Probably not, because its nearest relevant appearance was in 12 B.C., just outside the period in which Jesus' birth has been bracketed.)

The visit of 1456, surely seen by a couple of European kids named Christopher Columbus and Leonardo da Vinci, was described by Leonardo's tutor: "Its head was round and as large as the eye of an ox, and from it issued a tail fan-shaped like that of a peacock. Its tail was prodigious, for it trailed through a third of the firmament."

It worried people. The very word "disaster" means "evil star," as Nigel Calder points out in his most entertaining and useful book, "The Comet Is Coming!," the source for much of this information. Time and again, the experts predicted famines, wars and plagues. Queen Elizabeth's people tried to keep her from even looking at the comet of 1577, but she marched straight to the window and fixed it with her cold gray eye. Nothing happened, either to her or to the comet.

In the Dark Ages comets were hought to be the work of the Devil, and there were those who insisted they could actually smell a sulfurous odor in the night skies ("Halleytosis," says Calder). The 1664 comet inspired this dreadful prediction from one Englishman: "These Blazeing Starrs! Threaten the World with Famine, Plague & Warrs, to Princes, Death; to Kingdoms, many Crofses; to all Eftates, inevitable Loffes! to Herds-men, Rot; to Plowmen, haples Seafons. To Saylors, Storms; to Cittyes, Civill Treafons."

The following year, bubonic plague struck down a fifth of London's population.

Old-timey fears seem quaint to us. But in 1910, when it was learned the Earth would pass through Halley's tail, many people were panicked by the announcement that comet tails could contain poisonous cyanogen gas. Some citizens went around in surgical masks. Some took comet pills that serendipitously appeared on the market.

As recently as 1979, no less an authority than astrophysicist Sir Fred Hoyle suggested that all sorts of viruses and germs may be sowed on Earth by comets and spread by weather patterns. In his book "Diseases From Space," he points to the 1918 influenza strain that hit New England and India within a few days and cites other peculiarly patchy outbreaks of everything from smallpox to cholera. And what about Legionnaires' disease? And why are people over 75 often immune to new flu strains? Could this be related to the 76-year cycle of Halley's? And do our nostrils point down, rather than up, to protect us from space infections? And so forth.

Hoyle and his collaborator, Dr. Nalin Wickramasinghe, says Calder, "violate no laws of nature, but they do seem to stretch improbabilities of theory and evidence like bubble gum." He Hitched His Wagon

Edmond Halley got excited about comets in 1682, when a spectacular one blazed over Britain. He had already devoted some thought to the subject, concluding that they may well have caused Noah's Flood.

Because they seem to fly in a straight line, comets had puzzled observers for centuries. Besides, Aristotle had encouraged generations to assume that comets were very close, like rainbows.

The comet of that year became Isaac Newton's experimental proof of his theory of gravity, and it was the notion that a comet might be riding an extremely elongated gravity-controlled orbit rather than a straight line that made Halley famous.

For Halley was the first to work out the comet's correct path on a 76-year orbit and identify it as the same one that had passed through in 1531 and 1607. To dramatize his achievement, he told the world that it would return in early 1759.

It did.

Unfortunately for Halley, he was by then 16 years in his grave. How to See Halley's Comet

Scientists, perhaps burned a little by the Kohoutek fiasco, have been warning that Halley's won't be as flamboyant this time as in 1910 -- it will go behind the sun rather than in front of it.

But now comes word that the core is vaporizing into a coma much sooner than expected, making it up to 100 times brighter than it was supposed to be at this point. It will shine its brightest in February 1986, when it closes on the sun.

By March and April, the best viewing period, the tail should be 25 degrees, or 50 moon diameters long.

From the eastern seaboard one should look low to the southeast in the predawn sky, and to the sunset area in the late evening.

"The lightness can vary," says David De Vorkin of the Smithsonian -- where, as you might expect, a planetarium show called "Comet Quest" has opened at the Air and Space Museum -- "because, for instance, there could be an explosion in the nucleus. I think November '85 is a good guess for the earliest naked-eye sightings." But Eddie to His Friends

Most people say Halley as in alley. A distinct minority says Halley as in daily. Almost nobody says Halley as in crawly. Keeping Track

Earthlings will be studying every move Halley's makes, of course. The Japanese will send up two spacecraft to measure its hydrogen halo, the European Space Agency's craft Giotto will meet the comet March 13, 1986, and the Soviet Union plans two space shots, which will be aided by American antennas, in their attempt to close in on the phenomenon March 6 and 9 of that year. The Soviets' offer to share their observations came as a pleasant surprise to the world scientific community a year ago.

And in December American scientists announced that they would have a cosmic dust analyzer aboard the Soviet Vega-1 and -2 spacecraft in the first Soviet-American joint space operation since 1975.

Though the United States will not be launching a special comet satellite, owing to budget cutbacks, it will watch Halley's progress closely via the Pioneer Venus orbiter, the International Sun-Earth Explorer and the space shuttle.

Three astronomers will board the shuttle in early 1986 with a sophisticated new telescope that is expected to take superb pictures of the head and tail. Three different ultraviolet telescopes also will be used. Will It Hit Us?

People always asked this before they knew that comets don't travel in straight lines. (Since a faraway moving object isn't seen to change in size as it approaches, the observer gets no help from depth perception.)

The answer is no. But that is not to say a comet couldn't hit us. Consider the Tunguska Event.

Riders on the Trans-Siberia Express and a handful of Russian peasants saw a gigantic blue fireball just before dawn on June 30, 1908. It smoked a great arc through the sky and disappeared to the north with a shuddering explosion.

Investigators found the site by the Tunguska River without any trouble: For thousands of square miles the forest was flattened, trees blown over like dominoes, dead animals scattered about. Along a 70-mile streak, the trees had been obliterated. On either side, for hundreds of miles, branches had been bent down and tree crowns burned away.

The first thought was a meteor. But there was no crater. No chunks of iron under the ground. Only a cluster of small holes and grains of a meteorlike substance almost invisibly small, a tenth of a millimeter in diameter.

The explosion was calculated at 12 megatons five miles above ground.

Over the years, theories have come and gone. It was a crash-landing space ship. No, it was some kind of natural atom bomb. No, it was a hunk (antihunk?) of antimatter. No, it was a black hole that had bored through the Earth and come out in Siberia.

Finally somebody suggested that it could have been part of the comet Encke, through whose tail we were passing in 1908. British scientists, whose idea this was, calculate that a snowball fragment of only 50,000 tons -- say, 40 yards across -- would cause such destruction. An ice projectile would nicely explain why there are no iron chunks on the site. (Dick Tracy veterans will recall a strip about a guy murdered with a dry-ice bullet.)

The odds are a billion to 1 against a comet hitting the Earth. But it can happen. There is speculation that those rings around some moon craters could be the marks where a comet went in, followed by its tail.

Heads up, people. What Color Is a Comet?

Pink. So they say. Scratch Marks in the Sky

It was astronomer Fred Whipple who a few years ago decided the comet must be like a dirty snowball, an "icy conglomerate" of ice and various kinds of dust. The varying composition apparently has something to do with why some comets split in two, some explode, some burn up in the sun. At least 600 have been spotted from our planet, most of them on fabulously long orbits that bring them to us once in 100,000 or a million years or so and then back out past Pluto.

Some 100 shorter-term comets are known. The spectacular Encke has a 3.3-year orbit, which means it won't be around long, losing so much debris and gas in the sun that it will die after about 1,000 trips. Halley's is an "intermediate," one of only seven known comets with an orbit between 50 and 200 years.

Comets come from a deep-freeze storage area on the outer fringes of the solar system called the Oort Cloud, where as many as 2 trillion nascent comets are milling around. It was from this cloud, named for a Dutch astronomer, that Halley's was pulled by Jupiter's gravity.

Wherever it came from, wherever it is going, Halley's Comet will be seen this time by more people than ever before. It will be, as they say, the experience of a lifetime.

Even that isn't enough for some people.

One far-sighted manufacturer is turning out T-shirts that say: "I'll See It in the Year 2061 Too!"