Even now, some of the largest telescopes on Earth are peering deep into the constellation Canis Minor for the first physical sign that it's on its way.

So far, not even the 200-inch telescope on the top of California's Mount Palomar has seen anything, which comes as no great surprise to astronomers. Still halfway between the outer planets Uranus and Saturn, out about 1.3 billion miles from Earth and moving toward us at a speed of 20,000 miles an hour, Halley's Comet is no brighter than a star of the 25th magnitude. The most celebrated comet in history is not even a printout of light in the most powerful of earthy telescopes.

All that is soon to change. Early next year, Halley's Comet will be visible to telescopic observers for the first time on its current visit to our corner of the universe. By 1983, Halley will reach Saturn's orbit and in 1985 will have crossed Jupiter's orbit. Drawn ever faster by the gravitational pull of the sun, the comet will circle the sun on Feb. 9, 1986, the 29th time it will be seen to do so since Chinese astronomers first recorded it 240 years before Christ.

For the millions who witness it, the passage of Halley's Comet will be a spectacular sight. The comet will circle the sun at 140,000 miles an hour. Its head will grow to more than 100,000 miles in diameter, its tail will spread to more than 50 million miles in length. Lighted by the sun for four months on the way in and four months on the way out, Halley's appearance in 1986 will be seen by more people than any other celestal apparition in history.

Halley will be followed by hundreds of telescopes, thousands of rockets and balloons, at least three unmanned spacecraft and the astronauts on as many as four flights of the space shuttle. Thousands of amateur astronomers plan to photograph it.

As rewarding an experience as Halley-watching will be from any earthly vantage point, the reward will be greatest in the southern hemisphere, which will get the best view of the comet brilliantly lit by the sun in the night sky as it departs for the outer reaches of the solar system. Whatever your viewpoint, however, you won't marvel at the wondrous scene that Halley-watchers saw in 1910, when the comet displayed itself broadside to the Earth. This time the comet will pass behind the sun instead of in front of it.

This is one reason some people compare Halley's arrival in 1986 to Kohoutek in 1973, when a ballyhooed new comet never gave the performance it promised. So embarrassed were news editors many banned all mention of comets, thereby missing another new comet named West that split into four different comets before the very eyes of those who be happening to be watching it in 1976 in spite of the editors' edict.

"The people who beat the drum for Kohoutek didn't know comets very well," said Raymond Newburn of the Jet Propulsion Laboratory in Pasadena where he is mounting a global Halley watch. "Many new comets have things in them like carbon monoxide that vaporize far out from the sun and give us an appearance they don't sustain in close to the sun."

Even though it hasn't been seen since 1910, Halley is still the largest, brightest and best known of the comets that make predictable periodic visits to the reaches of Earth. Despite the Chinese sighting in 240 B.C., Halley's Comet got its name from 18th-century British astronomer Edmund Halley, who observed that the comets that appearerd in 1531, 1607 and 1682 behaved the same way and must have been the same comet returning at 76-year intervals.

"Halley predicted it would return again in 1758 and it did," said the JPL's Donald Yeomans, a Halley historian. "He was the first scientist to correctly figure that comets travel in predictable parabolic orbits around the sun."

Historically, Halley has a prominence few comets can claim. The comet appeared in 66 A.D. just before the destruction of Jerusalem and in 1066 at the Battle of Hastings, celebrated in cloth in the 271-foot-long Bayeux Tapestry.

In his Nativity scene, the 14th-century painter Giotto do Bondone depicted Halley as the Star of Bethlehem, though the comet's 12 B.C. visit misses by several years the 6 or 7 B.C. date that most New Testament scholars believe most probable for the birth of Jesus.

The Chinese began the practice of identifying comets with catastrophe, a custom soon adopted by others as well. The flood that Genesis reports drove Noah into the ark was blamed on a comet by some later commentators. Similarly attributed to noxious cometary influence were the plagues and famines of the Middle Ages and the great influenza epidemics of the 18th and 19th centuries. Even in 1910 people closed their doors and windows when Halley's Comet came lest the poisons stored in the comet's tail enter their homes. Summed up British author Nigel Calder: "Comets drive people dotty."

There is scientific evidence of at least one occasion when a comet actually may have caused havoc on Earth. In 1908, in what is still one of the 20th century's most puzzling natural events, the fall of the so-called Tungus "meteorite" destroyed 700 miles of Siberian forest. People for hundreds of miles around heard a noise like the rushing of a giant wind. When the wind had passed, a 70-mile strip of Khushma Forest has been stripped of its trees. On either side of the denuded strip, trees still stood but their crowns had been burned away and their branches bent down. Years later a team of Soviet scientists visited the forest to study the giant "metorite" that had wreaked such havoc. They found no signs of a meteorite. No craters, no iron and nickel fragments scattered in the soil. Only tiny, almost hollow spheres, a cosmic dust that could not be explained by an exploding meteorite.

"We were unable to discover traces of a ground explosion that would corresponsd to the energy of the 1908 event," the leader of the Soviet expedition wrote in 1960. "We found no traces of a collision with a meteorite."

Today scientists are convinced that the Tungus meteorite was a 100-meter-sized piece of the Comet Encke that broke loose from its parent and exploded in the atmosphere over Siberia.

"There's been a lot of silliness about anti-matter, black holes and alien spacecraft identified with the Tungus event," JPL's Yeomans said. "The beauty of the comet explanation is that it's such a fragile object that it would explode above the Earth and level all the trees but never leave a crater."

Comets are as old as the observable universe, but less has been known about these heavenly ghosts than anything else in the skies. The ancient Chinese made excellent observations of comets but put little thought into what they were and where they came from. The early Europeans gave cometary origins lots of thought but seldom backed it up with observation. g

"Part of the problem was Aristotle," says JPL's Yeomans. "Aristotle said comets were terrestrial phenomena, like rainbows. Nobody keeps records of rainbows."

All began to change in the 16th century when the Danish astronomer Tycho Brache proved that comets were not like rainbows and were, in fact, above the moon. Isaac Newton took cometology a giant step forward when he showed that comets didn't follow straight lines but parabolic paths with the sun at one focus. Then along came Halley with his prediction that the comet now bearing his name would circle the sun every 76 years.

When Halley's Comet made its last appearance in 1910, astronomers the world over observed it but failed to coordinate their efforts. Fewer than 20 percent of the 1910 observations were published, meaning the records of many were lost forever.

Larger telescopes and the space age brought new life to the study of comets.

It's only in the last 20 years that comets were found to be made of a cosmic mix of exotic duts, ice and snow. Knowledge of cometary ingredients makes it clearer why comets burn out from time to time and even disappear. Some comets have been seen to explode and others to lose so much of their mass circling close to the sun that they vanished.

In all, more than 600 have been observed inside the solar system, most of them one-timers that appear as infrequently as every 100,000 or even 1 million years and then drift far beyond the orbit of Pluto. There are about 100 "short period" comets known, with appearances as frequent as the comet Encke 3.3-year cycle. All began with longer periods, but then were pulled out of their original orbits by the giant planet Jupiter.

"Once a comet gets into a short-period orbit, it doesn't last too long," JPL's Raymond Newburn says. "Every time it comes around the sun, a short-period comet loses enough dust, gas and debris that after maybe 1,000 trips it's all used up."

Newburn says the solar system must pull in a new comet every century to keep its comet inventory intact. All comets come from the so-called Oort Cloud, an astral swirl of as many as 2 trillion comets 40 times as far from the sun as Pluto. The cloud is named for Jan Oort, the Dutch astronomer who discoverd it.

"We don't know if comets formed in the cloud or formed in the solar system and were kicked out to the cloud by the giant planets," JPL's Yeomans says. a"But if you look at the orbits of all the observable comets, most of them appear to come from a single distance in space which is the region of this Oort Cloud."

Halley's Comet doesn't fit the mold of most comets, being one of just seven "intermediate" comets whose orbits bring them back around the sun from the deep freeze of space every 50 to 200 years. Back before Christ, Halley was pulled from the Oort Cloud by Jupiter into the orbit that now takes it on a 76-year journey between the sun and some remote region between Neptune and Pluto.

When Halley reappears in 1986, it will be greeted by spacecraft from Japan, the European Space Agency, the Soviet Union -- and maybe the United States. Twice in the last five years, the National Aeronautics and Space Administration was given plans to intercept Halley's Comet. Twice, they were turned down, once by NASA for being too expensive and the second time by former President Carter for the same reason.

"We have a third plan that's ready that would do the job for $250 million," Dr. Bruce Murray, director of the Jet Propulsion Laboratory, said not long ago in an interview. "We're running out of time but this mission is still an option for the United States."

The way Murray's minions have devised it, a spacecraft the size of the 4,000-pound Voyager that flew by Jupiter and Saturn would be launced from the space shuttle in July 1985 on a curving path opposite to Halley's that would intercept the comet during its closest approach to the sun.

On the way to the meeting, the spacecraft's superb camera would continually photograph the comet, tracking it through space as it nears the sun and begins to boil and bubble. Less than five miles across, the core of Halley's Comet is believed to be made of some porous stone packed with a primordial snow that throws off geysers, fumaroles, sprays and upside-down snowstorms when warmed by the sun.

"We'll witness the head of the comet as it changes, we'll walk up and down the tail as it spreads out, we'll get the whole show for more than two months," says Halley project manager Kenneth Atkins. "'Star Wars couldn't do it better."

Just as important, the camera would track the comet as it appears through space like a balloon breaking air. With almost no gravity of its own, the tiny comet will dart here and there as it vents gas in almost every direction. The U.S. spacecraft would be the only one of the four to carry a navigating camera good enough to follow the comet's movements with the precision needed to intercept them.

Flying through the enormous and brilliantly lit coma, the cloud that surrounds the comet's nucleus, the spacecraft would sample the dust and gas streaming off the nucleus. Comets swarm with exotic dust particles -- complex molecules of sodium, carbon, oxygen, nitrogen and hydrogen that have been part of them since the dawn of time. Not even moon rocks exemplify the primordial matter of the solar system in as pristine a form as the contents of a comet's core.

Now less than 2,000 miles from the core of the comet and moving alongside and in front of it at 125,000 miles an hour, the spacecraft would jettison a topside mirror that served earlier as the camera's eye (the camera is behind a shield looking up at the mirror.) By now the reflective surface would be so covered with cometary dust that it would no longer function. The spacecraft would then roll to catch the reflection of the comet's sunlit side in a second mirror shielded from the dust.

As the comet appeared in full reflection, the spacecraft would rotate slowly to keep its image in view of the shielded mirror throughout its closest approach. If the comet's snow-and-dust storm didn't cover it with debris, the spacecraft would pass within 500 miles of front of the comet. After the passing, the spacecraft could then look back to photograph the full sunlit view of a comet whose origins go back farther in time than anything else man has seen up close.

Will it work? Only time will tell. Is it worth it? That will be up to President Reagan and his budget advisers.

"I venture to think that Halley would be in favor of such a mission," British astronomer Raymond A. Lyttleton said not long ago at a Halley celebration, "and I for one will express the hope that when 1986 comes and brings Halley back, the space fleets of all nations will be out to welcome it."