Space welcome mat out for Halley's comet.

Of all the comets in the sky There's none like comet Halley, We see it with the naked eye,

And peri-odi-cally.

This bit of doggerel has been making the rounds of the astronomical community to promote a particular pronunciation. But whether you rhyme its name with ''jolly,'' ''bailey,'' or, as the poem does, with ''alley,'' the famous comet is again coming into view. And scientists of the International Halley Watch already are hard at work to make the most of its reappearance.

P/Halley, as professional observers call it, has been under surveillance since G. Edward Danielson of the California Institute of Technology and David Jewitt (now at the Massachusetts Institute of Technology) got the first glimpse of it in October 1982. Then it was a billion miles away and 50 million times too faint to be seen with the naked eye. CalTech's team had the advantage of the 200 -inch Hale Telescope on Mt. Palomar, near San Diego. Currently Halley is about 507 million miles from the sun, just beyond Jupiter's orbit. It is racing toward its Feb. 9, 1986, solar rendezvous at nearly 35,000 miles an hour.

Europe, Japan, and the Soviet Union are preparing a squadron of five spacecraft to intercept it in March of that year, just after it has swept around the sun. At this writing, two Soviet Vega craft were due to be launched Dec. 15 and 21 respectively. The remaining three are to be launched next year - the European Space Agency's Giotto on July 2, Japan's Planet-A on Aug. 14, and its companion MS-T5 on Jan. 5. ESA's craft is named after the Italian painter Giotto di Bondone. The image he used to represent the Star of Bethlehem in his Adoration of the Magi fresco in the Arena Chapel at Padua is thought to be that of Halley's comet.

The international research now planned ''should lead to quantum leaps'' in knowledge, says Prof. Michael F. A'Hearn of the University of Maryland, one of the foremost cometary scientists in the United States. Also, for the first time, a comet will be studied by close-up measurements and sampling of its dust and gases rather than by distant analysis.

If comets are indeed relics of the primordial solar nebula whose substance has remained unchanged for 5 billion years, as many experts think, sampling these cosmic fossils may shed light on the birth of the sun. In fact, the relative abundance of two forms of the element lithium - lithium-6 and lithium-7 - was set in the first few minutes of the ''Big Bang.'' Its value in primitive comet dust may reflect conditions at the formation of the universe itself.

One way or another, with Earth-based and space-based metering, P/Halley seems destined to become the most intensely studied of comets. Yet throughout history, it has always enjoyed that status.

There are 28 recorded appearances of Halley from 240 BC to AD 1910. Only the comet's reappearance in 164 BC seems not to have been recorded. Chinese records are the best up to the comet's 1456 appearance, when European data began to pick up. Early Chinese astronomers were meticulous observers. They knew that the tails of comets - or ''broom stars'' as they called them - point away from the sun centuries before Europeans were aware of this fact. A dynastic history book, ''The Treatise of the Chin-shu,'' records: ''The imperial astronomers explain that the body of a broom star has no light of its own but it receives its light from the sun. It points away from the sunlight.''

Old Chinese records were good enough to help Donald K. Yeomans of the National Aeronautics and Space Administration's Jet Propulsion Laboratory and Tao Kjang of Dunsink Observatory near Dublin, Ireland, reconstruct Halley's course through time. Calculating back from current data they traced orbits to the appearance of 837. But in April 11 of that year, Halley made its closest recorded approach to Earth - a mere 3.7 million miles, about 15 times the distance to the moon.

Good Western historical data on Halley began with Johannes Kepler's crude naked-eye observations on Sept. 28, 1607, and end with a precise telescopic observation May 24, 1911. Although sparse, comet data were good enough by the late 17th and early 18th centuries for English astronomer Edmund Halley to make his celebrated prediction. Having deduced that the comets of 1531, 1607, and 1682 were the same body, he announced: ''I may, therefore, with confidence, predict its return in the year 1758. If this prediction is fulfilled, there is no reason to doubt that the other comets will return.''

The prediction was not precise. Halley knew that the major planets would perturb the comet's orbit. Calculations by French experts put perihelion (closest approach to the sun) at April 15, 1779, give or take a month. The comet was first sighted by an amateur astronomer in Germany on Christmas night 1778. It passed perihelion on the 13th of March.

As John C. Brandt and Robert D. Chapman of NASA's Goddard Space Flight Center have noted, Halley's successful prediction had ''vast philosophical implications for astronomy in general and comets in particular.'' They explain in their ''Introduction to Comets'' (Cambridge University Press, 1981): ''Comets were shown to be subject to the laws of physics. Their orbits could be calculated and their return predicted years in advance. At least some comets were members of the solar system. Any rational fear of comets as signs of disaster or evil should have vanished.''

Comets are solar-system featherweights. Although their masses have not yet been determined directly, they are estimated to range from 10 million tons to 10 ,000 billion tons. Halley weighs in at an estimated 100 billion to 10,000 billion tons. Earth is a billion times more massive. Today's favored model for a comet is the ''dirty snowball'' or icy conglomerate, to use the technical term. According to this concept - developed by Fred L. Whipple of the Harvard-Smithsonian Center for Astrophysics in 1950-51 - a comet consists mainly of a mixture of ices and small dirt particles, with water ice predominating. The dirt particles include silicates, which are abundant in terrestrial rocks. Typically, this mixture would be packed into a nucleus ranging from a few hundred feet to a few miles in diameter.

Comet nuclei are far too small to be seen from Earth at the distances at which they usually pass. Scientists are uncertain whether they are loose aggregations or may sometimes be solid rock. Last year, the now inoperative Infrared Astronomical Satellite (IRAS) discovered an asteroidal object - 1983 TB - that seems to be the solid remains of a comet that has lost its ices. It follows the same orbit as do the Geminid meteor showers. Most meteor showers are associated with comets.

Whether comet nuclei tend to be spherical or irregular remains a question. Radar echoes from Comet IRAS-Araki-Alcock, which passed within 2.9 million miles of Earth on May 11, 1983, suggest a nucleus shaped ''more like a cardboard cake box than a uniform sphere,'' according to Irwin I. Shapiro, director of the Harvard-Smithsonian Center for Astrophysics. Also, pictures of Halley made last January by Drs. Danielson and Jewitt show intensity variations that suggest the comet has light/dark patches.

The pictures have a reddish hue. That's the color of sunshine at what was then the comet's distance of 760 million miles. So the observers conclude Halley's surface may be white like snow. This was the first such study to be made of Halley at a distance where the sun's heat had not yet vaporized its ices to form the typical comet-head coma and sunlight pressure had not yet swept its dust into a spectacular tail.

These features tend to develop at about the distance of Jupiter. Halley crosses Jupiter's orbit next month. So Danielson and Jewitt will again be watching it to see if vaporization has begun. Typically, released gases expand into a coma some 60,000 miles in radius. Hydrogen expands farther into a vast low-density halo. Halley's coma grew to a maximum of about 125,000 miles just after it had swung around the sun in April 1910.

The expanding gases drag along the grit - tiny particles, a few ten-millionths of an inch in diameter. Resembling fine terrestrial dust, they have so little mass that they yield to the pressure of solar radiation. In effect, they ''feel'' less solar gravity than do heavier particles or the nucleus. So they take up new orbits a little farther from the sun than that of the nucleus and spread out into a shimmering tail. Halley can be expected to grow a tail millions of miles long.

Meanwhile, water molecules and other constitutents of the gases undergo complex chemical changes and are split apart by the sun's radiation. Electrically charged, these atoms are swept along by the particles and magnetic fields expanding outward from the sun as the solar wind. Comet gases form long straight, narrow tails that may show sharp kinks and a variety of detail. The dust tails reflect sunshine. Gas tails glow with their own emitted light.

Comets that visit the solar system repeatedly have short periods and revolve relatively close around the sun. Halley has one of the largest orbits in this class. It ranges from aphelion (farthest point) beyond Neptune, some 3 billion miles out - a point it should reach again in 2024 - to perihelion 54,568,400 miles from the sun on this visit. Its period of about 76 years depends on the planets' gravitational effects. The shortest return time on record was 74.42 years (1835-1910). The longest was 79.25 (451-530).

But most of the comets - several billion of them - orbit tens of thousands of times farther out. They travel randomly in what has come to be called the Oort cloud. Thirty-five years ago, Jan H. Oort at Leiden Observatory in the Netherlands developed the currently accepted theory that a large spherical cloud of comets envelops the solar system. It extends 1,000 times farther out than the orbit of Pluto - half way to Alpha Centauri, the nearest star.

Most comets stay there, too far away for even the Palomar telescope to see them. Occasionally, some of them will be thrown into the inner solar system by a passing star. A close stellar pass could send in hundreds of comets instead of the present 10 to 30 a year. Encounters with Jupiter and other major planets quickly settle new arrivals into short-period orbits or else eject them from the solar system altogether.

Staying in the Oort cloud - far from the sun and planets and kept cold enough to inhibit internal change - comets may be the purest samples of the original solar nebula now available.

Actually, comet dust is all around us. It contributes to the interplanetary dust whose scattering of sunlight makes the faint glows called zodical light and gegenschein. It produces meteors as it drifts past Earth. It shows up as fluffy, loosely structured particles on the sample plates of high-flying research aircraft.

Scientists can't be sure they have truly sampled a comet, however, until they collect the material in place. That is what some of the spacecraft that are to intercept Halley are equipped to do.

The USSR's Vegas take their name from Venera-Galleya, the Russian term for Venus-Halley. They are to fly by Venus and release landing craft on their way to the comet. Vega 1 is scheduled to pass within about 6,000 miles of Halley's nucleus March 6, 1986. Vega 2 is to swoop by at the same distance three days later on March 9. The craft carry mass spectrometers to analyze Halley's dust and a French infrared (heat radiation) spectrometer to study dust and gas in the coma. Two TV cameras should return pictures.

The Vegas also will provide ''pathfinder'' data for Europe's Giotto. Using this to fine-tune their guidance, Giotto controllers at the European Space Operations Center at Darmstadt, West Germany, hope to come within several hundred miles of the nucleus. There, the craft's instruments can sample dust and gas as it evaporates off the nucleus. ESA scientists also hope to get color photographs of the nucleus with a resolution down to 50 meters (about 31 feet).

This could be a kamikaze mission. Dust particles hitting at 150,000 miles an hour could ruin the spacecraft. In the worst case, data probably would be lost during the latter half of the flyby. It's a risk scientists consider ''well worth taking, however, because of the marvelous opportunity for a close-up view, '' says cometologist Roger Knacke of the State Univrsity of New York in Sky & Telescope magazine.

Japan's Planet-A, with a solar-wind sensor and ultraviolet-light imaging system, is to get images of Halley's hydrogen halo about 10 days before and after perihelion. It is to pass by the comet at about 125,000 miles distance March 8. Planet-A's companion - MS-T5 - may not come closer than several million miles.

Not to be left out, the US plans to have the Pioneer craft now orbiting Venus swing around and scan the comet as it swoops by.

The prospects for viewing the comet from Earth's surface are only moderate. It will be low in the sky in the northern hemisphere and obscured by air pollution and city lights. It will be best seen in a clear, dark sky from the southern tropics. In compensation, the comet will come moderately close next November (58 million miles) and again in April 1986 (39 million miles). Halley will next appear in the summer of 2061.

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