Here comes Halley's. Scientists and laymen alike have golden opportunity to `marvel at the star'

ISTI mirant stella. They marvel at the star. So reads the legend on the Bayeux Tapestry, which depicts the Norman conquest in the 11th century, for the scene where Halley's comet flashes by.

It could be a motto for our own time.

Around the world, more than 900 professional and 700 amateur astronomers and more than 100 observatories are engaged in the cooperative research program of the International Halley Watch. A fleet of five spacecraft from Japan, the Soviet Union, and the European Space Agency (ESA) are speeding to intercept the famous visitor next March. American astronauts are preparing to take an entire observatory of ultraviolet-light telescopes into space to study the comet next year.

It is the biggest coordinated international astronomical effort ever mounted. The professionals in this program have been watching the comet since it was first recovered -- first spotted since its 1910 visit -- on Oct. 16, 1982 by G. Edward Danielson of the California Institute of Technology and David Jewitt, who is currently at the Massachusetts Institute of Technology. Within range of binoculars

Now Halley has come within the range of binoculars and sharp-eyed sky watchers, so the rest of us can begin to marvel at it, too.

Halley's comet has a special appeal for professional observers, however. ``It is the brightest comet with a predictable orbit. What that means to us is that we can plan months or years in advance to observe the comet,'' says Arizona State University astronomer Susan Wyckoff.

She also explains that the scientific ``game'' of comet observing is aimed at ``understanding the physics and the chemistry of what's going on in the atmosphere [the coma] of this comet. . . . From the atmosphere, we infer what that [comet's] nucleus is made up of and what its structure is.'' A better understanding of comet structure and chemistry is expected to yield clues to our star system's formation.

Dr. Wyckoff led a team that was the first to see the comet begin to evolve its coma last February. Observations of that development and of the way Halley has brightened as it has approached, show that its nucleus is composed largely of water ice -- a discovery Wyckoff calls the most significant finding of the Halley watch so far.

Now indications of other substances, such as carbon dioxide and ammonia, are becoming evident. All told, the data so far show Halley to conform to the current theoretical concept of a comet as being made up of water ice, dry ice, and some other frozen gases, plus a scattering of metals and cosmic dust. In a phrase, it is a dirty snowball. It's a relic of the birth of the solar system.

Fred Whipple of the Harvard/Smithsonian Center for Astrophysics, who first proposed the dirty-snowball concept in the early 1950s, is looking forward to the spacecraft views of Halley. ``I want to see a picture of Halley's comet's nucleus. . . . I've been talking about that for 35 years, and I don't know what one looks like. It may be a highly irregular thing. It may be four miles in diameter. It may be eight miles in diameter. It may have great valleys on it. It may have great peaks. It may be an irreg ular body. It may be colored, spotted, mottled. But I haven't any idea what it's going to look like. And to me that's extremely important,'' he says.

At this point, the dimensions of Halley's nucleus are more uncertain than its composition. Scientists observed variations in the core's brightness when the comet was out near Saturn's orbit, before its coma formed to obscure the center. These variations suggest an irregular shape. The ratio of the longest to the shortest dimensions may be something like 2 to 1. Sizing up the comet

The typical size of those dimensions is uncertain by at least a factor of two. No earthly telescope has been able to show a sharp enough image for accurate measurement. So astronomers use an indirect method to estimate size. Assuming the nucleus is a sphere, they use a mathematical formula that relates the sphere's radius to the comet's observed brightness, the percentage of sunlight it reflects, and the angle between the directions from the comet to Earth and from the comet to the sun.

Peter A. Wehinger of Arizona State University says Halley's dirt-covered nucleus may reflect 5, 10, or perhaps 15 percent of the sunlight that hits it. Astronomers have to guess at the value. This puts similar uncertainty into radius estimates based on measurements of Halley's brightness. So, Dr. Wehinger says, the best current estimates are ``in terms of a ballpark figure, something between 2.5 and 5.0 kilometers,'' or about 1.5 to 3 miles.

As Halley comes closer, ground-based radar, which can penetrate the coma, may yield better size estimates. But the close-in pictures from spacecraft are the best means for reducing the uncertainty. That's why Whipple puts so much stress on them.

The Halley space fleet has a variety of equipment for studying the comet. Two Soviet Vega craft have instruments to measure cometary dust and gas, as well as TV cameras and other imaging instruments.

Vega 1 is scheduled to pass within 10,000 km. of the nucleus on March 6. Vega 2 follows on March 9 with a targeting distance that will depend on the Vega 1 results. The TV cameras have a best resolution of 200 meters (about 650 feet) at the 10,000 km. (6,000-mile) fly-by distance.

The Soviet data also will be relayed immediately to ESA to help in final guidance of its Giotto craft. The probe is named for Italian painter Giotto di Bondone, who depicted what may be Halley's comet as the Star of Bethlehem is his Adoration of the Magi fresco in the Arena Chapel at Padua, Italy. The heavily instrumented craft is to pass within 500 km. of the nucleus on March 13. One TV camera should be able to resolve features as small as 11 m. at that distance, while a color camera should have a reso lution of 50 m.

Meanwhile, Japan's two probes -- Suisei (comet) and Sakigake (pioneer) -- will have already encountered Halley. Suisei, the main probe, is due to pass within about 200,000 km. of the nucleus on March 8. Its main purpose is to take ultraviolet pictures of Halley's coma about 10 days before and after the comet's closest approach to the sun (perihelion) on Feb. 9.

Sakigake will pass within a few million kilometers of Halley and Suisei on March 11 to complement the latter's measurements by taking data on the electrically charged particles flowing from the sun, known as the solar wind.

All of the close-in probes will be in Halley's vicinity for only about four hours. Yet they will take millions of measurements as they whiz by at between 70 and 80 kilometers a second.

Meanwhile, scientist/astronauts on board the space shuttle in January and March will support the Halley probes by gathering additional data using a variety of instruments. Other US spacecraft involved

Other US space observations will be made by the Pioneer-Venus craft now orbiting Venus, by the Solar Maximum Mission satellite, which was rebuilt by ca zcometashuttle astronauts, and by the International Comet Explorer (ICE) craft that zipped through the tail of comet Giacobini-Zinner last September. ICE will take solar-wind measurements upstream from Halley.

With all that space-based observing power scrutinizing the comet, you might wonder what's left for earthbound astronomers to do. There is a great deal to do, according to Michael J. S. Belton of the National Optical Astronomy Observatories. In fact, he says, the spacecraft can't do their jobs properly without earthly help.

Right now, the comet's calculated course is accurate only to within about 2,000 km. Mission controllers need to predict the position of the comet better than that to target their space probes satisfactorily. So, Dr. Belton notes, observatories around the world are now working to refine their knowledge of the comet's orbit until, by next year, they should have it pinned down to within a few hundred kilometers.

Benton also points out that the probes are small craft whose instruments sample only a small region of space over small distances near the comet. From the ground, he says, ``we maybe can't see very clearly. But we can see big things. So we provide great complementary information.''

Looking ahead to the payoff from the Halley Watch effort, Belton says: ``. . . Cometary physics will be . . . on a firm foundation for the future. There will be a new page that will start to be written in our understanding of the early processes of the solar system after all this effort on Halley is finished and understood.'' Comet Halley's Timetable and distance from the sun INBOUND JOURNEY: Jan. 10, 1985: Mean distance of Jupiter. Nov. 28, 1985: Distance of Mars. Jan. 1, 1986: Distance of Earth. Jan. 21, 1986: Distance of Venus. OUTBOUND JOURNEY: Feb. 28, 1986: Mean distance of Venus. Mar. 20, 1986: Distance of Earth. Apr. 24, 1986: Distance of Mars. Mar. 12, 1987: Distance of Jupiter. CLOSEST APPROACHES TO EARTH: Nov. 27, 1985: 93 million kilometers (58 million miles). Apr. 11, 1986: 63 million kilometers (39 million miles). CLOSEST APPROACH TO THE SUN: Feb. 9, 1986: 87.8 million km (54.6 million miles). Speed: 54.55 km/sec (122,000 mph). FARTHEST DISTANCE FROM SUN (USING PREVIOUS POSITION DATA). Reached in 2024: 5.27 billion km (3.28 billion miles). Speed: 0.91 km/sec (2,000 mph).