Comet scientists work to pin down Halley's wobble and spin

Shaped like a peanut, blacker than tar, Halley's comet continues to intrigue astronomers as it heads for the outer part of our planetary system. In spite of close-up photos, which revealed one of the oddest-looking objects in the solar system, comet scientists still can't decide how fast Halley's nucleus spins. So they'll continue to study it from time to time for clues to its rotation.

The International Halley Watch (IHW), which marshalled a world-wide research corps of over 1,000 professional astronomers, plans to monitor Halley during the winter of 1988. The comet will be far enough from the Sun so that its activity will have died down. Astronomers will have a clearer look at it.

Then they may be able to decide whether it spins once in 2.2 days or once in 7.4 days. Both periods have their partisans. Some experts even suggest that the comet exhibits both motions. They envision a 7.4-day wobble superimposed on a 2.2-day spin, like a top that spins and wobbles at the same time.

Meanwhile, the IHW has begun archiving the masses of data gathered during Halley's visit. With only about 5 percent of those data turned in, it's already evident that the famous comet had some surprises for observers. There were sudden outbursts in what was expected to be a steady vaporization of its icy nucleus. Complex pulsations made lively variations in its brightness that researchers are trying to understand.

Then there's the intriguing view of the comet's nucleus. From Earth, we see the bright cloud of vaporized dust and gas that forms a coma many hundreds of thousands of kilometers wide and stretches out into a tail when a comet nears the Sun. For the first time ever, Soviet and European spacecraft have photographed the mass of dirty ice and gas that is the essence of a comet at the coma's center.

The European Space Agency (ESA) Giotto probe passed closest to the nucleus, coming within 600 kilometers (370 miles) last March. That was about a month after Halley had rounded the Sun.

They found an elongated object 14.9 km (9.3 mi.) long by 8.2 km (5.1 mi.) wide with an 87.8 square km (33.9 sq. mi.) surface area. A small mountain - 2 km (1.2 mi.) by 1 km (0.62 mi.) - protruded 400 to 500 meters (1,310 to 1,640 ft.) from one side. Partway around the object from there, a shallow depression, about 1.5 km (0.9 mi.) wide, dipped 100 to 200 meters (330 to 660 ft.) below the surrounding surface. There were many smaller craters.

Jets of gas and dust erupted from small areas. In fact, most, if not all, the escaping dust and gas seemed to come from such jets rather than diffusing from a wide zone.

Large amounts of amorphous carbon cover this nucleus. The action of cosmic rays, which impinge directly on the comet when it's far from the Sun, probably create this coating. Thus, at the heart of the brightest comets, there is likely to be one of the darkest objects in the solar system.

These and other preliminary findings were presented in late October and early November when researchers met first in Heidelberg, West Germany, for a special Halley symposium and then at an American Astronomical Society conference in Paris. More research is continuing to appear as teams publish their work. For example, papers in a recent issue of Nature show that erupting pockets of carbon dioxide may be at least partly responsible for some of Halley's explosive activity.

In spite of the scrutiny of six European, Japanese, Soviet, and United States spacecraft that monitored the comet and its interplanetary environment, Halley has not been an easy object to study. The uncertainty about its rotation is a case in point. Most of the information comes from watching variations in brightness as brighter or darker features rotate around. Some comes from a series of past and current photographs, including those taken by spacecraft.

Photos of successive positions of dust jets taken during Halley's 1910 visit indicate a 2.2-day spin. So do some of the Soviet Vega spacecraft close-up photos taken in March. Variations in intensity of light, including ultraviolet, from the comet's vast coma also have a 2.2-day beat. Yet a number of scientists studying these variations also find a 7.4-day rhythm. A great deal of subtle analysis will be needed to decide just how the comet rotates.

Scientists have a lot to do just to learn what the comet is made of as they sift through their data on the water, carbon compounds, and other material that came from its surface.

This has become the best observed comet in history. But the information gathered is only the beginning of the knowledge base scientists need to read the story of what they believe to be one of the most primitive bodies in the solar system - a story that may shed light on the very birth of that system.

Now they need detailed studies of other comets. And the Giotto spacecraft may help out. ESA controllers have put the craft on a trajectory that brings it within 22,000 km (13,670 mi.) of Earth on July 2, 1990, five years to the day after its launch.

Earth's gravity could then boost it onto a new course to intercept Comet Grigg-Skjellerup on July 17, 1992.

Giotto's camera may have been damaged during the Comet Halley flyby. ESA is considering whether to try turning it on for a full-scale test next September. If it's working well, then ESA is likely to give Giotto this new mission.

After getting that close to Halley's, scientists won't be satisfied merely to peer at comets through Earth-bound telescopes again.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.

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