Halley's Comet -- now the best known as well as the most famous of interplanetary wanderers -- is heading back toward the outer reaches of the solar system. It leaves behind an international cadre of scientists who are jubilant at the information they have gained. Close-in images and measurements taken by four spacecraft have confirmed the hypothesis that this comet -- and presumably most other comets -- is largely a dirty snowball.
The new data have also raised questions about the details of Halley's makeup. These include the question of why the comet's peanut-shaped nucleus is covered by a crust whose blackness is ``the darkest dark you can imagine,'' according to European Space Agency (ESA) scientist Horst Keller.
Now the international scientific team is settling down to many weeks of intense study to discover what the billions of bits of data they have gathered can tell them. It should establish a new benchmark against which they can test theories of the solar system's formation. Comets are thought to be unaltered samples of the primordial material from which the outer planets formed.
Equally significant, the scientists' work together is setting an encouraging international precedent. Burton I. Edelson, associate administrator for space science for the US National Aeronautics and Space Administration, calls the collaborative effort ``a triumph of international cooperation.''
Roald Z. Sagdeyev, who directs the Soviet Union's Halley exploration missions, is promoting the idea of an international unmanned mission to Mars as a follow-on project. He cautiously notes that, at this point, it is only a personal, not an official, suggestion. However, scientists from other countries have repeatedly praised the openness and cooperative spirit that have characterized Soviet participation in the Halley program.
Four spacecraft took part in the Halley encounter -- the Soviet Union's Vega 1 and Vega 2, Japan's Suisei, and ESA's Giotto. The two Vega craft flew within 5,500 and 5,125 miles of Halley's nucleus on March 6 and 9. Soviet experts quickly shared Vega 1 data, which helped Japanese controllers better target their own craft as it passed within 94,000 miles of the comet on March 8. Then, both Soviet and Japanese data enabled European controllers to refine their targeting. They narrowed the uncertainty in the relative position of spacecraft and comet from several hundred kilometers to a few tens of kilometers.
Giotto came within 335 miles of Halley's nucleus early on March 14 (European time). Finally, after centuries of speculation, scientists had a good look at a comet's nucleus. Dust bombarded the spacecraft at some 155,000 miles an hour. It eventually knocked out its camera and set the craft wobbling two seconds before its closest approach to the nucleus. This disrupted communication with earth for a little over half an hour. However, much highly useful information has been gathered. In the words of veteran comet scientist Fred Whipple of the Harvard-Smithsonian Center for Astrophysics, scientifically the project has been an ``absolute triumph.''
It's also a triumph for the ``dirty snowball'' theory, which Dr. Whipple developed more than three decades ago. The theory hypothesized that comet nuclei are comprised largely of ice and dust. Whipple notes that the surprising finding of a dark crust on Halley's nucleus is compatible with his theory.
Many scientists think comets formed in the outer solar system, perhaps in the range of the orbit of the planet Uranus. The recent inspection of Uranus by the US spacecraft Voyager found many particles in the planet's rings and many of its moons to be very dark.
These are also thought to be icy bodies covered with a black crust.
Halley's comet now is known to have a single nucleus about 9.3 miles long by at least 2.5 miles wide, according to early reports from the scientific team. Early Vega images suggested a double nucleus. But Dr. Sagdeyev later confirmed that only a single body could be seen.
The nucleus is surrounded by a vast cloud of dust and gases as expected. What was not expected was the presence of fist-sized rocks and boulders, which the spacecraft also found. The images sent back show jets of dust erupting from the nucleus and shooting out thousands of miles. In all, scientists found a highly active body at the core of Halley's Comet.
Meanwhile, the four spacecraft will go on circling the sun and, perhaps, send back more information on solar system conditions.
Giotto, which still has several working instruments, may eventually receive a new mission. ESA's Roger Bonnet said the craft will be brought to within 12,420 miles of earth by mid-1990. Then it could be given new research tasks.