Halley's comet appears to be a relatively solid ball of ice, about three to four miles across, of elongated shape, and perhaps covered with a ``cocoon'' of dust. That is the initial conclusion of a team of international scientists studying data from two space probes that intercepted Halley's comet as it streaked through space on its way back from a pass around the sun. The probes, launched by the Soviet Union in December 1984, carried scientific equipment from a number of countries, including the United States.
Scientists said the cooperative effort generated a wealth of data that will greatly enhance man's understanding of comets, of other cosmic forces, and -- eventually -- of the very origins of the solar system. More immediately, the data will aid other scientific efforts to study Halley's comet as it passes near the earth and then heads back into deep space, not to be seen again until 2061.
The information from the craft -- including the first-ever television images of Halley's -- already has bolstered the view that the comet has a core of ice, gas, and primordial debris left over from the creation of the solar system some 4.6 billion years ago.
Yet, Halley's refused to yield many of its secrets, spewing out microscopic dust particles that disabled sensors on two probes, causing five experiments to fail.
The second of two Vega probes came within 5,000 miles of the comet at 10:20 a.m. Moscow time yesterday. Like the first probe, which whizzed past Halley's last Thursday, Vega II was heavily damaged by the dust.
The damage sustained by the Soviet craft raised immediate concern about the fate of the European Space Agency's Giotto craft, which will pass even closer to Halley's this Thursday. But Roger-Maurice Bonnet, ESA director of scientific programs, said he was confident that the Giotto craft could withstand the dust thrown off by the comet long enough to get close to its core. He was among dozens of scientists gathered at the Soviet space research center to study the Vega data.
``It was worth a trip all the way to Moscow,'' exulted John Brandt, an astronomer from the Goddard Space Flight Center in Maryland. ``That's fantastic,'' he said, staring at black-and-white television images reaching earth some 8 minutes after Vega beamed them downward. ``That's just flat gorgeous.''
The first flyby of Halley's last Thursday, he explained, had strongly suggested that the comet had a solid nucleus. But there was still a ``residual doubt,'' he added. The latest images ``ought to put any doubt about a monolithic nucleus away forever,'' he said. ``It almost looks like a snowball, doesn't it?'' he remarked.
The words were carefully chosen. For Dr. Brandt and others, the images were vindication of a theory that Halley's is a sort of ``dirty snowball'' hurled through space at the instant of the solar system's creation. That notion was first put forward in 1950 by Fred Whipple, director emeritus of the Smithsonian Astrophysical Observatory in Cambridge, Mass. All of the data, he said, ``was in the range of expectation'' of his theory.
But the Vega probes have done more than that, carrying with them scientific equipment that recorded a wide variety of data about the composition of the comet and the forces at play in, upon, and out from it.
Sensors on the probes, for example, detected dust particles spewed out from Halley's as far as 137,000 miles away, according to John Simpson of the University of Chicago. Analysis of this dust was streaming back to Earth at the rate of 24 ``words'' (eight bits of digital information) a second, he said. The data should help scientists understand more of the composition of the comet and, presumably, the solar system itself.
Still other experiments studied the interaction between the plasma that surrounds the comet and the ``solar wind'' of particles streaming out from it.
Asked about the biggest surprise the Vega probes had produced, the ESA's Dr. Bonnet said it was ``the professionalism of the Soviets -- the high quality of the data they've gotten.''
Indeed, there was repeated praise for the proficiency and openness of the Soviet officials from the international team of scientists gathered here.
``They should be congratulated on a marvelous experiment,'' said Dr. Whipple.