The surface of Halley's comet is a surprisingly balmy 85 degrees even though the comet is largely made up of ice, according to data relayed to Earth by the first of two Soviet spacecraft to encounter the comet.
Vega 1 passed 5,518 miles from the comet late Wednesday night, sending back the first photographs ever taken of the nucleus of an active comet, and a second probe is due to pass even closer late tonight.
More than 100 scientists from around the world, who have been devouring data in Moscow from Vega 1, will now turn their attention to Vega 2, which has already sent back about 100 pictures as it speeds toward its rendezvous.
After Sunday, most of the scientists will move on to Darmstadt, West Germany, for the grand finale in what is the busiest week in the history of cometary science: the close encounter with Halley by a European spacecraft Thursday night.
The earliest photos from Vega 1 had indicated that Halley had a double nucleus. But Soviet scientists on Friday told reporters that what appeared in the photographs to be a second nucleus was more likely gas streaming from the comet.
"It's not two nuclei, I can give you my oath," said Vasily Moroz, head of the planetary physics department at the Soviet Space Research Institute.
Moroz told a Moscow press conference that "most probably the central part of the comet has a complex structure. The nucleus itself is of an irregular shape."
Data from the probe also indicates that the icy core of the nucleus is about three miles in diameter, surrounded by a deep layer of dust.
Although the primary constituent of comets is thought to be ice, Moroz said the surface of Halley is about 85 degrees, apparently because of solar radiation.
Soviet scientists also revealed that Vega 1 was damaged somewhat as it sped past Halley, raising new concerns about the fate of a European probe that is to pass within about 300 miles of the comet on Thursday.
The damage was caused mainly by dust that peppered the craft about 25 minutes after it made its closest approach to the comet. Vyacheslav Balebanov, deputy head of the Soviet space institute, said power from the craft's solar cells dropped 40%, apparently because of the impact of dust on their delicate light-collecting surfaces. Also, an instrument for measuring waves of charged particles was damaged.
Otherwise, the 15 experiments aboard the spacecraft were "all in excellent health," according to officials from the Jet Propulsion Laboratory in Pasadena who are in Moscow.
The damage to Vega 1 led to a friendly debate between the two Soviet scientists over whether the European space probe, Giotto, is likely to survive its encounter with Halley next week.
It Is a 'Kamikaze'
"In my opinion, it is a 'kamikaze,' " Balebanov said at a press conference. "The danger for it is very great."
Moroz disagreed, saying: "I take a more optimistic view. I think all will go well with Giotto."
While Giotto will pass 10 times closer to Halley than the Soviet probes, it has a double shield dust protector, and European officials have said that the odds that Giotto will survive the encounter are better than 90%.
European scientists can also take heart from the fact that although Vega 1 was damaged during its encounter, the dust around Halley was finer and sparser than had been expected.
The arrival of the international armada of five spacecraft comes just as Halley is beginning to look like a real comet during this generally disappointing visit to Earth.
"The comet has reached the point now where the ancients would have noticed it in the sky," said Steve Edberg of JPL.
"It's there," he added. "We've reached the point now where we have a comet that anybody would recognize."
What is needed, however, is a dark southern sky, about an hour before sunrise. The comet barely climbs above the horizon before sunlight begins to overwhelm it, so any kind of obstruction near the horizon will block the view, said Edberg, an official with the International Halley Watch.
For 79-year-old Fred Whipple, a distinguished American scientist who is known as "Mr. Comet" among his colleagues, the Moscow experience already has been sensational.
"I would say it is a huge success, huge," Whipple said.