A second Soviet spacecraft rocketed through the tail of Halley's Comet today, bringing electronic cameras and investigative instruments 500 miles closer to its core.
Vega 2's power system was crippled as dust particles battered its solar panels and caused damage to some instruments, the Soviets reported, but the spacecraft still came within 5,000 miles of the nucleus and was able to send back electronic images to the Soviet Space Research Institute here. The objective had been to come 750 miles closer.
Dust caused similar damage to the Vega 1 spacecraft as it spun past Halley's Comet from a distance of about 5,500 miles Thursday. But Vega 1's power supply was stabilized after scientists here cranked down demands on the system.
Scientists gathered at the institute for today's transmissions said the pictures were not clear enough to form instant conclusions about the shape of the comet's nucleus.
Comets, composed of matter thought to date to the start of the solar system, have never been examined at close range before. Theories about comets' nuclei -- principally the "dirty snowball" theory -- have been widely accepted but have not been proved.
The Vega 1 probe effectively has confirmed the comet's solid center, but scientists still are debating how the nucleus is formed -- and even whether there are two. According to Soviet scientist and commentator Sergei Kapitsa, the nucleus may be shaped like a dumbbell, hence the suspicion of two nuclei.
Either a cosmic ray or a dust particle knocked out the microprocessor system monitoring the exposure for the camera system on board Vega 2, causing the images to be slightly overexposed, scientists said, but they were hopeful that the images could be recovered later electronically.
Combined with information collected from the Vega 1 probe, data from Vega 2 already has shed some light on the mysteries of the composition of a comet. The two craft are virtually identical, but they approached the comet's interior from different angles.
Roald Sagdeyev, head of the Space Research Institute and chief of the project, described the comet as a "cocoon" with "a solid body and soft dust blanket, a few hundred meters thick and completely opaque."
"Most people think it is one of irregular shape, but so far we don't see the details," Sagdeyev said.
The Vega mission is part of a multinational project involving space vehicles from Europe and Japan. A key task for the two Soviet vehicles is to provide information to the European Space Agency to plot a trajectory for the Giotto probe, which is to swing close by Halley's Comet on Thursday.
The Giotto craft is due to come within about 300 miles of the nucleus and hence will be most vulnerable to damage from fast-flying dust particles. Initial findings from Vega 1 indicate that the dust is not as heavy or as dense as some had feared, but scientists say there is still cause for concern for Giotto.
About 700 pictures of the comet were taken by cameras on board Vega II. Scientists have declared the missions a success, although photographs taken from the Giotto craft may turn out to be more illuminating.