(2 of 2)
Early evaluation of data from Vega 1 showed that the craft encountered less dust than expected as it approached the comet. But Physicist John Simpson of the University of Chicago, who designed the only American instrument -- a dust detector -- aboard Vega, noted that as the spacecraft departed, it passed through a "huge spike of dust" with particles about the size of those in cigarette smoke. Simpson and other scientists interpreted the spike as a burst of dust and gas erupting from the surface of the nucleus. Other Vega instruments seemed to show that the icy cometary surface was being evaporated by solar radiation at a rate two to three times as fast as scientists had predicted.
Vega 1 also served as a pathfinder for two other craft in the Halley's flotilla. By helping scientists determine the comet's precise path, it enabled them to make accurate last-minute corrections in the courses of Vega 2 and Giotto. The second Vega was to pass within 5,000 miles of the comet on March 9, supplementing Vega 1's findings. Giotto's mission four days later was to swoop to about 300 miles of the nucleus, shooting close-up pictures as it passed. Precision pathfinding was less important for the Japanese craft. Suisei, designed to study the huge hydrogen gas cloud surrounding Halley's, was targeted to fly by the comet at a distance of almost 100,000 miles. Sakigake, studying the solar wind so that scientists can determine the wind's effect on the comet's tail, would not come closer than 4.4 million miles from the nucleus.
At week's end scientists were worried about Giotto's chances. As they continued to interpret Vega 1's data, they discovered that its passage through the dust jet had damaged 45% of the craft's solar panels. During Giotto's much closer encounter with the comet, the European probe was bound to pass through far thicker clouds of dust on what some scientists characterized as a "kamikaze mission."
FOOTNOTE: *The cameraless U.S. spacecraft ICE made the first close approach to a comet last September, when it passed only 5,000 miles behind the nucleus of Comet Giacobini-Zinner.