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Giotto's close visit to another comet.

Mission scientists are just beginning to analyze the wealth of data gathered when the European Space Agency's Giotto spacecraft flew past a comet called Grigg-Skjellerup on July 10. But information from two of the craft's instruments already indicates that the flyby established a new record: The craft apparently flew within 200 kilometers of the comet's nucleus, the closest visit yet to the core of a comet.

This surpasses the previous cometary benchmark, made when Giotto came within 600 kilometers of Comet Halley's nucleus during a much-publicized encounter six years ago (SN: 5/24/86, p.327). Unlike Halley, which passes near the sun once every 76 years, Grigg-Skjellerup visits the inner solar system every five years. Researchers speculate that such an orbit enables the sun to heat the icy body at a more constant rate, making it less likely that the comet would explosively eject as much gas and dust as Halley during its closest approach.

Giotto instruments did indeed indicate that Grigg-Skjellerup was losing far less dust and gas than Halley -- no more than 100 kilograms per second, compared with Halley's 30 tons per second. But Grigg-Skjellerup's loss rate is about twice what researchers had estimated, reports Giotto project scientist Gerhard Schwehm of Noordwijk, the Netherlands. The finding suggests that the comet may have somewhat higher activity than believed, he adds.

Giotto couldn't take pictures of Grigg-Skjellerup, because dust encountered during the Halley flyby damaged the craft's camera and two of its other 10 detectors. Scientists thus relied on other means to infer the amount of material ejected by the comet. For example, two plasma detectors found evidence of a bow shock -- a region where ions from the solar wind run into the heavier, slower-moving ions from the comet- as far away as 18,000 kilometers from the comet's nucleus. Researchers had mistakenly estimated that the comet's relatively puny ion emissions couldn't create a bow shock more than 6,000 kilometers from the nucleus.
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Title Annotation:European Space Agency comet flew within 200 kilometers of Grigg-Skjellerup
Publication:Science News
Article Type:Brief Article
Date:Aug 1, 1992
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