Science Papers Describe Deep Impact Mission To Comet Tempel 1

NASA's Deep Impact mission, which slammed an 820-pound "impactor" spacecraft into the icy heart of comet Tempel 1 last July, found a fragile ball of dust and ice held together mostly by gravity. Until the Deep Impact experiment, scientists' ideas about the interiors of these objects came primarily from theoretical models. The 8 September issue of Science Express and the 9 September issue of the journal Science feature three reports and a news story covering the first peer-reviewed analyses resulting from the Deep Impact mission.

In a news conference Tuesday, mission scientists said that the dust and debris discharged from the comet when spacecraft exploded into its powdery surface contained unexpectedly high levels of organic chemicals. That provided significant evidence to back the theory that comets delivered many of the chemical building blocks to "seed" the start of life on Earth, they said.

In an overview paper that presents results collected by the instruments aboard the impactor and the flyby spacecraft that launched it, chief mission scientist Michael A'Hearn and his colleagues describe some of Tempel 1's most distinctive features. Overall, they say there is no reason to believe that Tempel 1 is not typical of the Jupiter-family comets, though its overall shape and surface features are quite different from the two other comets whose nuclei have been studied, Wild 2 and Borelly.

The authors report that the comet undergoes frequent outbursts of dust, probably triggered by sunlight. It's also dotted with round depressions that appear to be impact craters, which have not been seen before on comets. The comet's surface seems to heat up quickly in response to sunlight, suggesting that the heat doesn't penetrate very deeply into the comet's interior. The comet consists largely of extremely fine particles that seem to be very loosely bound together-more like a pile of powder than a solid rock. It also contains a relatively high concentration of organic compounds; these were more abundant during and after the outburst than water and carbon dioxide, the authors report.

This false-color image shows comet Tempel 1 about 50 minutes after Deep Impact's probe smashed into its surface. Image courtesy NASA/JPL-Caltech/UMD.

Scientists using at least 70 different telescopes around the world watched the Deep Impact collision and its aftermath. In a second paper, Karen Meech of the University of Hawaii and her coauthors called it the largest such astronomical campaign ever. It provided important information about the effects of the impact and earlier, natural cometary outbursts as well, they said. Observers reported that the material ejected from the comet after the impact had a different composition than the material that was bursting out in small, natural eruptions before the impact. The new activity, which suggested differences between the comet's interior and surface materials, only lasted a few days. Researchers on Earth also recorded early changes in the brightness of the comet, which they hope will provide information about the formation and evolution of the new crater created by the impact.

The European Space Agency's Rosetta spacecraft was on its way to another comet around the time of the Deep Impact collision, so it trained its cameras on Tempel 1 for 17 days, from a distance of roughly 80 million kilometers away. In a third paper, astronomer Horst Uwe Keller of the Max Planck Institute for Astronomy in Germany describes the data collected on the gas and dust components of the impact cloud. The fastest particles moved with speeds of around 400 meters per second, though larger particles moved more slowly. According to preliminary estimates, the total mass of dust that left the comet nucleus was at least as large as that of water.

Though it weighs perhaps a billion tons, the meteor is "like a sponge, with a lot of cavities," Keller, an astronomer at the Max-Planck Institute for Solar System Research in Germany. If you could touch it, he said, it would crumble in your hands. Though the Deep Impact spacecraft penetrated as much as 150 feet into Tempel 1, it never hit the solid core that many scientists expected to find.

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9 September 2005