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Deep Impact: Early Results

"Although the collision of the comet and the impactor was not spectacular from the public's point of view, it is extremely interesting scientifically," stated IfA astronomer and Deep Impact Science Team member Karen Meech. In a talk to visiting science teachers and IfA staff at IfA Manoa on July 19, Meech discussed the early scientific results from the July 3 impact with Comet Tempel 1.

She reviewed the goals of the mission: to investigate the interior of a comet, which holds primitive debris left over from the first 100 million years of our solar system's existence, and to learn more about crater formation.

The early results show that most of the material ejected by the comet was a very fine and very opaque dust. So far, the spectra taken after impact have revealed water, carbon dioxide, carbon monoxide, and a variety of hydrocarbons. Scientists are intrigued that materials detected in a pre-impact outburst have not yet been found in the post-impact data.

Five minutes before impact.
Sixty-seven seconds after impact.

The opacity of the ejected material has made it difficult for scientists to see the crater, but at this point, it appears that the crater created by the impactor was on the high end of predictions, that is, about the size of a football field. Scientists had several theories about crater formation, with the most likely being that it is controlled by gravity. Early results seem to confirm this.

Deep Impact, NASA's eighth Discovery mission, represented a worldwide collaboration of major proportions, making Comet Tempel 1 the most intensively studied comet in history. In addition to the science team members, led by Principal Investigator Michael A'Hearn (University of Maryland), there were 130 registered observers at 70 major observatories who were able to communicate with each other in real-time during the impact event. The timing of the impact was optimized for observing from Hawaii, and Meech was the member of the Deep Impact team who coordinated all of the Earth-based observations. When asked who observed the comet, she replied, "Everybody on Earth with a large telescope. I am even getting data from people who were not registered observers."

In addition to the ground-based observations, many space observatories studied Tempel 1 at various wavelengths. They included the Hubble Space Telescope (optical), the Spitzer Space Telescope (infrared), the Chandra X-ray Observatory, XMM (also X-rays), and the Swift Gamma-Ray Burst Explorer (optical light, ultraviolet, X-rays, and gamma rays). Two radio observatories in space, the Submillimeter Wave Astronomy Satellite and Odin, looked for water in the comet. Even the Rosetta spacecraft, which is on a 10-year mission to land on Comet 67 P/Churyumov-Gerasimenko in 2014, observed Comet Tempel 1 between late June and mid-July from its position beyond Tempel 1's orbit.

Comet Tempel 1 90 seconds before impact. The image was taken by the targeting sensor on Deep Impact's impactor.

A camera on the impactor itself took some great close-up images as it approached Comet Tempel 1. The final image before impact taken from a distance of 19 miles from the surface shows features less than 13 feet across. The bright flash resulting from the vaporization of the impactor when it collided with the comet was not reported as an obvious sudden increase as seen from Earth, but the two cameras on the flyby portion of the Deep Impact spacecraft did record the event. After impact, the comet's brightness increased steadily for about an hour before leveling off. It maintained that level of brightness for about a day, but within two to three days after impact, its brightness returned to pre-impact levels. Scientists had hoped that the impact would create a long-lasting jet of material from deep inside the comet, but that did not happen.

In early August, the Deep Impact Science Team held its initial post-impact meeting in Hilo to analyze their data and discuss what they have learned. Many will present papers at the Asteroid, Comets, and Meteors meeting in Rio de Janeiro in August and at the annual meeting of the Division for Planetary Science of the American Astronomical Society in Cambridge, England, in September. The journal Science will feature Deep Impact in a special issue this fall. More detailed papers will appear in astronomy journals. We will update you on further scientific finds from this mission in later issues of this newsletter and in public talks.

What will happen to the Deep Impact spacecraft now that is has completed its primary mission? The science team has hopes of sending it to study another comet. NASA, citing budgetary constraints, had initially rejected this proposal. But the success of Deep Impact and the intense public interest in it has persuaded NASA to allow the Deep Impact team to make a formal proposal for an extended mission.