Deep Impact Rewrites the Book on Comets

When the world’s foremost expert on impact cratering mentions that his preconceived notions about comets have changed at least twice in the past week, it is time to rewrite the book on the subject. Dr. H. Jay Melosh, professor and a member of the Deep Impact science team, spoke before a packed crowd Saturday night, July 09, 2005, during a presentation at the University of Arizona entitled “Deep Impact: The first look inside a comet”.

“We did it!” Melosh exclaimed to loud applause. The Impactor spacecraft successfully separated from the Flyby spacecraft and collided with Comet Tempel 1 late Sunday, July 03, 2005 (Pacific Daylight Time.) Flyby captured the impact using its High and Medium Resolution Imagers and returned these spectacular images, along with data from a suite of other instruments, to the Earth over the next few days.

Melosh provided early results from analysis of this data. The surface of the comet consists of fine particles, similar in consistency to talcum powder. This dusty material was thrown out into space by the impact, resulting in a brilliant veil that hid some of the details behind it, including the plume of material rising from the impact site. While many people are eager to hear the size of the resulting crater, Melosh said much more work will need to be done before that can be determined, partly because of the material that obscured the impact site. It is not clear if images were taken of the crater. Flyby passed over the site quickly before it had to turn its cameras away to protect them from stray particles. When it was out of danger Flyby turned back around to capture the comet silhouetted by the still-growing plume of material.

One of the last images Flyby took of Comet Tempel 1 continued to show material reflecting sunlight. A false-color image using different colors to depict brightness levels indicates the bright plume and sunward facing side of the comet (in white) and the side facing away from the sun (in black). The other colors show the extent of the plume of material that spread out into space.

While he was not willing to provide detailed information about the crater size, Melosh said “think big.” What he could reveal was that the impact went deep, suggesting that comets are structurally weak. Impactor itself vaporized almost immediately upon impact, resulting in a bright split-second flash. The energy it imparted to the comet surface continued to travel in a shockwave that evacuated the crater, resulting in a gigantic explosion of material into space and the disturbance of surface material as the crater began to grow in diameter.

Although some of this activity was obscured by the plume and fine surface material, instruments onboard Flyby monitored the event in electromagnetic wavelengths that can see through dust. Scientists are busily examining the data and hope to share more results over the next few weeks.

Melosh hinted that current models for comets are wrong, but would not say how, since much more analysis and peer-review is necessary before reaching any conclusions. Deep Impact provided the highest resolution images ever of a comet, revealing complex and mysterious surface features, including round depressions that resemble impact craters, smooth regions that may indicate surface resurfacing, crevasses and steep slopes. Early compositional analysis of spectra taken during the event indicates the presence of water, CO2, and CO ices on the comet, according to Melosh.

Flyby has been redirected back toward the Earth, in preparation for a possible mission extension. While the spacecraft survived the encounter with Tempel 1 apparently unscathed and with half of its fuel unused, Flyby will require the gravitational help of the planet Earth to send it to another destination. Scientists are considering a return to Tempel 1 to capture new images of the impact area, or an encounter with another comet, both not expected to take place for another five years.

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