Eyeing the Impact

By Elise Kleeman|Tuesday, July 19, 2005

All eyes were on the sky early in the morning of July 4, as NASA guided Deep Impact’s 820-pound probe into a 6.3-miles-per second collision with comet Temple I—the first direct encounter with a comet. The probe and its mother ship captured some breathtaking images of the comet’s nucleus and the resulting explosion, but they were not the only ones collecting data that night. More than 60 observatories watched from Earth, along with several more from space. “Here’s an event that everybody knew exactly to the second was coming, and all these impressive instruments were able to train themselves on it,” said Gary Melnick, senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

What some of the groups witnessed:

The deep impact mother ship. Not surprisingly, the most dramatic views came from the Deep Impact mother ship, which came as close as 310 miles to the comet. It had less than 14 minutes to gather information before passing around the far side of the comet. Still, in that short time the ship took more than 4,500 images. They revealed that the impact ejected a cloud of very fine dust, closer in consistency to talcum powder than beach sand. Scientists are still trying to determine the diameter of the crater that was formed, but they believe it is on the high end of their original estimated range of 150 to 800 feet.

United Kingdom Infrared Telescope, Hawaii. The first large research observatory to detect the success of the mission, this telescope measured the effects of the encounter even before the confirmation signal from Deep Impact reached mission control at the Jet Propulsion Laboratory in Pasadena. The flare from the explosion was detected with the observatory’s visible-light camera, which measured the comet’s brightness directly, so astronomers did not have to wait for any data to be processed. 

Hubble Space Telescope. The Hubble captured six photographs of comet Temple I in the 24 hours after the collision. Although the comet’s potato-shaped nucleus, a mere 8.7 miles wide by 2.5 miles long, was too small for the Hubble to resolve, the glow created by the impact—10 times brighter than Temple I was before the explosion—was clearly seen. The series of pictures shows a fan of debris expanding from the impact site at about 450 miles per hour.

NASA’s Swift Satellite.  Swift measured the X-rays emitted when debris expanding outward from the comet was exposed to solar wind. In the week after the collision, the intensity of the X-rays continued to climb as dust migrated outwards. From preliminary data, researchers estimated that several tens of thousands of tons of material were ejected—enough to bury Penn State’s football field in 30 feet of dust, Swift researchers at Penn State were quick to note.

Submillimeter Wave Astronomy Satellite. SWAS had been hibernating for almost a year when researchers brought it out of retirement to study the Deep Impact event. Early data show that comet Temple I released only about 550 pounds of water per second after the impact—a rate very similar to what it was emitting before. This means the probe did not disturb the ice that many researchers thought existed a few feet below the comet’s surface. The result challenges the traditional idea that comets are made mostly of ice.

“It may still be a ‘dirty ice ball,’” says SWAS lead researcher Melnick, but to find out he will have to wait for other observatories to calculate exactly how much dust was ejected. “Then we’ll be able to put the dearth of water generated by that impact into better perspective,” he says.  “Probably most of what can be attributed to the impact has taken place—all the pieces of the puzzle have been dumped on the table. Now we’re witnessing what science does: turns the pieces over, shifts them around on the table, and fits them together.”

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