"Deep Impact" Comet Spewed Tons of Water, Study Finds

Adrianne Appel
for National Geographic News
April 4, 2006
Scientists are stumped by the behavior of the comet Tempel 1 after NASA sent the Deep Impact space probe hurtling into it last year some 83 million miles (133.5 million kilometers) from Earth.

(See photos of the collision.)

The flare that resulted from the July 4 collision was originally measured by optical telescopes as being a weak, five-day-long burst.

This suggested that only a small amount of water and other material had been released after impact.

But an analysis of x-ray images taken after the collision indicates that the flare lasted for a full 12 days and released about 250,000 tons (226,800 metric tons) of water in the form of ice-covered dust grains.

"There was disappointment that Deep Impact didn't cause a major optical outburst, but it did cause an x-ray outburst. Why that is, we don't know,'' said Dick Willingale, an astronomer at the University of Leicester in England whose team operated NASA's Swift X-Ray Telescope.

Willingate's team is puzzled as to why the x-ray observations of the impact's aftermath do not match the observations made by optical telescopes.

"When everyone said it was over, we were [actually] at the peak. We were just taking the data,'' Willingale said. "We realized we had something strange happen, but we didn't understand why.

"The important thing is [the flare] lasted for 12 days,'' Willingale continued. "It shouldn't have done that. The water was coming off so fast, it should've disappeared after a day or so. We don't know why [the strong x-rays] persisted for so long.

The Swift telescope was very far from the comet, about as far as the Earth is from the sun. By the time the comet material made it into Swift's range, the material had traveled 6,200 miles (10,000 kilometers).

"The fact that it took a few days means this material is moving very slowly,'' said Paul O'Brien, a member of Willingate's team.

Comets are believed to be balls of ice, gas, and dust that date back to the formation of the planets in our solar system about 4.5 billion years ago.

Studying their composition and behavior may lend clues about how our planets formed.

The Deep Impact collision, which was observed by more than 70 telescopes operated by teams the world over, was designed to mimic a meteorite smashing into a comet.

So far, scientists have learned from the collision that Tempel 1 originated from the area now occupied by Uranus and Neptune, and that it is a very fragile ball of dust and ice.

Only 6 percent of Tempel's surface is ice, and the rest is dust.

Willingale will present the his team's findings today at the National Astronomy Meeting in Leicester.

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