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NASA spacecraft unravels comet mystery
Stardust's flyby of Tempel 1 shows signs of erosion on the comet's surface, as well as the first clear pictures of the crater made by the Deep Impact probe.

But the encounter was not easy for the US space agency's Stardust-NExT mission spacecraft, which had to fight an onslaught of debris from the comet in order to snap dozens of revealing pictures.

"Comets, unlike any other body in the solar system, are unique when they are in the inner part of the solar system where the Earth is," says Don Brownlee, Stardust-NExT co-investigator.

"They are literally coming apart and sending tons and tons of gas and rocks and dust out in space."

"They don't just spew off things in a uniform way. They send off clods of dirt and ice and rock that come apart," says Brownlee, playing audio of the impact sustained by the spacecraft. The sound was like rapid firecracker bursts.

"A good analogy is thinking of a B-17 in World War II flying through flak - sometimes a large number of impacts in less than a tenth of a second - so it is a very dramatic environment."
Deep Impact crater

The pictures that Stardust snapped showed some erosion over the past five years, and for the first time allowed scientists to see the crater made by NASA's Deep Impact mission, an impact which was obscured by a huge dust cloud the first time around.

"We never saw the crater as we went by, it was there somewhere that created a lot of mystery, it also helped to create this mission," says co-investigator Pete Schultz of Brown University.

Deep Impact pummelled the comet in 2005 with a special impactor spacecraft and the material that came out was a surprise to scientists: a cloud of fine powdery material emerged, not the water, ice and dirt that was expected.

Deep Impact also found evidence of ice on the surface of the comet, not just inside it.

This time, the approach had to be carefully orchestrated so that the spacecraft could snap pictures of the right area of the comet at just the right moment.

"We planned it so on approach we would see the Deep Impact area," says principal investigator Joe Veverka of Cornell University. "That meant arriving at precisely the right time and the right place."

"We saw the crater, we really did see it," says Schultz, making a joke when an image of the crater failed to appear as prompted during a press conference.

"It is subdued. It is about 150 metres across and has a small central mound in the centre. It looks as if from the impact the stuff went up and came back down," he says.

"This surface of the comet where we hit is very weak. It is fragile so the crater partly healed itself."
Changing surface

Comet Tempel 1 is about 6 kilometres wide and travels on an orbit that brings it as close to the Sun as Mars and as far away as Jupiter.

Comparing pictures of the comet taken in 2005 to the latest ones, Veverka says experts could detect "erosion on a scale of twenty or thirty metres has occurred in the five years since we took this picture."

Other areas seen for the first time appear to show layers of material that have been deposited, a phenomenon that deserves further study, says Veverka.

"They have been interpreted as places where a very volatile gas from below the surface has erupted carrying with it small particles of ice and dust and while some of the stuff leaves into space some of it just flows downhill because a comet does have a little bit of gravity," he says.

"We are seeing changes that we have to spend time quantifying to understand what they mean."

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