Published online 15 February 2011 | Nature | doi:10.1038/news.2011.96

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Recycled spacecraft takes second look at comet

Stardust-NExT measures changes over one 'comet year'.

cometTempel 1, as seen by Stardust-NExT.NASA

In a late-night Valentine's Day rendezvous, a NASA spacecraft yesterday began to give scientists their first before-and-after images of a comet's passage around the Sun.

The spacecraft started life as the Stardust mission, which in 2004 flew past the comet Wild 2 and collected a canister of samples to send back to Earth. Its intended task over, the spacecraft was rechristened New Exploration of Tempel 1 (NExT) and put back into use when scientists realized that it had enough fuel to visit another comet.

The Tempel 1 comet was first photographed in 2005 as part of the Deep Impact mission, which analysed the interior of the comet by smashing a 370-kilogram slug of metal into it.

Deep Impact was designed to produce a dust cloud whose composition would provide clues to the comet's interior. But it also provided baseline images for last night's flyby, in which, at 4:39 a.m. GMT, the Stardust-NExT craft zipped within 181 kilometres of Temple 1 at a speed of nearly 11 kilometres per second.

It will probably be several weeks before the 72 Stardust-NExT images are fully processed, but scientists are already poring over the first of them, which were beamed back to Earth shortly after the flyby.

Eventually, the researchers hope to make detailed comparisons between these images and those taken by Deep Impact.

Once round the block

Since the Deep Impact mission, Tempel 1 has completed slightly more than one full orbit around the Sun — "basically, one comet-year", says Peter Schultz, a planetary scientist at Brown University in Providence, Rhode Island, and a member of the Stardust-NExT team.

The comet never comes much closer to the Sun than Mars, but that is close enough for the Sun to vaporize ice and the solar wind to blast material from it, causing the comet's radius to shrink by, on average, 25–50 centimetres for each passage around the Sun, says Michael A'Hearn, an astronomer at the University of Maryland in College Park.

Some of that material blows into space from the comet's tail. But the rest falls back to the comet's surface, like snow.

stardustArtist's rendering of Stardust-NExT spacecraft approaching comet.LMSS

"This is kind of like going to the northeastern United States and checking out, after one entire season, where the snow's come down, and where it's disappeared," says Schultz. "We're going to see if it is the same. It sounds like a basic question, but we've never been back to a comet before, after it's been once around the Sun."

The scientists also hope to spot the crater created by Deep Impact's 'bullet'.

In 2005, dust ejected by that impact obscured the crater, making it hard for researchers to determine exactly how big it was — information that would help them to work out whether the comet's surface is firmly or loosely packed. "Estimates range from 30 meters to about 150," says Schultz. A'Hearn says that it is "really important" narrow down that range to determine the hardness of the material that makes up the comet.

Researchers also want to know whether the crater has survived or has slumped into a level plain, visible primarily from the ring of material ejected from it. That, too, would reveal much about the characteristics of the comet's surface.

Different data

Stardust-NExT wasn't designed to take high-resolution photos and doesn't have the high-quality telescopes carried by Deep Impact. In fact, its only telescope is a navigation instrument capable of providing raw images with resolutions of just 35–40 meters per pixel, compared with about 2 meters per pixel for Deep Impact. "How much we can process the images to recover greater resolution we don't know yet," says A'Hearn.

But the Stardust-NExT mission has one advantage over Deep Impact: it carries a mass spectrometer capable of analysing the makeup of each grain of dust that enters its dust collector.

"That should tell us a lot," says A'Hearn, noting that the dust collector should be able to distinguish rocky dust from organic compounds, determine trace metal compositions and even tell apart different categories of organic molecules. Not only will this tell scientists much about Tempel 1's composition, but it will also tell them how it differs from Wild 2, Stardust's first target.

Ideally, all of this data will provide clues to the origin of the Solar System, and perhaps of life on Earth, says Joel Parker, an astronomer at the Southwest Research Institute in Boulder, Colorado.

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Some astronomers believe that comets are the source of Earth's water and the organic molecules that became the building blocks of life. And comets are believed to have originated beyond the outer planets, making them frozen remnants of the primordial Solar System.

Thus, says Parker, "any additional information you can get is of interest both for studying the origins of the Solar System and for the impact on life".

A'Hearn agrees. But the comets studied so far are so close to the Sun that they have been altered from their primordial states, he says. To work out what they originally looked like, he says, you must first "separate out" what has happened to them since they moved closer to the Sun — in other words, we need to see how they change with each orbit. 

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