THE clearest pictures ever taken of a comet鈥檚 nucleus are changing our ideas of how these cosmic geysers spew out the stunning tails that make them such a spectacular sight from Earth.
The images, taken by NASA鈥檚 Stardust space probe as it hurtled through the comet Wild 2 some 389 million kilometres from Earth, show for the first time how the signature jets of gas and dust are produced by pits on the comet鈥檚 surface.
These pictures are expected to provide a wealth of data that could take more than a year to analyse fully. But they are just the appetiser. The main course is due two years from now, when Stardust is scheduled to bring particles of comet dust from Wild 2 back to Earth for further study. This is the first extraterrestrial material collected for return to Earth since the Apollo 17 mission to the moon in 1972, although a mission called Genesis that will be collecting interstellar dust later this year will reach Earth first.
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The new images detailing the structure of the 8-kilometre-wide nucleus at the heart of the comet are an unexpectedly rich bonus. Stardust鈥檚 camera was intended primarily to guide the craft on its final trajectory to within 240 kilometres of the nucleus and far inside the bright coma, the highly reflective but tenuous cloud of gas and dust that extends more than 1 million kilometres in the comet鈥檚 wake.
The pictures are the first to show exactly where the geyser-like jets that spew cometary material through space actually come from. 鈥淲e are seeing processes we had never seen before,鈥� says the project鈥檚 lead scientist, Don Brownlee of the University of Washington in Seattle.
One big surprise is the appearance of the nucleus, which is nearly spherical but pockmarked by many deep pits. That is dramatically different from the only other comet nuclei imaged, those of comets Halley in 1986 and Borrelly in 2003. Their elongated, potato-shaped nuclei showed no sign of such pits.
On Wild 2, three of the five jets detected in the picture seem to emanate directly from the pits. 鈥淔or the first time, we can actually correlate those jets with features on the surface,鈥� says Ray Newburn, head of the Stardust imaging team. 鈥淲e are seeing what the places they come from look like.鈥�
Astronomers had long suspected that the gas emissions from comets were very localised, coming from only about 5 per cent of its surface area, but this is the first time they have confirmed what these regions look like. 鈥淐omet mechanisms are still a bit of a mystery,鈥� Newburn adds.
As well as analysing the dozens of pictures taken during the close encounter, scientists will spend the next year assimilating data from an onboard mass spectrometer and a device for monitoring the quantity and sizes of dust particles.
But the project鈥檚 most interesting results may come from particles of the comet collected by Stardust. The particles, collected in a tennis-racket-shaped matrix packed with an ultra-light silicon-based foam, will be returned in a capsule due to land in the Utah desert on 15 January 2006.
This landing will be a useful test case for possible future returns of samples from Mars, says John Rummel, the NASA official in charge of 鈥減lanetary protection鈥�, the prevention of contamination of Earth by biologically active material from elsewhere, and vice versa. But in this case, he says, there is no potential risk, as 40,000 tonnes of cometary material already enter Earth鈥檚 atmosphere every year.
Such cometary particles, left over from the formation of the solar system 4.5 billion years ago, are thought to be essentially unaltered specimens of primordial material spewed into space by the death throes of earlier stars. By studying these particles scientists will be 鈥渢ravelling back in time, literally reaching for the stars鈥�, says Brownlee. And because the sun and planets formed from similar primordial dust, the returned samples may provide insight into Earth鈥檚 origins.