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Nasa's US$212 million ($273 million) Stardust mission was designed to gather some of the raw material that formed the sun and planets and escaped to the cold outer reaches of the solar system before the sun was formed.
Instead, the mission yielded material that was altered by the early sun and later thrown to the outskirts. "The hope was that Stardust would bring back some of this primitive, unaltered, pristine material," said physicist Hope Ishii of Lawrence Livermore National Laboratory, lead author of a report in the January 25 issue of Science.
The pristine dust may have held clues to the birth of the solar system and perhaps to the origin of life itself. The Stardust spacecraft travelled almost five billion kilometres in seven years to snatch some dust from the tail of comet Wild 2, which had been to the the Kuiper Belt, and send it back to earth in a capsule two years ago. It was the first mission to successfully return a sample to earth since the Apollo missions brought back moon rocks 35 years ago.
More than 200 scientists around the world have been studying the tiny comet bits which are less than 100th the width of a human hair. The team at Livermore Lab, led by Ishii and co-author John Bradley, used a high-powered electron microscope to figure out what the bits are made of.
But the precious cargo has proven disappointing. "The Wild 2 sample is looking a lot more like meteorite material from the asteroid belt," Ishii said.
In addition to the inner solar system particles they found, the sample is missing some of the telltale ingredients of early solar system material, or stardust, which are known from comet dust collected from Earth's stratosphere over decades by high-altitude aircraft.
"In the sample from Wild 2, the abundance of stardust appears to be a lot less than even in the samples of meteorites, which is a [surprise]," Mr Bradley said. The mission isn't a total bust, however. Despite its disappointing makeup, Wild 2 is still technically a comet because it has a tail formed by vaporising ice. This means it formed far enough out in the solar system to be frozen.
It has shown that the line between asteroids and comets is blurry, and there's more of a spectrum between the two. And the material in the Stardust sample is the first solid evidence that inner solar system materials were hurled out to the fringes, which means the formation of the solar system was more violent than previously thought.
And scientists have only looked at about 5 per cent of the dust collected.
"We are almost certainly likely to find more pre-solar grains," Bradley said
