London, August 7 : Scientists studying material collected by NASA's Stardust mission during its seven years in space have been unable to find any sample of interstellar dust till date.
The Stardust probe launched in 1999 and visited a comet called Wild 2, collecting space dust in a honeycomb matrix of gel.
According to a report in New Scientist, new data showed that Stardust's first six candidate dust particles originated much closer to home, with most coming from Earth or the spacecraft itself.
Most of the dust collected comes from the comet, but NASA scientists estimate it may have also picked up as many as 45 pieces of dust that originated outside the solar system.
Since the gel and its samples returned to Earth in 2006, the Stardust@home website has recruited an army of almost 30,000 volunteers to sift through cross-sections of the gel, looking for telltale holes - just 20 microns across - left by these dust impacts.
Scientists expect interstellar dust to be made of primitive materials left over from earlier in the universe's history.
These would be relatively deficient in heavy elements, and unaffected by heating and other processes that altered the composition of objects that formed in our solar system.
But studies of six of the candidate particles at Lawrence Berkeley National Laboratory in the US and the European Synchrotron Radiation Facility (ESRF) in France showed no such signs.
The team blasted tiny wedges, or "keystones", of gel containing the particles with powerful X-rays that reveal the energy signatures of metallic elements.
Three of the six candidates showed large quantities of zinc - a metal that is rare in the cosmos, suggesting the dust came from Earth. Another was high in cerium, an even rarer metal that flaked off Stardust's solar panels in space.
The fifth candidate was a piece of alumina, or aluminium oxide, an impurity deposited when the gels were being made on Earth.
And the last candidate was "invisible" to the beam.
"It's most likely an organic particle made of carbon, hydrogen, nitrogen and oxygen; possibly from the Wild-2 comet," said Andrew Westphal, director of stardust@home.
But the team is not discouraged.
According to Westphal, "This is not at all unexpected. We have around 100 candidates, and we've only looked at the first six."