By Paul Rincon
Science reporter, BBC News, The Woodlands, Texas
Stardust landed back on Earth in January 2006
Scientists may have identified the first specks of interstellar dust in material collected by the US space agency's Stardust spacecraft.
A stream of this dust flows through space; the tiny particles are building blocks that go into making stars and planets.
The Nasa spacecraft was primarily sent to catch dust streaming from Comet Wild 2 and return it to Earth for analysis.
But scientists also set out to capture particles of interstellar dust.
The material was gathered by the Stardust probe in a seven-year, 4.8-billion-km (2.9 billion miles) interplanetary voyage.
It extended a retractable device containing cells filled with a material called aerogel, a porous substance designed to trap dust molecules.
A capsule containing the precious samples was then returned to Earth in January 2006.
Team members have now reported the possible discovery of two contemporary interstellar dust grains in the Stardust Interstellar Dust Collector (SIDC) deployed during the mission.
Dr Andrew Westphal, from the University of California, Berkeley, announced the find at the Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas.
The discovery was made by a member of the public, using the Stardust@Home internet application, which invited participants to search the aerogel collection medium for tiny particles of the dust.
"There are two particles, but they are in the same track. So when they hit the aerogel, they were together - they are two components of the same particle," Dr Westphal told BBC News.
"But they are very different from each other. That in itself is interesting, because if this does turn out to be interstellar dust, then it is a bit more heterogeneous than people thought."
The initial speck, known as particle 30, was spotted by Bruce Hudson, from Ontario in Canada. Under the agreement made between the science team and participants in Stardust@Home, Mr Hudson was allowed to choose a name for the particle; he called it Orion.
After preliminary analyses, the scientists found another grain upstream, which Bruce Hudson named Sirius.
But Dr Westphal stressed that the find "could be a false alarm".
"The right way to say it is we're cautiously excited," he told me.
"We have very limited data on it so far and the reason is deliberate. The analyses we are doing have the potential to do some minor damage to the particles. We don't think it will and we'll be careful to limit our analyses.
"So far this particle is unique... if we drop it on the floor, it will cost $300m to get another one."
Scientists have identified 28 definite impact "tracks" in the interstellar dust collector. But most of these come from angles indicating they are little particles of debris from impacts with the spacecraft's solar panels. However, particle 30 is one of seven with ambiguous trajectories.
Interstellar dust is formed when gas is ejected from stars and condenses to form grains. This dust then has to survive in the interstellar medium - the matter which exists between stars - where it is battered by cosmic radiation and shock processes.
It carries with it the heavy atoms that go into making the stars and planets. Our own Solar System was also constructed with these building blocks.
Stardust flew by Comet Wild 2 in 2004
The possible dust grains were collected as Stardust travelled with the interstellar dust stream which passes through our Solar System.
The spacecraft's chief scientist, Dr Don Brownlee from the University of Washington in Seattle, told BBC News: "All the heavy atoms in this room were in interstellar dust... so we want to know what this stuff is."
He added: "This dust, once it's formed, and once it's heated or changed [initially] it is set for billions of years.
Dr Westphal told BBC News: "It is very fine-grained material, which is what you'd expect for interstellar dust. It has an elemental composition which is consistent with what you would expect for interstellar dust. And it has a composition for other elements which are not inconsistent, but a bit surprising."
The researchers have so far analysed magnesium, aluminium, iron, chromium, manganese, nickel, copper and gallium from the particles.
A new mineral found in a type of particle known as interplanetary dust has recently been named Brownleeite after Dr Brownlee, who is regarded as a founder of the field of cosmic dust research. The discovery has been published in the journal American Mineralogist.
Though highly prized by Stardust's team, interstellar dust can be a nuisance in optical astronomy, because it can obscure objects in regions of the sky targeted for observation.