NASA's Stardust mission captured flecks of interstellar dust. Credit: NASA

Scientists say they have caught the first pieces of interstellar dust — the fundamental building blocks of the Sun, Earth and the rest of the Solar System. The discovery required an army of volunteers, including a Canadian man who spent 15 hours a day studying images online and eventually won the interstellar lottery.

The minute specks of dust were collected by NASA's Stardust spacecraft, which launched in 1999 with the aim of catching pristine interstellar grains and bringing them back to Earth.

The discovery validates four years of effort from more than 27,000 volunteers around the world, who searched 71 million images of the material captured in the Stardust collecting trays.

The two probable dust particles found so far could mark the beginning of an analysis of what stars and planets really are made of, and also offer a way of charting the chemical evolution of the Milky Way.

"The interstellar dust is fundamentally the stuff we're made of," says Andrew Westphal, a physicist at the University of California at Berkeley who announced the discovery on 3 March at the Lunar and Planetary Science Conference near Houston, Texas. "We're trying to understand our own origins."

The Stardust mission was sent on a path through the Solar System that crossed the route of comet Wild 2. Westphal and his colleagues wanted to collect dust from Wild 2 because they thought at the time that comets were frozen samples of primordial interstellar matter, essentially unchanged since they formed billions of years ago outside the Solar System. But analyses of these comet grains several years ago showed that they were forged in hot environments near the Sun and had been substantially altered during the Solar System's formation (see 'Comet Born of Our Own Sun').

However, Stardust also captured some non-cometary grains. Before the encounter with Wild 2, Stardust opened another collecting tray to space. The researchers hoped to catch 100 or so interstellar grains from the weak but continuous flux in open space. The elements in these grains were forged in stars, but coalesced into grains in the empty space between stars, where they were mixed and rocked by supernova shock waves and cosmic rays.

The grains were far harder to catch than the comet particles. Not only was the flux much lower, but the interstellar particles were smaller than the comet grains and were moving several times faster — up to 30 kilometres per second.

The Stardust researchers say that the interstellar grains nabbed by their spacecraft may provide a unique way to study the matter between stars. "It's kind of a grand thing," says Don Brownlee of the University of Washington in Seattle, who was principal investigator for the mission. "We're catching a piece of the galaxy."

"I'm cautiously excited," says Westphal, who adds that the researchers must conduct more tests to ensure that their particles are truly interstellar grains, rather than micrometeorites or even pieces of the spacecraft knocked loose by debris.

It took four years of searching to identify the two potential interstellar dust particles, stuck at the ends of tunnels they had bored in the Stardust collector, which consists of a wispy material called an aerogel. The researchers carefully extracted the first particle and sent it to three microprobe facilities around the world for analysis. The results offered hints of a glassy, amorphous shape that is rich in aluminium.

One of the most powerful aspects of the project was the distributed army of volunteers who have searched for the tiny tracks in the aerogel. The online effort created by Westphal, dubbed Stardust@Home, has relied on the work of thousands of self-described 'dusters', who have scanned millions of pictures of candidate tracks.

Bruce Hudson from Midland in Ontario, Canada, scored the first catch and, as per the rules of the project, got to name the first piece of stardust: Orion. Reached by phone, Hudson, 46, a former groundskeeper for a Catholic shrine, said he thought the result was "so cool". Hudson suffered a stroke in 2003, and he turned to the Stardust@Home project in 2006 as a productive way of passing the time. For a year or so, he spent as much as 15 hours a day scanning thousands of pictures, five seconds per slide. "I kept doing that over weeks and months," he says. "It was all I could do, really." He at one point cracked the top ten ranking among the dusters, which reflects productivity and efficiency.

Westphal says he will start a new phase of the @Home project in a few weeks, using the newly identified candidate tracks as a way of calibrating the way that dusters do their searches.