Researchers have recovered seven interstellar dust particles -- the ancient building blocks of the solar system. It’s the first time scientists have held primordial material that have not been altered during the violent events surrounding the Milky Way’s birth.
The particles -- weighting just a few trillionths of a gram -- were captured by a tennis racket-shaped collector aboard NASA’s Stardust spacecraft, which was launched in 1999 to collect samples from the tail of the comet Wild 2. Comets are ancient frozen bodies, and they’re expected to be a repository of the primordial ice and rock that goes into building a solar system. However, the minerals in the cometary samples collected have been transformed by heat before they were incorporated into comets.
To catch pristine star stuff, scientists turned to the dust streaming in from interstellar space. For this mission, Stardust stuck out its dust collector panel (pictured above) for a total of 200 days during 2000 and 2002. To capture interstellar particles traveling faster than 15,000 kilometers per hour, the racket-shaped collector panels contained blocks of “frozen smoke” -- or aerogel made of silica that’s 99.8 percent air. (For the CSI-viewers out there, it’s like that gel they shoot into in ballistics.) These helped to slow down and retain dust particles without altering their shape and composition, or vaporizing them entirely. When it hits the aerogel, the hypervelocity particle buries itself in the material, creating a carrot-shaped track up to 200 times its own length. Since aerogel is mostly transparent (with a smoky blue tinge), scientists can see tracks and follow them to the tiny particles. About 85 percent of the collector consisted of aerogel; the remainder consisted of aluminum foils. The collector and their embedded particles came back to Earth via the Sample Return Capsule that was ejected in 2006.
Stardust team members enlisted more than 30,000 volunteers worldwide to search for any particles that may have embedded in the aerogel. These “dusters” examined online microscopic images taken through the aerogel for cone-shaped tracks left by speeding particles.
One hundred million searches and 71 tracks later, Stardust team members found seven “probable” interstellar dust particle impacts. Two particles, Orion and Hylabrook, were captured at low speed (much slower than 10 kilometers per second); they weighed about 3 trillionths of a gram each. A third particle, Sorok, was captured at very high speed (much greater than 10 km/s); no residual particle survived the impact. And then there were four impacts in the aluminum foils; enough of the particles were retained that their composition and structure were still measurable.
To be sure that the grains are truly interstellar, the researchers will have to transfer the dust from inside the aerogel onto instruments that can analyze their isotopes. Developing the techniques “will be fundamentally boring but necessary,” says Andrew Westphal of UC Berkeley, according to Science. “It would be very easy to lose them.”