“We do this by using a weird material called aerogel,” Westphal explains. “It’s a low density foam made of silicon dioxide. We chose it because it can capture particles moving at many kilometers per second, while leaving them practically intact. About 85% of the spacecraft’s collector area is aerogel [which is arranged in a honeycomb-like pattern on both sides of the collector array], and the other 15% is made of aluminum foil. The foil was not originally envisioned as a capture medium, but it turned out to be useful as a complementary medium to the aerogel.”
The aerogel on one side of the collector was exposed to the material streaming from Comet Wild 2 during its encounter with the spacecraft. The aerogel on the other side was exposed for 200 days during Stardust’s journey en route to the comet. The hope was that this side of the collector would gather material not from the comet, but interstellar dust that is constantly streaming through our Solar System.
“It’s a bit like going down a freeway in a snowstorm,” says Westphal. “You see snowflakes coming at you from the opposite direction to your direction of motion. The Solar System is moving at about 26 kilometers per second with respect to local interstellar space, so when we were moving in the right direction relative to this stream, we exposed the interstellar-dust side of the collector to collect these particles.”