The fourth in NASA's Discovery program of low-cost spacecraft with highly focused science goals, Stardust was comprised of a spacecraft and capsule that completed a seven-year, 3-billion-mile journey to a comet in 2006. The capsule returned to Earth, carrying an aerogel collector that captured particles from comet Wild 2 as well as interplanetary dust. The Stardust capsule is now on display at the Smithsonian's National Air and Space Museum.