Dr Peter Tsou holding an aerogel sample tray from the Stardust mission, during a press conference at Johnson Space Center, USA, on 19th January 2006. The aerogel was carried on the Stardust spacecraft to within a few hundred kilometres of the comet Wild-2, which it met in January 2004. Aerogel is a substance with very low density, which helped it to trap cometary dust particles without damaging them. The aerogel was returned to Earth in a capsule, landing on 15th January 2006. Tsou is an investigator for the mission. At left is Donald Brownlee, another investigator. Comets are relics left over from the formation of the solar system some 4.5 billion years ago. Studying their composition should provide information on its history and formation.