A composite image of the nucleus of Comet Wild 2 and its jets. The Stardust camera combined two exposures. The first short exposure reveals details on the nucleus. The second exposure was taken just 10 seconds later, but was much longer and shows the jets of gas and dust that make up the cometary tails. Both images were captured at closest approach on 2 January 2004. Images: NASA / JPL NASA's Stardust probe was the first mission to collect dust and carbon-based samples from a comet and return them to Earth. Launched on 7 February 1999 it travelled past Comet Wild 2 (pronounced Vilt 2) in January 2004. During its journey to the comet, it probably collected a few tens of grains of interstellar dust – believed to consist of ancient interstellar grains and remnants from the formation of the solar system. It is hoped that analysis of these grains may yield an insight into the evolution of our Sun and planets.

The spacecraft was launched aboard a Delta II rocket, and used the Earth's gravity to send it on its trajectory towards Wild 2. Travelling in 3 loops around the Sun to minimise fuel and weight on-board the spacecraft, it intercepted the comet on the second loop travelling at a mere 6.2 km/s – slow in comparison with previous missions. The probe flew past the comet's nucleus at a distance of just 150km, allowing for the collection of 'fresh' particles from the coma – a region of gas and dust surrounding the comet's nucleus.

Asteroid Annefrank

Stardust image of asteroid Annefrank, just prior to closest approach. Image: NASA / JPL En route to Wild 2 in November 2002, Stardust flew past the asteroid Annefrank, allowing scientists to test the probe instruments before the main encounter. Annefrank turned out to be fainter and larger than expected, with a diameter of 6 km. In common with other asteroids, it has an irregular shape and is heavily cratered.

Encounter with Wild 2

Stardust flew past Wild 2 on 2 January 2004. The on-board camera was used to study the nucleus at close range, so that astronomers now have a detailed map of its surface.

Wild 2's nucleus is about 5 km across, with prominent depressions that may mark the site of gas vents or impact craters.

Stardust image of the 5-km wide nucleus of Comet Wild 2. Image: NASA / JPL Deeper images show the jets of gas and dust that are most active when Wild 2 is near the Sun. These are swept back by the pressure of sunlight and the solar wind and go on to form the comet's tail.

During the encounter, perhaps a million dust particles (mostly a few millionths of a metre across) were brought to a halt in the Aerogel collector. This ultra-low density material stops and collects dust particles travelling at enormous speeds without destroying them.

Throughout the journey the probe should have captured a few tens of particles of interstellar dust. These will be the only (reasonably pristine) samples we have of material that originates from beyond the solar system. Finding these grains in the aerogel will be hard so NASA is asking the public to help – see the Stardust@Home page for more details.

Return journey

A particle of cometary dust embedded in the aerogel collector. Image: NASA / JPL After its rendezvous with Wild 2, it took 2 years for the Stardust samples to arrive back on Earth. On 15 January 2006 the re-entry capsule landed in Utah, west of Salt Lake City, where the samples were collected and taken for analysis by NASA scientists.

Results

The initial findings of the Stardust sample analysis team should be announced in mid-2006. Images of the aerogel collector show a large number of embedded cometary particles, some of which carved holes almost as large as a finger.