Credit: NASA/JPL

In January this year, NASA's Stardust spacecraft flew within 237 km of the comet Wild2. Stardust took 72 close-up shots during the flyby, described as an “unqualified success” at the Lunar and Planetary Science Conference in Houston, Texas, last week. After a few months' analysis, the Stardust team has now released stunning pictures of the comet's 5-km nucleus.

The Wild2 comet is ‘fresh’. It is believed to have spent billions of years in the Kuiper belt, out beyond Neptune, until gravitational encounters kicked it into orbit closer to the Sun. In 1974, a close encounter with Jupiter knocked Wild2 into its present orbit, inside that planet's. Since then, Wild2 has passed close to the Sun only five times and so has suffered less exposure to solar radiation than other comets. Its surface bears a well-preserved record of its early history in the Kuiper belt.

The short-exposure image at the centre of this composite shows the cometary surface. The surface is markedly different from those of the comets Halley and Borrelly, and of Jupiter's ice-rich moons Callisto and Ganymede: instead of a continuum of crater size, Wild2's craters are mostly large; some are likely to be impact craters, others not. A long-exposure image taken 10 seconds later reveals a glowing halo of gas, thrown out from the nucleus in as many as 20 highly collimated jets.

Stardust's mission doesn't end there. The spacecraft is now heading for Earth once more, carrying particles from the comet's coma that were trapped during the flyby in centimetre-size aerogel cells. The samples will arrive on 15 January 2006.