Comet Wild 2 is more than just rubble. Credit: © Science

NASA'sStardust spacecraft has surprised scientists with the most detailed picture of a comet yet. The first results from the probe, which swooped over the comet Wild 2 on 2 January this year, prove that not all comets are born equal.

All comets were once thought to be little more than lumps of loosely packed icy rubble. Stardust has revealed that Wild 2 bucks the trend with a rigid core that is able to support near-vertical cliffs and spires of rock.

"It's completely unexpected," says Donald Brownlee, Stardust's principal investigator and an astronomer at the University of Washington, Seattle, whose results are published today in Science1. The comet's pock-marked surface records impacts dating back billions of years, showing just how sturdy the dirty snowball is.

Stardust is the first mission destined to bring samples of a comet's dust back to Earth. The probe is scheduled to deliver its precious payload on 15 January 2006, by parachuting a capsule into the Utah desert. The comet dust dates from the time when the solar system was first forming, and may hold clues about the origins of Earth, and even of life itself. The most exciting scientific results from the mission will have to wait until then.

But early onboard analysis already shows that the comet's particles contain a variety of organic molecules that are rich in nitrogen2. Their presence in the comet suggests that these chemicals were around right at the birth of the solar system, and similar particles may have seeded our own, infant planet with some of these chemical building blocks of life. However, Brownlee and his colleagues point out that the dust contains no amino acids, which are the basic chemical units that make up proteins.

Cosmic duster

Stardust captured dust from around the comet with an aerogel, a type of glass that is actually 99.8% air. When the dust particles hit the aerogel they were travelling up to six times as fast as a rifle bullet. "These things were like a thunderbolt," says Anthony Tuzzolino, a space scientist from the University of Chicago's Enrico Fermi Institute, who helped to build Stardust's onboard dust monitor.

The lightweight aerogel was specially designed to slow the particles gently, without breaking the dust into smaller pieces. At one point the craft recorded more than 1,100 impacts in one second. The largest particle measured during the fly-by was roughly 50 micrometres across, about the width of a human hair. Tuzzolino estimates that Stardust is bringing back roughly 3,000 dust particles from Wild 23.

The onboard aerogel stops particles in their tracks. Credit: © Nasa

Stardusthas also showed that life on a comet is much more violent than anyone had suspected. The craft's sensors detected dust particles swirling in distinct clusters around the comet, which indicates that hefty chunks of material measuring a metre across are breaking off the comet and fragmenting as they move away4. This surprised the scientists: "In general, one thinks of a comet emitting gas and dust in a nice, uniform steady state, sort of like a hose," says Tuzzolino.

Wild times

Astronomers think that Wild 2 spent billions of years in the Kuiper belt, the region just beyond Neptune's orbit that contains millions of aspiring comets. Some gravitational shuffling pushed it closer to the Sun, before a close encounter with Jupiter just 30 years ago set Wild 2 on its present course. In all that time it has only passed close to the Sun on five occasions, so its icy surface has suffered little melting and carries a much better preserved record of its impact history than more weathered bodies, such as Halley's comet.

These things were like a thunderbolt. Anthony Tuzzolino

Thecomet's craters are also free of the dust and debris that litter the Moon, because Wild 2's gravity is so weak. Once something hits the nucleus, the brittle material just breaks away and flies off into space. Photographs of the comet show the incredible detail of surface features, many of which date from its time in the Kuiper belt.

The five-kilometre-wide comet was first seen by Swiss astronomer Paul Wild in 1978, and it orbits the Sun once every six and a half years or so. Its elliptical path around the Sun keeps it swinging between the orbits of Mars and Jupiter. Stardust began its seven-billion-kilometre round trip when it blasted off from Cape Canaveral in 1999.

Previous comet encounter missions include Giotto, which surveyed Halley's comet in 1986, and Deep Space 1, which studied comet Borrelly in 2001. The European Space Agency's spacecraft Rosetta, which blasted off in March this year, carries a probe that is destined to make the first controlled landing on a comet when it meets Churyumov-Gerasimenko in May 2014.