Trail of the lonesome comet

Deep Space 1's exploration of the Comet Borrelly, say Heather Cooper and Nigel Henbest, might explain why life arose on Earth

On October 24 1998, a spacecraft with a difference blasted off from Cape Canaveral. While most probes rely on tried-and-trusted technology, Deep Space 1 positively bristled with cutting-edge hardware, to be tested for the first time in the unforgiving space environment. Instead of heavy conventional rocket fuel, Deep Space 1 had a new propulsion system which relies on electric particles. It also boasted a solar array which could focus sunlight to obtain extra power, and innovative software to enable the probe to make intelligent decisions for itself.

Deep Space 1 was brilliantly successful in achieving all its technological goals. But the team at California's Jet Propulsion Laboratory (JPL) who controlled it had more ambitious plans for their little pioneer. They were going to persuade it to do some science.

Enter Comet Borrelly. This not exactly well-known comet was discovered by Alphonse Louis Nicolas Borrelly in 1904. In the 19th century, it passed close to Jupiter, whose gravity "tweaked" it into an elongated orbit that brings it round the Sun every 6.9 years. The JPL team realised the comet was due to reach its closest approach to the Sun on September 14 2001 – and that Deep Space 1 could intercept it.

Comets are "dirty snowballs": leftover building blocks of the Solar System swarming billions of miles away from the Sun in deep freeze. Most of them stay that way, but if they fall in towards the Sun, they develop a huge head of steam (the coma) and two tails: one of blown-off sooty dust, and the other of evaporating gas. To many astronomers, comets are the Rosetta Stone that will help us unlock the mystery of our origins. Because they are made of material unaltered since the birth of our Solar System nearly 5 billion years ago, they could shed light on what conditions were like then, and even why life arose here.

Nasa HQ gave the go-ahead to re-route Deep Space 1 towards Comet Borrelly in late 1999. And that's when disaster struck. Its navigational system – ironically, not one of the cutting-edge technologies – packed up. The spacecraft was now 200 million miles from home. The team's only chance of contacting it was to mobilise the huge dishes of the Deep Space Network. They sent a stream of commands to an experimental camera on board to instruct it to act as a star tracker – thereby restoring navigational facilities. It was a nail-biting task; one wrong command could have killed the mission. And the scientists also realised that when Deep Space 1 entered Comet Borrelly's coma, to search for the tiny "core" giving off all the steam, it wouldn't be able to do two things at once – to navigate and image the core. With only one camera, the navigation had to go.

Many considered that the encounter with Borrelly would be a journey to oblivion. Unlike the European Space Agency's Giotto probe, which intercepted Halley's Comet in 1986, Deep Space 1 had no protective shielding. Travelling at nearly 40,000mph through a hail of dust and gas, just one impact with a tiny particle could be lethal.

But on September 22, Deep Space 1 pulled the encounter off in spectacular fashion. It flew just 1,250 miles from the comet's core, and returned images that are even more detailed than those that the purpose-built Giotto took of Comet Halley. They reveal an extremely dark, potato-shaped lump five miles long – half the size of the nucleus of Halley's Comet. Like Halley, the solid core of Borrelly is spouting brilliant jets of gas. There are different patches on the surface, suggesting areas with different chemical compositions. Deep Space 1 was also able to sample the environment around the comet.

Experts are saying that our knowledge of comets has "doubled overnight". For instance, Borrelly seems to be much less watery than thought – an "icy dirtball", as opposed to a "dirty snowball". But with less than a week since the data came in, it's too soon for any conclusions.

Sadly, plucky little Deep Space 1 will be reaching its own conclusion very soon. The hydrazine fuel that fires its attitude-control thrusters is now nearly exhausted, and the probe will soon be unable to orientate itself in space. By late November, its innovative, adventurous mission will be over.