Bullet-proof spacecraft set to probe the frozen hearts of comets

The countdown has begun to the launch of a space probe with a special bullet-proof "vest" that will allow it to kiss the icy heart of a comet without being damaged.

The countdown has begun to the launch of a space probe with a special bullet-proof "vest" that will allow it to kiss the icy heart of a comet without being damaged.

Space engineers are putting the Contour spacecraft through its final paces before they send it to the launch-pad of Kennedy Space Centre in Florida, where the lift-off is scheduled for next July. Contour – the Comet Nucleus Tour – is the first spacecraft designed to get up close and personal with three passing comets, the wandering vagrants of the Solar System.

Comets are thought to be the "builders' rubble" left over from the formation of the planets more than 4.5 billion years ago. They contain water, in the form of ice, and much of the organic material thought to be essential for life. Some scientists say life may even have begun on a passing comet.

So precarious is the mission, with the spacecraft going within 60 miles of the comet's central "dirty snowball" of ice and dust, that scientists have built a special protective shield of Kevlar and Nextel, the material of bullet-proof vests.

On only two previous occasions has a spacecraft passed close to a comet. The first was in 1986, when the Giotto spacecraft glimpsed the nucleus of Halley's Comet, the omen seen before the Battle of Hastings as depicted on the Bayeux tapestry. The second came in September, when the Deep Space 1 probe made a fly-by of Comet Borrelly during an unscheduled rendezvous orchestrated as a last-minute venture by scientists on Earth.

Neither encounter, however, will rival that of Contour, which is expected to take photographs at least 25 times more detailed than any taken before of a comet's core.

Mary Chiu, Contour's project manager at Johns Hopkins University in Baltimore, where the probe is being built, said: "The images we have got back before were good, but not really that detailed. With Contour we plan to get much closer than ever before. One of the challenges is to get in really close without any significant damage, because the central core of a comet gives off jets of gas and dust travelling at very high speeds."

At the heart of every comet is a central chunk of mixed-up ice and dust often measuring only a few miles across, even though the dusty "tails" can extend for millions of miles.

Contour, about the size of a family car, is scheduled to rendezvous with Comet Encke in November 2003, with an approaching speed of 28km (18 miles) a second. It will then fly on to meet Comet Schwassmann-Wachmann 3 in June 2006.

Each rendezvous is designed to take place when the probe and the comets make their closest pass to the Sun during their elliptical solar orbits. That will ensure the probe encounters the cometary cores during their most active phases, when solar heating explosively evaporates water and gases from the icy nucleus.

The probe is expected to rendezvous with the third comet when it makes its third- closest pass to the Sun – scientists have yet to identify which comet this will be. The US National Aeronautics and Space Administration (Nasa), which will operate the probe, said: "Contour's mission design is so flexible that the spacecraft can be retargeted to intercept an unexpected cometary visitor.

"The appearance of such a comet cannot be predicted in time to plan a space mission, but Contour will take advantage of the opportunity if a 'new' comet passes close enough to Earth's orbit."

The next five years are expected to produce an explosion in our understanding of comets. In addition to Contour, Nasa is sending another probe, called Stardust, to Comet Wild 2, which it is expected to reach in 2004. Unlike Contour, Stardust will investigate the cometary "tail" by collecting particles in a soft aerogel material, which will then be returned to Earth in 2006 for detailed laboratory analysis.

Yet another mission, called Deep Impact, aims to blast away the crust of Comet Tempel 1 in 2005 with a half-ton copper projectile to create a huge crater, exposing the primordial matter that makes up a comet's icy heart.