The joint Nasa/European Space Agency Cassini-Huygens mission to the planet Saturn will be launched this autumn. John Zarnecki, of Kent University, has spent the past seven years heading a team building an instrument which, if everything goes to plan, will send just three hours of data back to earth. The instrument will not reach its target, the surface of Saturn's largest moon, Titan, for seven years, by which time instruments on Huygens will be over 15 years out of date.

"Titan has a thick atmosphere," says Dr Zarnecki. "There are some similarities between it and the early atmosphere of earth. That is one reason for going. The second is to investigate the surface of Titan. It is 100 per cent covered with haze. Voyager took 3,000 pictures but saw nothing. The surfaceis the largest area of the solar system as yet unexplored."

Relying on the thick atmosphere, saucer-shape design and a parachute, it is hoped Huygens will land safely.But Zarnecki knows he only has one chance. "With lab experiments you can gradually revise, but here it's all or nothing. It means you have to be very rigorous about building - that's why it has taken so long.

"Anything we measurewill be great. It would be seven years building, and seven years to get there, for three hours of data. But it would be absolutely wonderful, unique data which we would kill for. If you are in the space sciencebusiness you have to be a littlebit crazy, but the pay off is great if it works."

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...and failure

Exploring the unknownis an uncertain business; that is surely part of its popular appeal. Yet when the Cluster mission, designed to take a three-dimensional look at the effect of the sun on the earth's magnetic field, blasted off from French Guyana last summer the possibilities offailure were far from the minds of those who had devoted more than a decade to realising their dream.

Leicester University'sStan Cowley, involved in Cluster since day one, was at homeat the time of the launch.

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"I was listening to the news and then all of a sudden it was this great drama from French Guyana. I was stunned.I walked around the house knocking my head against the wall. All those years we had been planning, and then it goes up in smoke. We had all the infrastructure, done all the arguing, and then there was no data." Since last summer's disaster, blamed on a fault in the rocket Ariane, space scientists have been pushing to rebuild Cluster. Earlier this year, ESA gave the Cluster replacement the go ahead. Launch is now due for the year 2000.