Q&A: Rosetta mission

Rosetta will probe the origin of the Solar System

By Helen Briggs BBC News Online science reporter

A daring European Space Agency (Esa) mission to visit a comet should launch in 2003. BBC News Online looks at the facts of the mission.

How novel is this mission?

There have been only a handful of missions to comets and asteroids. Spacecraft have flown through the tails of comets or passed close to asteroids capturing images.

The big difference with Rosetta is that it will attempt to land a robot on a comet for the first time - an ambitious technical challenge.

The closest we have come so far was when the US space agency (Nasa) landed the spacecraft Near on the asteroid Eros in February 2001.

The lander will anchor itself to the comet's surface

Rosetta is Esa's second cometary mission. It follows Giotto, which visited Halley's comet in the 1980s.

"Giotto was a fleeting glimpse," says Dr Christopher Lee of Imperial College London.

"This is even more ambitious because we need to catch up with the comet, stay with it, orbit it and then land a lander on it."

Rosetta's primary target, Comet Wirtanen, was ruled out when concerns surfaced about the spacecraft's launch rocket and the mission was delayed. But a new target, when it is chosen, will probably be a similar body with an orbit that takes it out to planet Jupiter.

"The European Space Agency has never done anything that has gone as far as Jupiter before," says Dr Chris Carr of Imperial College London.

"That's been in the realm of the Americans - so it's quite a departure."

What other mission to comets have there been?

• The first spacecraft to visit a comet was the International Cometary Explorer, led by Nasa. It passed through the tail of Comet Giacobini-Zinner in 1985.
• Six space probes visited Comet Halley in 1986: the USSR-led Vega-1 and Vega-2, two Japanese probes - Suisei and Sakigake - and the International Cometary Explorer.
• In 1992, Giotto became the first spacecraft to visit two comets when it passed within 200 km (124 miles) of Grigg-Skjellerup.
• In September last year, the Deep Space 1 probe got within 3,000 km of Comet Borrelly to produce some astonishing images of a cometary nucleus - the best since Giotto's pictures of Halley.

Are any other comet missions in the pipeline?

Yes - a Nasa mission known as Deep Impact. It aims to blast a crater in a comet and analyse the debris.

Deep Impact is heading for Comet Tempel 1, another member of the Jupiter family of comets. It is scheduled for launch in July 2002.

Another Nasa comet mission, Contour, was lost in 2002. It was meant to rendezvous with at least two comets over the course of four years on a 48-million-km journey.

Contour was supposed to get within 100 km of Comet Encke in 2003 and Comet Schwassmann-Wachmann 3 in 2006.

Images courtesy of Astrium.