A mission to Mercury, the planet closest to the sun, will be discussed by an international group of scientists next week at Imperial College, London.

The planet has puzzled scientists since 1974, when the Mariner 10 spacecraft flew past Mercury and detected a magnetic field which ought not to exist according to current theories of the solar system.

The mission, which is likely to form part of the European Space Agency's Horizon 2000+ programme, would have to be sufficiently robust to cope with temperatures that vary from 700C to -100C depending on which side is facing the sun.

The scientists will work out the most burning questions about Mercury and, therefore which would be the best instruments to put on the spacecraft. They will also consider the technological problems of getting a mission to the planet.

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Andre Balogh, professor of physics at Imperial College, said that his priority was to solve the mystery of Mercury's magnetic field. If a planet has a magnetic field this implies that it has a liquid metal core. But Mercury is so small that current theory implies that its huge iron core must have solidified early on in its formation.

Another priority is to map Mercury's surface. "Its surface is like the moon's but the pock-marking is different," said Professor Balogh, who led a team that explored the science of the mission when it was first under consideration by the ESA.

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Studying Mercury's craters would give some idea of the size of the projectiles that crashed into it early in its life, and therefore some insight into the early history of the solar system.

The mission will compete with other proposals to form part of Horizon 2000+. Imperial is hoping to play a key part because of its experience at measuring magnetic fields.