They are members of an exclusive club - every one of them has hurtled once round the Earth and returned through the atmosphere.

More than 100 astronauts and cosmonauts from the Association of Space Explorers touched down in Edinburgh yesterday and warned the citizens of Earth that our planet is in grave danger.

Prominent spacemen and spacewomen at the ASE's annual conference include the Russian Alexei Leonov, the first man to walk in space, and Helen Sharman, the first British woman in space.

Some 50 years since Sputnik was launched, the explorers are in Edinburgh to discuss ways in which space programmes can help protect the planet - including from collision with near earth objects. Mr Leonov, 73, said: "You do not have to have a lot of imagination to think of what would happen if the Tunguska meteorite hit not a rural area like Siberia but a capital like Moscow or Paris. It would be a catastrophe and we have to think very seriously about it."

The conference is a celebration of the achievements of 50 years of space research and the ways in which it has enhanced our everyday lives - from weather to cancer diagnostics. As India and China and the US draw up plans to prepare to send astronauts to the moon and to Mars, Britain must play its part, or risk missing out, according to Helen Sharman, who in 1991 visited the Mir space station.

Ms Sharman, 44, said: "My own mission was a completely commercial thing.

"Britain needs to push its boundaries otherwise it is going to be left behind as everybody else goes out into space."