But don't advertise ourselves to aliens, warns Hawking

Suppose there is someone out there

Professor Stephen Hawking and other scientists have a new warning: space probes and radios may be hazardous to our health. The reason is that if extraterrestrial life exists, and uses those probes or radio signals to detect and pinpoint us, and then travels to visit us, that species will probably be more advanced and more desperate - and more aggressive - than we are.

Earlier this week scientists at the American space agency Nasa finally lost touch after 25 years with Pioneer 10 - now the most remote spacecraft in the universe, still moving away from us and now 67 times further from the Sun than the Earth. It carries a plaque intended to show aliens where it originated, and what sort of species built it.

But a new CD-Rom featuring Professor Hawking, famous for his theories on black holes, warns that the arrival of intelligent alien life would be bad news for humanity.

He believes it would be an experience comparable with the American Indians' encounter with Christopher Columbus. "I don't think they were better off for it," he said. Instead, many were wiped out by new diseases and wars over territory.

Malcolm Young, professor of psychology at the University of Newcastle, agrees wholeheartedly. "It is very, very expensive to do any sort of crewed interstellar travel," he said. "If anybody, or anything, ever does show up in the solar system then it must be because they really wanted to get here. Goodwill seems an unlikely motivation.

"But if your home star's exploding or your planet's dying, then it would be worthwhile. But in that case, you're not going to be interested in sharing. It will be like the film Mars Attacks - all the world leaders want to negotiate peace, but the aliens just shoot them."

He points out that dead probes such as Pioneer 10 are unlikely to be found, let alone to indicate our origins. "They're just interstellar junk."

But old radio broadcasts will now have reached any star within 70 light years - which includes hundreds of thousands of star systems. "It only needs one of them to be able to crack the code for its origin," he said.

Professor Hawking prefers to think that aliens have accidentally missed Earth, but is not looking forward to any time when they correct that oversight. "It could be very nasty," he said.

Pioneer 10 is one of four deep space probes now heading out from Earth. It carries a plaque which shows the star it came from, relative to 14 highly energetic stellar radio sources called pulsars, and to the centre of the galaxy. A sufficiently intelligent race could trace its source. Other probes, such as Voyager 1 and 2, and Pioneer 11, are also headed slowly, and in contrasting directions, towards other stars - though they will take at least 30,000 years to arrive. Over the next million years they will pass stars in our close neighbourhood, light years distant. But we will lose touch with them over the next 30 years.