Aliens have left the Moon littered with space debris, a meeting on the search for ET was told this week.
The conference, in London, heard that the lunar soil was likely to be full of fragments of alien spaceprobes.
Dr Ian Crawford said there could be as many as 260 micron-sized bits of ET’s technology in every square kilometre of moonrock. (That is 673 per square mile).
It showed that the Moon was one of the best places to look for evidence of extra terrestrials.
Dr Crawford told the meeting, organised by the Royal Astronomical Society, that 100 billion civilisations had probably existed at some time in our Milky Way galaxy alone.
It was possible that some had left obvious relics like the monolith in the movie 2001: A Space Odyssey.
He said: “That idea is an extreme example – it is a longshot but still a valid one.
“Intelligent aliens in star systems close to our own would soon have realised that there was something interesting about planet number three in our solar system – the Earth.
“They would have seen that an airless body like the Moon would be a good place to leave evidence because they would be preserved for ever.”
Dr Crawford, of University College, London, said the Moon was like a time capsule. He added: “Space-faring civilisations cannot avoid generating space debris.
“The more space-faring civilisations there has been, the more interstellar space debris will exist. Some would fall on small airless worlds like the Moon.”
Dr Crawford, who is president of the UK’s Society for Popular Astronomy, said that future missions to the Moon should look for bits of ET while probing the lunar surface.
Similar alien artefacts that fell to Earth would have been destroyed by our atmosphere or geological activity.
He said that if one star in a billion harboured a technologically able civilisation then the nearest to us today would like about 1,000 light years away – just on the limit of searches being carried out by radio telescopes today for the SETI project.