Scientists Fine-tune Hunt for Et

Radio astronomers are to focus on 150 locations in space next week in the search for ET. They have narrowed the hunt for extraterrestrial civilisations to a selection of star systems, thanks to the world's biggest computing exercise.

Seti@home is a screensaver package which has been downloaded by more than 4 million computer users. When no one is using their computer, it works on data from the radio telescope at Arecibo in Puerto Rico, which is sent to it over the internet.

In the course of three years, the program has clocked up more than 1m years of computing time, looking for signals that might be evidence of intelligent life. According to Seti scientists, there are now at least 150 radio sources which deserve a closer look.

"I give it a one-in-10,000 chance that one of our candidate signals turns out to be from ET," said Dan Werthimer, of the University of California, Berkeley. Dr Werthimer, who runs Serendip - the search for extraterrestrial radio emissions from nearby developed intelligent populations - will next week use the 300-metre dish at Arecibo to check 150 locations in 24 hours.

Since television was developed more than 50 years ago, there is now a "bubble" of signals stretching out from Earth for a distance of 50 light years, carrying episodes of Coronation Street and Stars in their Eyes to Alpha Centauri and beyond.

The logic of that is that ET may be sending similar signals towards us. The trick is to recognise them across distances of millions of light years, amid the sound of collapsing black holes, pulsing neutron stars, and colliding galaxies.

This sky search - in effect, tuning in to the whole universe - would have been beyond any single computer system.

But David Anderson, the director of the Seti@home project, also based at Berkeley, is anxious not to raise hopes too high. "If there is any possibility at all of finding an extraterrestrial signal, it is probably much less than 1%," he said.

However, he added: "This is the culmination of more than three years of computing - the largest computation ever done. It's a milestone."