By Dr David Whitehouse BBC News Online science editor

Some of the ET candidate sources in the sky

The scientists behind the world's biggest distributed computing project are about to take a closer look at the most promising radio signals so far collected in the search for alien beings.

For four years, millions of people around the world have been running a special screensaver program on their desktops, sifting data for unique patterns that might represent an intelligent transmission.

Now, the most interesting radio sources picked out by the Seti@home project are to be re-observed using the giant Arecibo telescope in Puerto Rico.

Researchers have about 150 radio sources they want to examine in the next three days.

Curious remain

Since it started in 1999, Seti@home (Seti stands for the Search for Extraterrestrial Intelligence) has become a massive hit with computer users.

Several million volunteers from more than 200 countries have downloaded the screensaver program that uses idle time on a PC to analyse data obtained by radio telescopes that scan the skies for unusual signals, possibly from intelligent lifeforms.

The giant Arecibo observatory will look for lifeforms

In so doing, Seti@home has become the largest computation ever done on this planet, having accumulated more than a million years of computing time.

Now, in a phase known as the "Stellar Countdown", the project will use the Arecibo radio telescope to re-observe the most interesting radio sources thrown up by the screensaver search.

David Anderson, Seti@Home's Project Director, said: "After the re-observations of our Stellar Countdown help us eliminate candidates that are random noise or terrestrial radio interference, we will be very curious to see what candidates remain."

Serendipity data

The Seti@home software downloads data from the Search for Extraterrestrial Radio Emissions at the Nearby Developed Intelligent Populations (Serendip) project at the University of California at Berkeley, US.

The Seti@home screensaver has been a huge success

The odds that it has succeeded in identifying a real alien transmission are very long.

Since 1960, there have been over 50 searches for intelligent signals from space, initially at radio wavelengths but latterly looking for laser pulses. Neither has produced definite detections.

Even optimistic scientists put the chance that Seti@home will find an extraterrestrial signal at less than 1%.

On-the-spot analysis of data during the Arecibo observing run will allow the astronomers to re-target any especially promising signals.

A more detailed assessment of the Stellar Countdown results will be conducted offline after the Seti@home team returns to the University of California at Berkeley.

Best bets

Dan Werthimer, chief scientist of Seti@home, will lead the team conducting re-observations at Arecibo.

The researchers will observe the sky eight hours each day, staggering the time of day for each session to cover as much sky as possible.

The list of the most promising signal candidates far exceeded 150, but the project was allotted only 24 hours from March 18 to 20 to use Arecibo, making it impossible to examine all of the leads at this time.

The candidate radio sources were chosen on the basis of several criteria:

• number of times the radio source was detected
• how closely different observations resemble each other
• strength of radio source
• proximity to known stars
• type of star
• the presence of known planets