By Dr David Whitehouse
BBC News Online science editor
Astronomers have completed their most sensitive search yet for radio signals from intelligent life in space.
Arecibo scanned the skies for ET
They believe the best way to find ET is to look for a radio signal. Such signals can travel vast distances.
The Arecibo radio telescope in Puerto Rico, supported by Jodrell Bank, searched over a period of 10 years.
The scientists looked at 800 nearby stars with no evidence of a signal from ET. They say they have learned a lot, and plan another search next year.
From the ashes
The last star scrutinised by Project Phoenix - the most powerful search for intelligent life in space ever carried out - was HD 169882, a fairly ordinary star lying just 88 light-years away.
The result was that no signals indicative of an intelligent origin are coming from it, at least during the time it was observed.
So if there are any aliens on a planet circling that star then perhaps they are not interested in signalling, or are doing it in a way we cannot yet detect.
Project Phoenix was so-named because it rose from the ashes of a US space agency (Nasa) initiative to search for intelligent life in space that was cancelled by US Congress in 1993.
Despite this setback, the scientists involved were determined to carry out their search.
"When the 'termination' order came from Washington, most of the equipment was on lab benches. We were immediately faced with three challenges: raise private money, get Nasa to loan us the equipment and get it working," Peter Backus, project manager for Phoenix, told BBC News Online.
After the initial scramble, the scientists managed to get an Extra-Terrestrial Intelligence (Eti) search system built and used it on the Parkes radio telescope in Australia in February 1995, just one month later than the original Nasa plan.
Much of Project Phoenix's time was spent on the world's largest radio telescope, the 330-metre dish at Arecibo, which takes advantage of the natural topography of Puerto Rico's mountains.
"Over the years we have observed about 800 nearby stars over billions of frequency channels at high sensitivity," says Backus.
Peter Backus plans more searches
"No other search covered as many frequencies or achieved the same sensitivity. It was the only search capable of detecting ET transmitters with power comparable to our own military radars."
One of the problems in looking for signals from intelligences in space is that signals from Earth can interfere, so the scientists have to have a reliable way of discriminating between ET and terrestrial interference.
Phoenix pioneered a technique of "real-time interference monitoring" using a second radio telescope to determine if any suspicious signal was actually coming from deep space.
No suspicious signal survived that test, but the astronomers are not down-hearted; they know that ET could be detected tomorrow, in a thousand years, or never.
They say a search with an outcome that could be one of the biggest scientific discoveries of all time is worth the effort.
"We've learned a lot about searching for Eti. We'll carry those lessons and the new search system to the Allen Telescope Array (ATA)," Backus adds.
"Later this year, we'll be using the ATA with 32 small dishes. As the array expands, we'll start a new targeted search covering several hundred thousand stars.
"As I look back over the past 10 years I'm very proud of what we have achieved - the most sensitive and comprehensive search of our galactic neighbourhood.
"Conclusion: we live in a quiet neighbourhood."