Seti@home, developed at the University of California at Berkeley and sponsored in part by the Planetary Society, officially launched last May 17 - although beta testers and Unix users got an early crack at it. Since then, the number of personal computers running the program has rapidly increased.
Users from 226 countries and territories around the world have devoted more than 260,000 years of computer time to the effort, organizers say. Versions of the program are available for Windows, Mac, various flavors of Unix, the Be operating system, OS/2 and more.
Here's the basic scheme behind SETI@home: Radio signals received by the world's biggest telescope dish - the 1,000-foot Arecibo Observatory - are transmitted to Berkeley, carved into 350-kilobyte "work units" and distributed by a central computer to users over the Internet. The free client software works like a screen-saver, analyzing the data in the background or when your computer is otherwise idle.
After a few hours or a few days of data-crunching, depending on how high-powered your computer is, the results from the work unit are uploaded, then a new work unit is downloaded to start the cycle again.
When the results are returned to Berkeley, particularly interesting signals - for example, strong and steady tones in a narrow frequency band - are flagged for further analysis. For 40 years, SETI scientists have been engaged in an increasingly sophisticated search for such signals, believing that they could represent an intentional transmission from an advanced civilization beyond our solar system.
Berkeley astronomer Dan Werthimer, who is SETI@home's chief scientist, said this month that analysts have followed up on some strong signals flagged by the software but hadn't come up with any unexplained spikes. He said the spikes turned out to be either intentional test signals, radio interference from earthly transmitters or orbiting satellites - or spurious computer data that didn't match up with the actual Arecibo readings.
In the past, some SETI researchers have expressed optimism that, with the rush of technology, alien signals could be found within a decade. But Werthimer is thinking more in terms of 50 years.
"I would say right now earthlings have a long way to go," he said in an interview last summer. "We're a fairly primitive civilization. Also, if you look at the parts of the spectrum that we're covering ... it would be really easy to miss signals."
This first run at SETI@home was scheduled to run two years, and Werthimer expects that the program would take a break for more detailed data analysis. There would be almost certainly be an expansion of the program, he said, bringing in more data from a SETI project in the Southern Hemisphere and/or widening the spectrum being searched.
Project director David Anderson agrees that the odds of finding an alien signal this time around are, well, astronomically low.
"My most likely scenario is that 50 years from now, we'll have a giant radio telescope out in the outer solar system, and it's quite possible by then that the desktop computer will be as fast as all the (mainframe) computers we're using in SETI@home," he said. "I think when we get those two things the odds will go to 50 percent."
Werthimer is also involved in an optical SETI project, which looks for flashes in the visible-light spectrum instead of radio wavelengths; and the 1HT project, which seeks to build a network of backyard-style radio dishes with the observing power of a single dish measuring 1 hectare (2.47 acres) in area. But thanks to his student aides, Werthimer is able to keep those projects going while he continues to blaze a trail with SETI@home.
"I think this may be the way that we'll do SETI in the future," he said.