SETI@home: The Screensaver That Searched for Aliens for 21 Years

Radio SETI produces far more data than any one institution can analyze, and in the late 1990s the world's idle desktop computers were the largest unused computer on Earth. SETI@home, launched on May 17, 1999 by the University of California, Berkeley's Space Sciences Laboratory under David Anderson and Dan Werthimer, put the two together: a screensaver downloaded a small chunk of radio-telescope data, searched it for narrow-band signals while the computer sat idle, and sent the results back.

The data came from the SERENDIP receiver piggybacking on the Arecibo Observatory in Puerto Rico, recording whatever the giant dish happened to be pointed at. Each work unit covered about 107 seconds of a narrow band; the client searched it for Gaussian-shaped rises in power, pulses, and triplets that a sky-fixed transmitter would produce as the beam swept past. In 2002 the software was rebuilt on BOINC, the Berkeley Open Infrastructure for Network Computing, which went on to host dozens of other volunteer science projects.

More than five million people in over 200 countries ran it, making it the largest computation in history at the time. It found no alien signal. The nearest thing to a result was a 2004 candidate, SHGb02+14a, near the hydrogen line, which New Scientist reported as the best candidate yet and which the project team quickly downplayed as an instrumental or noise artifact; it was never seen again. Like every SETI effort before and since, the project's scientific product was a set of limits: what is not out there, at what strength, in the sky Arecibo saw.

In March 2020 the project stopped sending out new work, announcing that it had reached a point of diminishing returns and needed to focus on analyzing twenty years of results. Arecibo's instrument platform collapsed that December. The search moved to Breakthrough Listen, the $100 million program launched in 2015 with the Green Bank and Parkes telescopes and far more computing power than volunteers could offer, and BOINC continued with projects such as Einstein@home. SETI@home remains the moment the public was invited to join the search, one screensaver at a time.