The SETI at home project, one of the largest citizen science experiments in history, is approaching its conclusion. For 21 years, millions of volunteers around the world helped search for signs of extraterrestrial intelligence by processing data from the Arecibo radio telescope. Now astronomers are using the world’s largest radio telescope, FAST in China, to examine the final 100 most intriguing candidates. If none of them proves to be an alien signal, the project will officially close one of the most ambitious chapters in the history of the search for extraterrestrial life.

The scale and results of the project

SETI at home was launched in 1999. Participants downloaded a small program that used their computers’ idle processing power to analyze radio data collected by the Arecibo telescope. Over the years, the project identified about 12 billion narrowband candidates, short bursts of energy at specific frequencies coming from specific points in the sky.

David Anderson, a cofounder of the project and a computer scientist at the University of California, Berkeley, explained that these were brief energy spikes at a particular frequency originating from a precise location in the sky.

For a long time, the team did not know how to handle such a massive volume of data. Only by 2016 did algorithms appear that were capable of filtering out radio frequency interference. At the Max Planck Institute supercomputing center in Germany, the 12 billion signals were first reduced to one million, then to one thousand. After manual inspection, only 100 candidates remained. These are now being studied by FAST, the only telescope in the world capable of such detailed follow up observations after the destruction of Arecibo in 2020.

The last chance to find extraterrestrial intelligence

Observations with FAST began in July 2025 and are still ongoing. Experience from previous decades shows that almost all candidates eventually turn out to be local radio interference. Nevertheless, scientists are treating the final stage with the utmost seriousness.

Anderson said that even if nothing were found, the project would at least establish a new level of sensitivity, because any signal above a certain power would have been detected.

Eric Korpela, another cofounder of the project, noted that they had carried out the most sensitive narrowband search over significant portions of the sky and had the best possible chance to find something, adding that it was naturally disappointing that nothing had been discovered so far.

Lessons for the future

SETI at home became not only the largest citizen science project, but also an important lesson for the entire field. In 1999, computers were thousands of times weaker than today’s machines, and the developers had to make compromises in how data were analyzed.

Korpela admitted that some mistakes were made and that these were conscious decisions dictated by the limitations of computer performance at that time.

He believes that in the future it would be worth returning to the SETI at home data using modern algorithms and machine learning, because something might have been missed.

The results of the project were published in two papers in 2025 in The Astronomical Journal. One focused on the analysis methods, and the other on data collection and processing.

In brief

SETI at home, which operated from 1999 to 2020, identified 12 billion candidate extraterrestrial signals. After years of filtering, only 100 of the most promising remain. They are now being checked by the Chinese FAST telescope, representing the final chance to find something real. Even if nothing is detected, the project will have set a new sensitivity benchmark. Scientists emphasize that even without discovering extraterrestrial intelligence, SETI at home provided invaluable experience and demonstrated how enormous volumes of data can be analyzed with the help of millions of volunteers.