Extraterrestrial Intelligence: An International Petition

In a paper in 1982 [1], astronomer and science communicator Carl Sagan, together with 68 other leading scientists in the fields of astronomy and biology, published a letter in the prestigious research journal Science petitioning for an international coordinated systematic search for extraterrestrial intelligence (ETI) to begin immediately.

Here are my notes on this letter. You can find my notes on other classic SETI papers in my roundup of the technosignatures literature.

The result of a thorough search for ETI would be profound

Sagan et al. argued that public support existed for a search for ETI and that either a positive or negative result would have profound implications for our view of the universe. A systematic international effort would be low cost and a million times more thorough than any search done thus far.

The technology for detecting ETI is already available

Technology in the 80s was already able to detect radio signals from extraterrestrial civilisations, with an equivalent level of technology to our own, 1000s of light years away.

This is still true today and when the square kilometer array (SKA) is operational it will increase the searchable volume of our galaxy by a factor of 1,000.

The absence of evidence for advanced extraterrestrial civilisations is not enough

By 1982, there were several publications that argued that because intelligent extraterrestrial beings were not already here, they do not exist [2,3]. Sagan et al. argued that this line of reasoning relies heavily on extrapolation of technological capabilities. A radio search, however, assumes technology no more advanced than our own. The authors concluded that a priori arguments are not a substitute for an observational program.

The search should begin now (in the 80s)

The authors argued that detecting radio signals from ETI will become harder as time goes on due to increasing radio frequency interference (RFI) generated by humanity. They petitioned for an observational program to begin immediately.

Certain radio frequency bands are still protected for radio astronomy (including bands 1400–1427 MHz and 1660.6–1670.0 MHz that sit in the ‘water hole’, which is 1420 to 1662 MHz and argued to be a strong candidate band for beacons from ETI). This protection is orchestrated by the International Telecommunication Union (ITU). The increase in RFI has nevertheless certainly come to pass and, unforeseen in the 80s, has been exacerbated by the deployment of satellite constellations in low Earth orbit (you can see a 3D visualisation at this tracking site) – there are over 10,000 currently in orbit with many more planned. These have intentional transmissions in the unprotected radio bands as well as unintentional emissions in the low radio frequency range [4] – both can interfere with detecting faint radio signals from space.

Summary

This short letter petitioned for a systematic international search effort to detect radio signals from extraterrestrial intelligence. It summarised the main arguments and was signed by leading scientists across the globe.

References

1. 1. Sagan C: Extraterrestrial Intelligence: An International Petition. Science 1982, 218:426–426.
2. 2. Hart MH: Explanation for the Absence of Extraterrestrials on Earth. Quarterly Journal of the Royal Astronomical Society 1975, 16:128.
3. 3. Tipler FJ: Extraterrestrial Intelligent Beings Do Not Exist. Quarterly Journal of the Royal Astronomical Society 1980, 21:267–281.
4. 4. Vruno FD, Winkel B, Bassa CG, Józsa GIG, Brentjens MA, Jessner A, Garrington S: Unintended Electromagnetic Radiation from Starlink Satellites Detected with LOFAR between 110 and 188 MHz. Astronomy & Astrophysics 2023, 676:A75.