Radio SETI – the “water hole”

  • In the vernacular, a “water hole” can refer to a place where people come to meet and talk.
  • In radio SETI, the “listening” must focus on parameters including a radio frequency /frequency band and a target/hypothetical source.
  • Suppose that the radio SETI target, and so the “aim”, has been selected (no small task in itself).  Now … what frequency to listen to?  This can call for a guess – what frequency (or frequencies) might an intelligent race beyond the Solar System choose to broadcast from?
  • An intriguing “guess” in this regard is a frequency band known as the water hole / waterhole – potentially, a “place” (frequency band) “where” interstellar intelligent neighbors can “meet” and “talk”.  This band has been the focus of much “listening” in radio SETI.
  • The waterhole (a term coined by Bernard Oliver in 1971) is the frequency band between 1420 and 1666 megahertz, which corresponds to wavelengths of 21cm and 18cm respectively.
  • Why this frequency band?  In radio telescope “listening”, these two wavelengths correspond to the strongest spectral lines for the hydroxyl radical, HO (the neutral form of the hydroxide ion HO− ) at 1420 megahertz, and the hydrogen atom, H. at 1666 megahertz.  Combined, these two form water (H2O), which is essential to all life on Earth.  The selection of the water hole is based in part on the guess that water is also essential to (hypothetical) life beyond the solar system (or at least recognized by intelligent life beyond the solar system as being important to at least some forms of life in the universe).  As such, the guess is that our hypothetical interstellar neighbors might, operating on similar reasoning to our own, choose to broadcast at this frequency band.  Moreover, the water hole is an especially “quiet” band, so there is relatively little “noise” to interfere with hypothetical intentionally-sent communications.
  • Resources include:,,
Microwave window, as seen from a ground-based system.  From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence
Microwave window, as seen from a ground-based system. Public domain image, from NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence, available at

© 2015 Fosdick EDS  ☾><(((°>

Leave a Reply