In 1960, Cornell University astronomer Frank Drake pointed for the first time a radio telescope to the stars Tau Ceti and Epsilon Eridani starting the search for possible alien civilizations named Project Ozma.

He used the 26 meters Green Bank radio telescope tuning the receiver to the 1420 MHz frequency (wavelength of 21 cm), corresponding the spectral line of the hyperfine transition of neutral hydrogen. The reasons of this choice are essentially two: first of all the atmosphere of the Earth is almost transparent to that wavelength and, secondly, since the hydrogen is the most abundant chemical element in the Universe, it has been thought that other intelligent civilizations would have likely used that frequency to send pulsed signals to space.

One year later, once again in Green Bank, the first conference dedicated to SETI (Search for ExtraTerrestrial Intelligence) was held. The project, that is still active, has the main aim of looking for alien messages. During that conference, Drake wrote down the famous equation that is associated with his name, providing for the first time an estimate of the number of advanced civilizations that could coexist with us in the Milky Way.

In 1974 a message was sent from the Arecibo radio telescope through the globular cluster M13: in this message, composed in binary code and arranged as an array of 23 x 73 elements, crucial indications about our civilization were present:

  • the numbers from 1 to 10
  • the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus
  • the chemical formulas of sugars and basis of DNA nucleotides
  • a graphical representation of the DNA double helix
  • a graphical representation of a human and the dimensions of an average man
  • the Earth population
  • a graphic representation of the Solar System
  • a graphic representation of the Arecibo radio telescope

On 15th August 1977 a radio signal lasting 72 seconds has been received by the Big Ear radio telescope. This message possessed some of the features possibly related to an artificial origin, first of all the absence of replication and such a strong intensity that has never been observed by that telescope. This sequence is often called "Wow signal!", from the word that the astronomer Jerry R. Ehman wrote on the side of the printout, and its origin has been the subject of a long standing debate. However, recent studies favour a natural explanation for that signal.