The Wow! signal: detected in 1977, it has never been repeated (Image: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO))

In 1950, Nobel prizewinning physicist Enrico Fermi posed his famous paradox: if extraterrestrial intelligence exists, why haven’t we found it?

Why indeed? It is not as if we haven’t been trying. The search for extraterrestrial intelligence (SETI) has been going on for over half a century. It has mostly drawn a blank. But once in a while there is a flurry of excitement. Here are some of the highlights.

Read more: “Is the answer to life, the universe and everything 37?”

First contact

On 8 April 1960, Cornell University astronomer Frank Drake pointed a 26-metre radio telescope at two nearby stars. The telescope – based at the US National Radio Astronomy Observatory (NRAO) in West Virginia – was tuned to a frequency of 1420 megahertz, the wavelength of radiation naturally emitted by hydrogen in space. Thus began Project Ozma, the first experiment explicitly designed to look for aliens.

Drake was hoping to detect radio waves sent by an extraterrestrial civilisation. He chose the emission frequency of hydrogen because it is the most abundant element in the universe, and hence an obvious signal for any intelligent civilisation trying to get itself noticed by another.

Although the stars – Tau Ceti and Epsilon Eridani – were considered promising candidates, being nearby and sun-like, Project Ozma detected nothing in over 150 hours of observation.

In 1972, astronomers at NRAO had a second go, this time using a bigger telescope that collected as much data in a minute as the older one …