Project Ozma

In 1960, Frank Drake carried out the first experiment to detect radio signals from from an extraterrestrial intelligence (ETI) based on a strategy proposed by Cocconi and Morrison in 1959 [1]. Drake published a rationale for his experiment, which he named Project Ozma, together with its method and results in an article in Physics Today in 1961 [2].

Here are my notes from reading this classic article.

I am tracking all of the papers I’ve read and want to read on this topic in this roundup of the technosignatures literature.

Search strategy

In one of the first SETI papers, Cocconi and Morrison suggested searching nearby sun-like stars for radio signals with frequencies around 1420 MHz. This frequency they argued is likely to be universally meaningful since it corresponds to the emission line of interstellar hydrogen. It is also possible for us to receive this frequency through the Earth’s atmosphere and distinguish it above background radiation for a sun-like star.

Drake uses this strategy to perform the first SETI experiment.

Rationale for SETI experiments

Drake presents the rationale for conducting his experiment.

Exoplanets must exist

At the time, no exoplanets had been discovered but astronomers expected them to exist. One reason for this is the low rotational velocity of nearby singular stars, which implies that the angular momentum they had when forming was transferred to planets in orbit. At the time, it was thought that 40% of stars were binary systems and that the angular momentum was transferred to the second star so, Drake argues, 60% of stars have planetary systems instead.

The ingredients for life arise spontaneously

Meteorites and laboratory experiments (the now-famous Miller-Urey experiment that simulate early earth’s atmosphere and generated amino acids) suggested that the ingredients for life arises spontaneously under the right conditions.

Drake discusses the habitable zone around other stars (the region that is the right temperature for liquid water) and hypothesises that stars with much lower luminosity than the sun would have a very narrow zone for planets to be habitable and therefore the chance of finding life around these stars is low.

Intelligent life elsewhere has had enough time to evolve

Stars are forming all the time and most of the nearby stars are young and metal-rich like the Sun and are therefore likely to possess planets with the potential to carry the ingredients for life.

Humans took approximately 5 billion years to evolve and other nearby stars have been around longerr than this so enough time has passed for a civilisation to emerge.

Focus on two nearby stars

Drake used the Green Bank Telescope (GBT) in West Virginia, USA, to listen for signals from two sun-like stars, Tau Ceti and Epsilon Eridani, both about 11 light years away from Earth.

A total time of 150 hours was used to observe these stars using a receiver that switches between two feed horns: one focussed on the star and the other the space near the star. This “on/off” method allows radio interference from Earth to be removed, since it will appear in both.

Results

No signals were detected during the experiment. Drake admits that the probability of finding a signal is low and that early success would be unexpected.

Summary

Project Ozma was the first SETI experiment. It looked for narrow band signals at the frequency of the hydrogen line from two nearby sun-like stars using 150 hours of telescope time. It was a small but pioneering experiment that was not expected to succeed but stimulated important discussions and further efforts in the search.

References

1. 1. Cocconi G, Morrison P: Searching for Interstellar Communications. Nature 1959, 184:844–846.
2. 2. Drake FD: Project Ozma. Physics Today 1961, 14:40–46.