By Alan M. MacRobert and Govert Schilling

In 1961 astronomer Frank Drake wrote the equation that put the search for alien
civilizations on a scientific footing and launched the modern SETI movement.
How do the numbers look today?

Searching for extraterrestrial life has become a hot topic among astronomers, biologists, and the general public. But not many remember how the subject was jump-started more than 40 years ago.
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published a landmark article in the British weekly journal Nature with the provocative title, "Searching for Interstellar Communications." Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 centimeters (1,420.4 megahertz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe. Other intelligences might see this as a logical landmark in the radio spectrum where searchers like us would think to look.

Seven months later, radio astronomer Frank Drake became the first person to start a systematic search for intelligent signals from the cosmos. Using the 25-meter dish of the National Radio Astronomy Observatory in Green Bank, West Virginia, Drake listened in on two nearby Sunlike stars: Epsilon Eridani and Tau Ceti. His Project Ozma (named for L. Frank Baum's story Ozma of Oz) slowly scanned frequencies close to the 21-cm wavelength for six hours a day from April to July 1960. The project was well designed, cheap, simple by today's standards, and unsuccessful.

Following the Ozma experience, Drake organized a meeting with a select group of scientists to discuss the prospects and pitfalls of the search for extraterrestrial intelligence — nowadays abbreviated SETI. In November 1961, ten radio technicians, astronomers, and biologists convened for two days at Green Bank. Young Carl Sagan was there, as was Berkeley chemist Melvin Calvin, who received news during the meeting that he had won the Nobel Prize in chemistry.

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