Early SETI: Project Ozma, Arecibo Message
Project Ozma
In 1960, radioastronomer Frank D. Drake, then at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, carried out humanity's first attempt to detect interstellar radio transmissions. Project Ozma was named after the queen of L. Frank Baum's imaginary land of Oz -- a place "very far away, difficult to reach, and populated by strange and exotic beings." The stars chosen by Drake for the first SETI search were Tau Ceti in the Constellation Cetus (the Whale) and Epsilon Eridani in the Constellation Eridanus (the River), some eleven light years (66 trillion miles) away. Both stars are about the same age as our sun.
From April to July 1960, for six hours a day, Project Ozma's 85-foot NRAO radio telescope was tuned to the 21-centimeter emission (1420 MHz) coming from cold hydrogen gas in interstellar space. A single 100 Hz channel receiver scanned 400 kHz of bandwidth. The astronomers scanned the tapes for a repeated series of uniformly patterned pulses that would indicate an intelligent message or a series of prime numbers such as 1, 2, 3, 5 or 7. With the exception of an early false alarm caused by a secret military experiment, the only sound that came from the loudspeaker was static and no meaningful bumps superimposed themselves on the formless wiggles on the recording paper. After Project Ozma's pioneering steps, systematic searches for the technological manifestations of civilizations on the planets of other stars became a feasible scientific objective.
Arecibo Message
In 1974, the most powerful broadcast ever deliberately beamed into space was made from Puerto Rico. The broadcast formed part of the ceremonies held to mark a major upgrade to the Arecibo Radio Telescope. The transmission consisted of a simple, pictorial message, aimed at our putative cosmic companions in the globular star cluster M13. This cluster is roughly 21,000 light-years from us, near the edge of the Milky Way galaxy, and contains approximately a third of a million stars.
The broadcast was particularly powerful because it used Arecibo's megawatt transmitter attached to its 305 meter antenna. The latter concentrates the transmitter energy by beaming it into a very small patch of sky. The emission was equivalent to a 20 trillion watt omnidirectional broadcast, and would be detectable by a SETI experiment just about anywhere in the galaxy, assuming a receiving antenna similar in size to Arecibo's.
The message consists of 1679 bits, arranged into 73 lines of 23 characters per line (these are both prime numbers, and may help the aliens decode the message). The "ones" and "zeroes" were transmitted by frequency shifting at the rate of 10 bits per second. The total broadcast was less than three minutes. A graphic showing the message is reproduced here. It consists, among other things, of the Arecibo telescope, our solar system, DNA, a stick figure of a human, and some of the biochemicals of earthly life. Although it's unlikely that this short inquiry will ever prompt a reply, the experiment was useful in getting us to think a bit about the difficulties of communicating across space, time, and a presumably wide culture gap.
Visit the Arecibo Observatory Homepage