The attempt to detect artificial transmissions from other civilizations in space. The first such attempt, called Project Ozma, was made in 1960 by the American radio astronomer Frank Donald Drake (1930– ), who observed two nearby Sun-like stars, Tau Ceti and Epsilon Eridani, with the 26-m dish at the National Radio Astronomy Observatory, West Virginia. Since then nearly 200 searches have been made with increasing sensitivity and enlarged coverage.
The two most sensitive searches have been Project SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) and Project Phoenix. SERENDIP is an ongoing all-sky survey that operates on the back of normal astronomical observations at the 305-m Arecibo Telescope in Puerto Rico, while Phoenix was a targeted survey between 1995 and 2004 which observed more than 800 likely stars within about 240 light years. Phoenix used the 64-m radio telescope at Parkes, Australia, the 43-m dish at the National Radio Astronomy Observatory, and the Arecibo Telescope in conjunction with the 76-m Lovell Telescope at Jodrell Bank.
The majority of searches have concentrated on that part of the radio spectrum where the background noise is a minimum, between the radio spectral lines of hydrogen (H) at 21 cm wavelength and hydroxyl (OH) at 18 cm. Since H and OH together make H2O, this region of the radio spectrum is termed the water hole. Radio signals deliberately sent to attract attention over interstellar distances are expected to be of narrow bandwidth, 1 hertz or less, and specialized receivers with millions of channels have been developed to detect them. Project BETA (Billion-channel ExtraTerrestrial Assay) scanned the water hole across all the sky visible from the 25.6-m radio telescope at Harvard University's Oak Ridge Observatory, Massachusetts, in 1995–9.
Another possible mode of communication is by brief (nanosecond) pulses of laser light which could outshine their parent star by 10 000 times. Searches for such visible pulses, termed Optical SETI, or OSETI, are now underway, notably an all-sky search using a purpose-built 1.8-m telescope at Oak Ridge Observatory which began in 2006. In 2007 the Allen Telescope Array, the first radio telescope designed and built specifically for SETI observations, began operations with 42 dishes. When completed it will consist of 350 individual antennas with a collecting area equivalent to that of a 114-m antenna and be capable of observing many stars simultaneously.
SETI can succeed only if there are civilizations elsewhere in the Galaxy transmitting towards us. The Drake equation attempts to estimate how many such communicative civilizations there might be. Early estimates were optimistic, suggesting that perhaps tens of thousands of them existed in the Galaxy. Most present-day estimates are more conservative, no more than a few hundred, and some astronomers suggest that we might well be the only advanced civilization in the Galaxy at present.
Although simple life may well be widespread throughout the Galaxy, intelligent life on Earth has taken billions of years to evolve, requiring long-term stability of climate. Planets that meet this condition may be very rare, and consequently any advanced civilizations would be few and far between. To have a realistic chance of making radio contact with distant civilizations would require an instrument more sensitive than any yet built. Future radio telescopes such as the Square Kilometre Array, scheduled for completion in 2024, may be required before there is a realistic chance of contact.
Subjects: Astronomy and Astrophysics.