The Search for Extra-Terrestrial Intelligence (SETI) dates from 1960, when Frank Drake first used a radio telescope to look for signals coming from the direction of the sunlike stars Tau Ceti and Epsilon Eridani in Project Ozma. Since that time, more than a dozen searches have been carried out, looking for radio signals at different wavelengths.
In 1992 NASA initiated the High Resolution Microwave Survey. This aimed to simultaneously scan millions of radio channels in the microwave region of the spectrum. It used a 34 metre antenna in California and the 305 metre antenna at Arecibo in Puerto Rico to begin a survey of the whole sky as well as selected individual targets.
One year later, the US Congress withdrew funding and the project closed down. Project Phoenix re-established the targeted search in 1995 but is now entirely funded through private donations. It initially used the 64 metre antenna at the Parkes Observatory in Australia and now uses the refurbished Arecibo telescope in conjunction with the 76 metre Lovell telescope at Jodrell Bank in England (above) to eliminate unwanted terrestrial signals from the search.
To date (July 2000) no convincing extra-terrestrial signal of artificial origin has been detected. However, even a single unambiguous detection would be of such enormous value that many astronomers feel a continuing search is worthwhile.
PC owners are now able to assist with SETI projects. The University of Berkeley runs a SETI program called Project SERENDIP which uses data collected with the Arecibo telescope. In order to accelerate the processing of data from this project, they encourage home computer owners to download a SETI@Home screensaver. This software works as a normal screensaver, but processes the raw data to search for signals and sends it back to the project base once each data set is analysed. The project leaders promise to give credit to any individual whose screensaver discovers a signal from an extraterrestrial civilisation.
Other methods of interstellar communication are less promising. Currently there is little prospect of humans travelling the enormous distance to even the nearest stars to make contact with other civilisations, as the resources required are so great. There is also no evidence that intelligent extraterrestrial life has visited the Earth.
However, signals from television and radio stations have been broadcast into space for more than 70 years. This means that they could be detected out to a distance of 70 light years from the Earth, giving an impression (perhaps not the most accurate one!) of the life of its inhabitants.
The only deliberate attempt to broadcast into space took place in 1974, when the Arecibo telescope was rededicated. The signal was directed to M13, a globular cluster in Hercules containing nearly 200,000 stars. Unfortunately it lies at a distance of 24,000 light years from the Earth so any reply will not be received for nearly 50,000 years!