The search for alien life is no longer the province of science fiction. Alastair Gunn reports on a new Anglo-American collaboration which, this week, takes us one bold step nearer to meeting our intergalactic neighbours
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ASK A SCIENTIST what humankind's most important discovery could be, and somewhere in the top five will be the detection of extraterrestrial life. Far from being beyond the bounds of respectable science, the Search for Extra-Terrestrial Intelligence (SETI), has been an active research project for nearly 40 years. This week, scientists in the UK and the USA begin the most sensitive and comprehensive search for extraterrestrial civilisations yet undertaken. The University of Manchester's huge 76-metre Lovell radio telescope is about to play host to astronomers from the SETI Institute in California in their attempt to detect faint radio signals from intelligent life elsewhere in our galaxy.

The question of whether we are alone in the universe is at least as old as history itself, but only in the modern era have scientists been able to attempt its answer. In 1959, Cornell physicists Giuseppi Cocconi and Philip Morrison published a cornerstone paper pointing out the potential for using radio waves to communicate across space. The possibility that life could have evolved elsewhere in the universe and reached the technological development necessary to attempt communication using radio waves was taken very seriously. The next year, a young radio astronomer, Frank Drake, conducted the first search for extraterrestrial signals. Drake's experiment targeted two nearby stars, but failed to detect any repeating patterns that might indicate an intelligent origin. In the Sixties and Seventies, as the perception grew that SETI research had a reasonable chance of success, both Soviet and American scientists conducted comprehensive research programmes. In 1988, after years of planning, Nasa adopted a strategy for examining the 1,000 nearest Sun-like stars, in addition to performing a systematic survey of the entire sky. The observations began, as planned, on 12 October 1992, the 500th anniversary of Columbus's arrival in the New World. Unfortunately, within a year Congress terminated the project's funding. It was careful to point out that SETI was an interesting project and the quality of the science and technology was high, and the reason for the cut was essentially budgetary.

The SETI Institute, originally established to help run the American programme, took over much of the advanced technology designed and built by Nasa. The organisation is based in Mountain View, near San Francisco, California and is funded by private donations. Today, the SETI Institute has more than two dozen research programmes underway, including the world's most comprehensive and sensitive targeted search for extra-terrestrial radio signals. Frank Drake, the young man who began the hunt for ET intelligence, is now president of the Institute.

The new British/American collaboration starting this week is a continuation of the Institute's research programme called Project Phoenix. The University of Manchester's Nuffield Radio Astronomy Laboratories at Jodrell Bank are home to the Lovell telescope, the second largest fully-steerable radio telescope in the world. What makes the project so fascinating is the way in which SETI scientists are attempting to discriminate between signals of an extra-terrestrial nature and those that come from within our Solar System. The process involves the 305-metre US radio telescope based at Arecibo, in Puerto Rico. As Jodrell Bank's Ian Morison explains, "local signals can be eliminated by making simultane- ous observations at Arecibo and here at Jodrell Bank. Due to the rotation of the Earth and the large separation between the telescopes, a signal has to come from a great distance, from at least the outer part of the Solar System, to be seen by both telescopes simultaneously. In addition, any local interference will only be seen at one telescope and therefore easily eliminated." Jill Tarter, Director of the SETI Institute, believes that "these large telescopes and the new techniques give us the best chance of success we've ever had."

The questions of where to look for ET life, and more importantly, what kind of signals to look for, have occupied scientists for many years. Astronomers widely expect that other civilisations would be most likely to arise on planets moving around stars similar to the Sun. Although there is, as yet, no evidence for Earth-like planets around other stars, it is only relatively recently that planets such as Jupiter have even been found. There are plenty of stars stable enough to allow the development of life, should they have planetary systems with suitable planets. There is also the question of age. As Jill Tarter, who is heading Project Phoenix, explains, "we exclude stars younger than about 3 billion years so that if evolution elsewhere takes as long as it did on Earth, there is a chance that technology will be present." The Project Phoenix search concentrates on stars from which a signal would be strongest by targeting those within about 200 light years of Earth. SETI scientists often choose to tune their radio receivers to a frequency of 1420MHz. This is the natural emission frequency of hydrogen gas, the most common and simplest element in the universe. Astronomers reason that, to extra-terrestrial civilisations, this would seem a logical frequency at which to transmit. There are other sources of radio waves in the sky, including natural emissions from celestial objects and, unfortunately, man-made interference. SETI research is trying to identify those signals which originate outside the Solar System and which could not be the result of a natural process. Such a signal might very well be highly monochromatic, ie transmitted within a very narrow range of frequencies, and would repeat in a predictable way. The simultaneous use of telescopes at Jodrell Bank and Arecibo makes the job of identifying possible ET signals much easier.

The sophisticated radio receivers used in Project Phoenix swallow a large chunk of the spectrum of radio waves which can pass through the Earth's atmosphere. This frequency range (1000MHz to 3000MHz) is split up into hundreds of millions of very narrow channels, each one only 1Hz wide. A purpose-built computer searches each channel simultaneously for patterns in frequency or time that indicate whether a continuous or pulsed signal is present. Any prominent candidates are then looked at more closely to determine their origin and nature.

Ask SETI scientists about the chances of detecting aliens and they are naturally cautious. In their original paper of 1959, Cocconi and Morrison stated: "It is difficult to estimate the chance of success, but if we don't search, the chance of success is zero." That is the sentiment of most SETI researchers today, although, as Ian Morison points out, "our search is only sensitive to relatively small distances from the Sun so ET civilisations would have to be quite commonplace for us to detect them."

But there are theoretical grounds for believing that the universe may harbour other life-forms. In 1961, Frank Drake published a now famous equation which identifies specific factors thought to play a role in the development of ET civilisations. The Drake equation seeks to estimate the number of technological civili- sations that might exist by taking into account factors such as the rate of formation of suitable star systems, the fraction of possible planets that could harbour life and the lifetime of civilisations currently communicating by radio. Although many of these factors are essentially guesswork, optimistic guesses still indicate there would be hundreds of thousands of detectable civilisations within our own galaxy. Given the size of the universe there is little chance of discovering extra- terrestrial intelligence by direct physical contact and the attempt to overhear transmissions that have leaked away or been deliberately beamed into space from other planets is, at present, our only hope.

There is a common misconception that humankind's own radio transmissions are polluting the universe and could be detected by extra-terrestrial beings. However, since commercial broadcasts have only existed since about 1920, such civilisations would have to be within 78 light years for the signals to have reached them by now. By comparison, the Sun is more than 25,000 light years from the centre of our own Galaxy. Furthermore, television and radio broadcasts have relatively low power so their detection would require technology far more capable than our own. However, some defence radar systems generate high-power signals that could be detected by technology comparable to ours as far away as the limit of Project Phoenix. It is the SETI scientists' hope that we may pick up similar deliberate or accidental transmissions from other civilisations.

Astronomers are naturally reticent about the form that extra-terrestrial life might take, since their guess is as good as anyone else's. However, scientists are well aware that the discovery of extra-terrestrial beings would have a profound global impact. Jill Tarter predicts that, "if we detect another civilisation, its members will be so vastly different from us that the discovery will trivialise the dis- tinctions among humans that today lead us to hatred and violence." SETI researchers are also quick to point out that they have no hidden agenda nor obligation to keep quiet about the results of their experiments. The Institute recognises that its research is in the common interest of all humankind and has strict protocols for announcing results to both the scientific community and the media. Their ground rule is: "tell everybody". Studies to assess the public's likely reaction to such a discovery have concluded there would be confusion and excitement but little panic or hysteria. The long-term effects are more difficult to predict but will surely be profound.

The latest phase of Project Phoenix is another chapter in Jill Tarter's quest for the ultimate scientific goal. Having worked for the SETI Institute and the original Nasa programme, she is the holder of two Nasa Public Service Medals and a Lifetime Achievement Award for Women in Aerospace. Her career is strikingly similar to Ellie Arroway's - the scientist played by Jodie Foster in the film Contact - but Jill is cautious in drawing too many parallels: "I've experienced much of what happened to Ellie in the film, including the death of my father when I was young and the termination of government funding. Carl Sagan wrote the book about a woman who does what I do, not about me." Although Sagan's depiction of humankind's greatest discovery generally meets favour among SETI researchers, Jill adds with whimsical resignation, "I have not yet detected a signal!"

Despite the apparent lack of success so far there have been some close calls. In 1977, a telescope run by Ohio State University detected a very high-power pulse, now known as the "Wow" signal after the comment written by the observing scientist, Jerry Ehman, in the margin of the data printout. It had all the hallmarks of a signal from a remote source in space but was never seen again, despite many follow-up observations. Nevertheless, SETI scientists are excited by the possibility of success that new advances in technology have brought. Professor Andrew Lyne, Director of Jodrell Bank, says "This would be the most dramatic discovery ever made and we are glad we can make a contribution to this exciting quest." But Ian Morison, who is co-ordinating Jodrell Bank's participation, remains philosophical: "If after many years of searching we do not make any detection, then it could mean that life-bearing planets like ours are rare and we should do our best to nurture the Earth." Whatever the outcome, the collaboration between the SETI Institute and the University of Manchester opens an exciting new chapter in our civilisation's greatest quest.