AMATEUR radio hams are usually excited by the faint buzz of a distant shortwave station, but a group of scientists believe they have received a message from extra-terrestrials.
Astronomers think that a signal picked up by a radio telescope last year shows the highest probability yet that ET’s family may have returned his call.
In February 2003, scientists involved in the search for extra-terrestrial intelligence (SETI) pointed the huge radio telescope in Arecibo, Puerto Rico, at about 200 sections of the sky.
Unexplained radio signals had been detected twice by the same telescope in these areas and scientists were trying to confirm the findings.
It may sound fanciful, but a report in the journal NewScientist reveals how the team has now finished analysing the data, and all the signals seem to have disappeared - except for one which has got stronger. Detected on three separate occasions, the signal is "an enigma", say researchers.
So far, explanations have included conjecture that it could be generated by a previously unknown astronomical phenomenon, or may even be something far more pedestrian, such as an artefact on the telescope itself interfering with measurements.
But the astronomy team says that it also happens to be the best candidate yet for a contact by intelligent aliens in the six-year history of the SETI@home project, which uses programmes running as screen-savers on millions of personal computers worldwide to sift through signals picked up by the Arecibo telescope.
Dr Dan Wertheimer, a radio astronomer at the University of California (Berkeley) and the chief scientist for the project, said: "It is the most interesting signal from SETI@home. We are not jumping up and down, but we are continuing to observe it."
Named SHGb02+14a, the possible alien communication has a frequency of about 1420 megahertz - one of the main frequencies at which hydrogen, the most common element in the universe, readily absorbs and emits energy.
Some astronomers have suggested that aliens trying to announce their presence would be likely to transmit at this frequency, and SETI researchers regularly scan this part of the radio spectrum.
The unexplained signal appears to be emanating from a point between the constellations of Pisces and Aries, where there is no obvious star or planetary system within 1,000 light years, and the transmission is also very faint.
Dr Eric Korpela, of the research team, said: "We are looking for something that screams out ‘artificial’. This just doesn’t do that, but it could be because it is distant."
So far, the telescope has managed to pick up the signal for only about a minute in total, which is not sufficient for astronomers to analyse it fully.
Dr Korpela believes that it is unlikely the "message" is the result of any obvious radio interference or noise, and it does not resemble any known astronomical object.
Others, however, are more sceptical, saying the current lack of explanation does not mean that it could only have been produced by aliens.
Dr Jocelyn Bell Burnell, of the University of Bath, said: "It may be a natural phenomenon of a previously undreamed-of kind - like I stumbled over."
It was Dr Bell Burnell who, in 1967, observed a pulsed radio signal which the research team at the time believed was from extra-terrestrials, but which later was confirmed as the first sighting ever of a spinning collapsed star.
Other questions arise over the signal’s frequency, which oscillates by between eight and 37 hertz a second.
Paul Horowitz, a Harvard University astronomer who looks for alien signals using optical telescopes, believes that the drift in the signal makes it "fishy".
David Anderson, the director of the SETI@home project, is also sceptical but curious about the signal. He told NewScientist: "It is unlikely to be real, but we will definitely continue to observe it."
Meanwhile, a new analysis of interstellar communications claims that, rather than sending radio signals, aliens would find it far more efficient to send a "message in a bottle".
Scientists at Rutgers University in New Jersey claim that beaming a radio signal that can be detected 10,000 light years away would demand a million billion times as much energy as just shooting out matter on which the data is inscribed.