Wednesday, March 31, 1999 Published at 06:44 GMT 07:44 UK
A signal from ET?
Earth satellite or alien signal?
By BBC News Online Science Editor Dr David Whitehouse
This radio signal has been shifted in frequency to bring it into the audio range.
It was detected by an amateur space watcher using a small radio dish and PC-based equipment to analyse the signals.
The amateur is a member of the SetiLeague, a world-wide grouping of over a thousand alien hunters that operates a network of radio telescopes.
All day, every day, they are searching.
In reality, this signal is almost certainly from a man-made satellite orbiting the Earth. But as there are so many satellites up there, no one knows for sure what caused this strange signal. It has never been positively identified.
Most astronomers believe that the best way to send a message in-between the stars is to transmit a radio wave.
There is also some merit in the argument that flashes of laser light may be the best way to signal between the Earth and the very closest stars to us.
But it is with radio telescopes that the main searches have been undertaken. So far nothing has been detected.
That is not to say no suspicious signals have been picked up - just that none of them have passed the stringent criteria to be considered as being from an intelligent alien source.
Astronomers have many strange signals in their files that they cannot explain, but none of them are persuasive.
Perhaps the most impressive was detected in 1977 by the so-called 'Big Ear' radio telescope at Ohio State University.
When Jerry Ehman, the duty astronomer saw it come off the printer he scribbled "wow" next to the readout - since then it has become known as the Wow signal.
Alas it has not been repeated.
As well as the searches organised by the SetiLeague, professional astronomers are using the world's largest radio telescope to try to pick up a signal from the cosmic static. Again, nothing important has shown up so far.
But with the rapid advances in electronics and signal processing techniques, the searchers of the skies are able to double their sensitivity to signals every 200 days or so.
Because of this, many of them believe that if a detection is going to be made, it will be made soon.
However, if after a few more years of searching, the cosmic silence is held, it will be an indication that intelligent life may be frustratingly rare in the cosmos.
Either that or the aliens are not transmitting.
Next month, a new initiative to search for ET is launched. It is called Seti@home and it will use a modified screen saver.
PC users all over the world will have access to software that will look for alien signals in segments of data downloaded from the world's largest radio telescopes.