Tuesday marks the 25th anniversary of the first deliberate radio message to the stars. The intention was to make contact with alien life - but we weren't really expecting an answer.

The signal was sent to celebrate a major upgrade of the Arecibo radio telescope in Puerto Rico on the afternoon of 16 November, 1974.

The message was just under 3 minutes long and contained some very basic information about the human race.

It included a representation of the fundamental chemicals of life, our genetic molecule DNA, and crude diagrams of our Solar System, a human being and the Arecibo itself.

Of course, the signal is still out there - it is just 25 years into a 13,000-year journey to the M13 globular star cluster which consists of 300,000 stars.

"It was strictly a symbolic event, to show that we could do it," explains Donald Campbell, Cornell University professor of astronomy, who was a research associate at the Arecibo Observatory at the time.

Tremendous power

The purpose of the message was to call attention to the tremendous power of the newly-installed radar transmitter at Arecibo and the ability of the telescope's giant dish antenna to project a powerful signal into space.

Would aliens be impressed with the message?

But many of those present took the event seriously, according to Harold Craft, Cornell's vice president for services and facilities, who was then director of the Arecibo Observatory.

"We translated the radio-frequency message into a warbling audio tone that was broadcast over speakers at the ceremony. When it started, much of the audience spontaneously got up and walked out of the tent and gazed up at the telescope."

While the audience that had gathered beside the huge Arecibo dish was impressed by the idea of sending messages to space, others were critical. Some actually suggested that sending such a message was dangerous, because it might attract the attention of hostile aliens.

But the chances of the message being detected by some extraterrestrial intelligence are extremely small.

So far the message has travelled only one thousandth of the distance to the globular cluster, or about 236 trillion kilometres. There are stars closer to our Solar System than that, but none of them is in the path of the message.

Computer modems

Ironically, the globular cluster at which the signal was aimed will not be there when the message arrives. It will have moved well out of the way in the normal rotation of the galaxy.

But the real point of sending the signal is that another version of the Arecibo telescope anywhere in our galaxy would be able to detect it. In one sense, the message marked our joining the club of space communicating species - and, for the moment, humans are the only known members of that club.

The 1974 message was transmitted on a frequency of 2380 MHz. It consisted of 1,679 binary bits representing ones and zeros, sent by shifting the frequency of the signal up and down over a range of about 10 Hz - a method similar to that used by computer modems to send binary code over a telephone line.

If the ones are translated into graphics characters and the zeros into spaces, the message forms a symbolic picture 23 characters wide by 73 long.

Seti (Search for Extra-Terrestrial Intelligence) is reaching an important phase. With increasingly powerful electronics and data processing capabilities, scientists searching for ET are able to carry out significant searches and draw definite conclusions about the possible distribution of life in the cosmos.