It will be one of the greatest moments in science, and also one of the greatest moments in history. After decades of searching, a signal from extraterrestrials is received by a radio telescope on Earth. SETI (Search for Extraterrestrial Intelligence) analysts quickly check the transmission using other instruments, and prepare to announce the great discovery. The media descends on the story and soon millions of people around the world are reading the news. Then what?

Exactly how the world would react to the discovery of extraterrestrial intelligence has been the subject of much speculation. There could be a mixture of excitement, fascination, fear, confusion, disbelief, indifference and panic. Like emergency planners preparing for a catastrophe, scientists regularly assemble to consider ways that world at large would respond to such an event, and how to plan for the day when a discovery comes.

Psychologist and SETI scholar Doug Vakoch has been exploring this question for years, and recently gathered an eclectic team to explore the issue. The result is a large and detailed book, "Astrobiology, History and Society", which was recently released by the academic publisher Springer. A free preview of the book has been released online here.

Scientists and journalists have struggled with this problem for a long time.

"One of the best ways to prepare for the discovery of life beyond Earth is to understand how we've dealt with false alarms in the past," explains Vakoch. "History is rich with incidents when life beyond Earth was reported and widely believed. In the early nineteenth century, the astronomer John Herschel reportedly discovered intelligent "bat-men" on the Moon. The news spread widely, and the public was energized. There's only one problem: it was a hoax!"

"As we move from science fiction to science fact, we learn the same lesson: be cautious! When astronomers first discovered pulsars--super-dense stars that emit regular pulses as they spin rapidly--these scientists wondered whether they may have finally detected signals from extraterrestrials. After all, the signals they detected looked unlike anything they'd ever seen nature make before."

It is not entirely clear when the next big alert will come, but Vakoch hopes that we will be a little wiser than before.

"If we do detect a signal from extraterrestrial intelligence, we should expect some ambiguity. It probably won't be clear overnight whether or not this really is a signal from an advanced civilization. So our best preparation for the actual discovery of extraterrestrial life--whether intelligent or microbial--is to be patient, as scientists sift through the data to see whether it really points to the existence of life beyond Earth."

The discovery of a real transmission would probably provide no immediate answers to one of the most frequently asked questions about SETI: What do the aliens look like? The SETI community is usually reluctant to speculate on this issue, but this won't stop the public from using their own imagination.

Public perceptions of extraterrestrials are influenced by a variety of educational and cultural factors. It's fair to say that many people have their preconceptions of aliens shaped by Hollywood movies, which generally present them as hostile or dangerous. Traditional legends of monsters, humanoids and supernatural creatures stalking the Earth could also influence the way some people and cultures would react to a discovery.

The rise of online and social media is creating new dilemmas for the SETI community. Online channels will help to quickly circulate news of a SETI discovery, but they could also distort or corrupt the message.

Online hoaxes have already plagued the SETI community, and some have even fooled the mainstream media. Some analysts fear that pranksters could cause panic by suggesting that the extraterrestrials are planning or even carrying out hostile activities. False stories could go "viral" very quickly as people panic. The rise of cyber-attacks around the world has also raised fears of potential attacks on SETI computers and online accounts.

Issues such as cultural traditions and religion will also affect the way a discovery is perceived by different people.

One of the most controversial questions that will be raised after a discovery is the issue of sending a reply. This remains hotly contested amongst scientists and SETI researchers.

SETI is becoming more important today as scientists discover hundreds of "exoplanets" orbiting other stars. This suggests that solar systems that could support life could be more common than some scientists previously suspected.

While we wait for a signal, the SETI community will sharpen their theories in preparation for the big event.

Dr Morris Jones is an Australian space analyst who has written for SpaceDaily.com since 1999. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.