Right now, radio telescopes are scanning the galaxy for a transmission from extraterrestrials. The SETI Institute and other organizations around the world have been listening for roughly half a century. The Search for Extraterrestrial Intelligence (SETI) has become more important with the discovery of hundreds of new planets around distant stars. Somewhere out there, there could be another civilization.

The whole concept of SETI relies on the idea that extraterrestrials are transmitting signals into the universe. Mostly, it's assumed that these signals are deliberate attempts to communicate with other worlds. ET wants to talk to us. Or so we think. As with so many issues in SETI, there are plenty of questions and not enough answers.

We don't know if extraterrestrials are willing or able to talk to us. But we're even less sure about whether or not we should be talking to them. SETI has no problem with listening in to the universe, and even potentially eavesdropping on transmissions that were never meant to reach us. However, the SETI community is more divided on whether we should send our own messages, before or after we are contacted by extraterrestrials.

Messages have been beamed out to space in the past. There's a famous digital pictogram sent out from the huge Arecibo radio telescope in 1974, which was aimed at the M13 globular star cluster. It will reach its target in around 25,000 years.

In recent times, someone has deliberately sent a message out into space roughly every two years. Some of these have been scientific outreach programs for young people. The most entertaining example came in 2008, when NASA transmitted the Beatles song "Across the Universe" into space using the huge dishes of its Deep Space Network!

Moving much slower than the speed of light, we have messages on spacecraft heading into interstellar space. Pioneers 10 and 11 carry stylised images of humans, a DNA helix and information on our solar system. Voyagers 1 and 2 carry gold records with sounds and images of Earth. There's a faint chance that at some point in the future, someone will find them.

It seems natural to assume that if we want to hear from extraterrestrials, we should be trying to talk to them ourselves. It hardly seems fair to expect them to do all the work. And if nobody talks, then how can anyone expect to hear from anyone else?

Nevertheless, sending messages into space is controversial. How do we decide what to say? And who should make the decision? So far, there has been no national or global coordination of these transmissions. Rules and codes of conduct have been drawn, and while they have been adopted by some major SETI groups, they have no legal force.

Despite the lack of solid rules, a certain level of caution has still been practiced. All message projects have tried to avoid controversial subjects, focusing mainly on sending greetings, messages of goodwill and scientific data. Issues such as war and other negative subjects are avoided. Censorship is a controversial and contested subject when humans communicate amongst themselves.

How can we decide what should and should not be said to the rest of the universe? Organisations such as the SETI Institute study the issue of how to communicate with extraterrestrials very carefully. The issues of how and what to say are regularly examined in books, journals and conferences on SETI. They may think wildly at times, but most SETI scientists aren't so trigger-happy with their transmitters.

Some of the reluctance to transmit is strategic. In this school of thought, the Earth remains hidden and safe as long as we stay silent. To announce our existence to the galaxy is to invite an attack by hostile aliens. This theory is losing its appeal as we consider the amazing discoveries that have been made of planets in other star systems.

"We haven't been searching for long, and we already know of hundreds of distant worlds. When our technology improves, we will be able to see them in more detail, and examine them for evidence of life. Advanced alien societies will probably be able to do the same. It seems increasingly difficult to hide the Earth. Let's not forget that even if we don't send any special transmissions to ET, we are still pumping out radio signals from our own telecommunications.

Right now, the general perspective amongst those who listen for ET is not to talk. This partially seems to be motivated by a desire to avoid controversy. Simply listening for aliens is controversial enough in some circles. The US government axed funding for NASA's own survey. Trying to talk back would generate more questions from more people than are currently debating our search strategy.

Working out what to say is somewhat moot if we do not know how to encode a message. This is one reason why some messages are so simple in their structure. Apart from language differences, there are cultural and cognitive issues that could divide us. Some scientists fear that any extraterrestrial civilization we discover could be dead by the time we receive their message.

It could take centuries or even longer for messages to cross interstellar space. In the meantime, a civilization could destroy itself through war, disease, anarchy or any of the other apocalyptic scenarios we can imagine. Space itself poses all sorts of risks from killer asteroids to gamma ray bursts. Simply staying alive in a hostile universe could be an achievement in itself. If this is so, a series of transmissions to the galaxy could be a long-lasting legacy for the human race.

Should we transmit now, before we hear from anyone else? Should we remain silent even if we receive a signal? What should we say? What shouldn't we say? Who should control the transmissions? These questions won't be resolved to absolute satisfaction in the future. They will continue to remain controversial as long as our interest in the universe continues.

Dr Morris Jones is an Australian space analyst writing for SpaceDaily.com since 1999. He is a contributor to the new SETI book "Astrobiology, History and Society" from Springer. Read a free preview here.