We've tried phoning aliens - Nasa

Cosmologist Stephen Hawking says it's too risky to try to talk to space aliens, but he's too late - Nasa has tried it.

The US space agency and others have already beamed several messages into deep space, trying to phone extra-terrestrials.

Nasa, which two years ago broadcast the Beatles song Across The Universe into the cosmos, has been discussing its latest search strategy for life beyond Earth.

In its missions Nasa is focused mostly on looking for simple life like bacteria in Earth's solar system rather than fretting about potential alien overlords coming here.

Just days ago, Prof. Hawking said on his new TV show that a visit by extra-terrestrials to Earth would be like Christopher Columbus arriving in the Americas, "which didn't turn out very well for the native Americans".

The world-renowned physicist speculated that while most extra-terrestrial life would be similar to microbes, advanced life forms would probably be "nomads, seeking to conquer and colonise".

The comment reinvigorated a three-year debate festering behind the scenes in the small community of astronomers who look for extra-terrestrial life, said Seth Shostak, a senior astronomer at the SETI Institute, which looks for aliens.

While some people think broadcasting into the universe is "like shouting in a jungle, not necessarily a good idea", Mr Shostak asked: "Are we to forever hide under a rock? That to me seems like no way to live."

There is a big difference of opinion in astronomy about the issue, says Mary Voytek, a senior astrobiology scientist at Nasa headquarters.

"We're prepared to make discoveries of any type of life, of any form," she said in a Nasa teleconference.

Much of the search for intelligent life was privately funded by groups like SETI, she said.

About 20 years ago, Nasa held a conference on the issue. Back then, most of the experts were worried about attracting the wrong type of aliens, said Christopher Kraft, the former Nasa Johnson Space Centre director who created Mission Control.

But Mr Kraft, a Nasa legend who received a lifetime achievement award yesterday from the Smithsonian Institution, said he would welcome aliens.

"I might just learn something," he said.

The SETI Institute in Mountain View, California, takes a passive approach, listening for any signals from aliens.

For more than a quarter of a century, various groups have been purposely sending out signals to other worlds. The most famous was a three-minute broadcast from the Arecibo Observatory in Puerto Rico in 1974, Mr Shostak said.

The Canadians made a series of broadcasts using a Ukrainian antenna in the 1990s. The now-defunct Team Encounter of Houston and a prominent Russian astronomer made public and distinct "cosmic calls" out to the universe, including one just from teenagers.

Nasa beamed Across The Universe to the star Polaris in 2008 to promote the space agency's 50th anniversary, the 45th anniversary of the Deep Space Network and the 40th anniversary of the Beatles' song.