Cosmologist Stephen Hawking says it's too risky to try to talk to space aliens, but he's too late - Nasa's tried it.
The US space agency and others have already beamed several messages into deep space, trying to phone extraterrestrials.
Nasa, which two years ago broadcast the Beatles song Across The Universe into the cosmos, discussed its latest search strategy for life beyond Earth.
"The search for life is really central to what we should be doing next in the exploration of the solar system," said Cornell University planetary scientist Steve Squyres, chairman of a special National Academy of Sciences panel advising Nasa on future missions.
The academy panel is looking at 28 possible missions, from Mars to the moons of Jupiter and Saturn. And 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, Professor Hawking said on his new TV show that a visit by extraterrestrials 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 extraterrestrial life would be similar to microbes, advanced life forms would probably be "nomads, looking to conquer and colonise".
The comment reinvigorated a three-year debate festering behind the scenes in the small community of astronomers who look for extraterrestrial 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, said 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.