SAN JOSE, Calif. — For more than 50 years, scientists have been listening for signals from extraterrestrial civilizations, to no avail. Now some experts on the search for extraterrestrial intelligence, or SETI, think it's time for a sustained effort to send signals out to extraterrestrial civilizations, even though there's no sign of their existence. Others think that's a terrible idea.

The debate over the risks involved in "Active SETI" isn't new: It's been simmering beneath the surface for years, if not decades. What's new is that both sides in the debate want to bring their arguments to the wider public.

"We should encourage ongoing, international discussions about Active SETI, even after the launch of sustained Active SETI projects," SETI Institute researcher Douglas Vakoch says in his proposal for communicating with aliens.

Vakoch wants to send repeated radio transmissions of messages to hundreds of stars that lie within 82 light-years (25 parsecs) of Earth — using a focused beam from the Arecibo Observatory in Puerto Rico, the world's largest radio telescope.

Physicist Stephen Hawking has argued against that strategy, saying that the signals could attract the bad kind of aliens depicted in movies ranging from "War of the Worlds" to "Independence Day." He imagines that our first contact with extraterrestrials could be like the Native Americans' first contact with Europeans, "which didn't turn out very well for the Native Americans."

In Hawking's view, it's better to lie low, and science-fiction author David Brin agrees. During sessions at this week's annual meeting of the American Association for the Advancement of Science in San Jose, Brin faced off against Vakoch and the director of the SETI Institute's Center for SETI Research, Seth Shostak.

"We've had some disagreements lately," Brin admitted.

He said the reason we haven't been hearing radio traffic from our extraterrestrial brethren could well be because "they know something we don't know." Brin wants a moratorium on Active SETI for now, but he also wants a wider discussion of the potential risks and benefits, as well as the possibilities for life far beyond our solar system.

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"For heaven's sake, this is a mystery," Brin said. "Let's embrace it, let's discuss it."

Decades of beaming to E.T.

Over the decades, there have been a good number of signal sessions directed at distant aliens. The best-known is the Arecibo message, an intricately coded message that was sent in 1974 from the Puerto Rico dish toward a star cluster 25,000 light-years away. The Evpatoria radio telescope in Crimea sent out four "Cosmic Calls" between 1999 and 2008. Other projects have broadcast whale songs, Beatles tunes and Craigslist postings to the stars.

Several years ago, astronomers such as Shostak were saying that most of those transmissions, along with "I Love Lucy" and all the other TV shows that have been beamed out over the past seven decades or so, were probably too weak to be picked up by extraterrestrials. Now they're not so sure.

Our own radio-sensing technologies have improved so much in the past decade that, if a parallel civilization improved its technologies at the same rate, they might have heard from us already, Shostak said. "If they're only one, two or three centuries ahead of us, they can easily pick up our leakage. ... If you're going to worry about this, don't worry about this, because it's too late to worry about that," he said.

Brin called that the "barn door excuse." In his view, the worries about Active SETI shouldn't be dismissed just because our radio transmissions have been escaping through a metaphorical barn door. He noted that the Active SETI plan calls for a significant increase in the frequency of targeted, high-powered transmissions.

"What we have been asking for is to talk about it," Brin said. "That process has been done in many fields — the genetic engineering field, for example. ... I don't understand why these guys don't want it, because it would be worldwide, it would involve all of their peers, and it would certainly involve the world. Half a billion people would probably tune in. It would be exciting, it would be fascinating, it would be instructive, and it would probably draw an awful lot of new donations to SETI."

Who OKs the message?

In the end, it comes down to this: If researchers with access to a radio telescope want to beam a message to a particular star, possibly piggybacking on radar observations of near-Earth objects, should there be someone else who decides whether or not the effort is too risky?

Shostak would hate to see high-powered radio transmissions vetoed by a popular vote.

"This is going to hamstring every future generation of Homo sapiens, because we've had radio for 100 years, only 100 years. What are we going to be using it for 10,000 years from now?" Shostak said. "It's probably going to involve some high-powered transmissions, because we're going to start populating parts of the solar system. Do you want to hamstring all that activity, not for the weekend, not just shut down all the radars next week, or Active SETI next year, but shut down humanity forever?"

David Grinspoon, an astrobiologist and senior scientist at the Planetary Science Institute, said the planetary protection protocols for Mars missions may provide a useful example for the debate. Such protocols are drawn up by a panel of experts to avoid earthly contamination of Martian samples, and to protect Earth from any biological hazards that might be contained in material from the Red Planet.

'"It's not a perfect analogy, but it is an analogy," Grinspoon said.

Update for 7:10 p.m. ET Feb. 13: Brin and Hawking aren't the only ones concerned about Active SETI: This week the team behind the SETI@Home data analysis project put out a statement declaring that the "decision whether or not to transmit [messages aimed at extraterrestrials] must be based upon a worldwide consensus, and not a decision based upon the wishes of a few individuals with access to powerful communications equipment."

Among those signing the statement are SpaceX's billionaire founder, Elon Musk; Berkeley SETI researcher Dan Werthimer; John Rummel, the former head of NASA's planetary protection office; and planet-hunting astronomer Geoff Marcy.

Marcy voiced further concerns about Active SETI on Friday during an AAAS seminar on exoplanets: