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In Britain Dr Marek Kukula, public astronomer at the Royal Observatory in Greenwich, said, "Part of me is with the enthusiasts and I would like us to try to make proactive contact with a wiser, more peaceful civilisation."
But he is concerned at the risks.
"We might like to assume that if there is intelligent life out there it is wise and benevolent," he said, "But of course we have no evidence for this. Given that the consequences of contact may not be what we initially hoped for, then we need governments and the UN to get involved in any discussions."
The issue will be among several topics raised at a two-day meeting at the Royal Society in London this week, under the heading, "The detection of extraterrestrial life and the consequences for science and society".
Among those speaking will be Professor Simon Conway Morris, a Cambridge University evolutionary biologist, who says there is good reason to think aliens exist, and that they may even have chemical and biological similarities to us.
"My basic argument is that, contrary to most neo-Darwinian thinking at the moment, evolution is much more predictable than people think," Morris said. "In particular, I would argue that the emergence, by evolution, of intelligence, cognitive capacity and all that stuff is an inevitability."
Many scientists are enthusiastic about searching for aliens.
"My personal view is that being more active is a worthy strategy," said Douglas Vakoch, director of interstellar message composition at the SETI [search for extraterrestrial intelligence] Institute in California. "There is more serious talk of it, though not at the institute level."
At an astrobiology conference in Texas in April, SETI enthusiasts will discuss new methods of discovering extraterrestrial life, including the sending of interstellar messages. Alexander Zaitsev, a Russian scientist who has already beamed out four carefully composed signals to nearby stars, has been invited to attend.
While some warn of dangers in attracting hostile extraterrestrials, the chances of finding any life outside Earth seems more remote than ever.
"My main worry is that our technology is simply not up to the task. Earth came into existence four to five billion years ago, but the oldest stars are twice that age. They may have life, but that life may be far more advanced than we are," said astronomer Heather Couper.
Conway Morris suggests that there may simply be no alien civilizations out there, perhaps because planets with biospheres are very rare.
Weak electromagnetic signals have been transmitted from Earth for nearly 100 years in the form of radio, television and other transmissions. Some should have traveled 100 light years out into space. But as digital technology develops, Earth is becoming quieter.
U.S. astronomer Frank Drake, who has been seeking radio signals from alien civilizations for almost 50 years, said it is becoming less likely aliens will pick up Earthly transmissions. "The trouble is that we are making ourselves more and more difficult to be heard," said Dr Drake.
"We are broadcasting in much more efficient ways today and are making our signals fainter and fainter."
In the past radio and television transmissions were broadcast at thousands of watts. Satellites typically beam signals of less than 75 watts, and with localized digital broadcasting this may be even less.
"For good measure, in America we have switched from analogue to digital broadcasting and you are going to do the same in Britain very soon," Drake said, "When you do that, your transmissions will become four times fainter because digital uses less power. Very soon we will become undetectable."
This may be welcome news for those scientists fearful malevolent aliens might be attracted to the Earth if they learn of our presence.
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