In 2008, NASA beamed the Beatles song "Across the Universe" into deep space, sending a message of peace to any extraterrestrial who happens to be in the region of Polaris, also called the North Star, in 2439.

"Amazing! Well done, NASA!" Paul McCartney said. "Send my love to the aliens."

Who could argue with such a well-meaning, positive initiative?

Quite a few, actually.

As the citizens of Planet Earth strive ever more enthusiastically to reach E.T., some experts say numerous messages zipping through the cosmos are confusing or little more than space spam.

Others ask who has the right to represent our world to the galaxy -- or question the wisdom of bellowing out our presence in what may be a hostile neighbourhood.

"A lot of the stuff is very responsible, but I do wonder about some of the other stuff that's being transmitted," Albert Harrison, a professor of social psychology at the University of California at Davis, said at a conference at the Royal Society in London on Monday.

"There's pictures of celebrities, of two political candidates -- one identified as good, the other identified as evil -- snack-food commercials, love letters to rock stars and so on."

He added: "When you start broadcasting and drawing attention to yourself, you have to be very cautious about the image you give. We might appear as a threat to them.

"We don't know what will be made of these messages and it could be years and years before we find out."

The thirst for contact with alien civilisations has a long history.

The US probes Pioneer 10 and 11, launched in 1972 and 1973, bear plaques of a naked man and woman and symbols seeking to convey the positions of the Earth and the Sun.

Voyager 1 and 2, launched in 1977 and now on the outer fringes of the Solar System, each carry a gold-plated copper phonogram disk with recordings of sounds and images on Earth.

But, relative to the vast distances of interstellar space, these four scouts are crawling along.

It will take around 40,000 years for Voyager 1, the most distant man-made object in space, to get anywhere close to a star.

No-one knows if there is any intelligent life there to pick up the time capsule... or whether our species will still be around to get a reply.

As a result, the electromagnetic spectrum offers a far quicker channel.

For the last 50 years, enthusiasts have been listening out for signals, discernible in the background noise of space, that might point to another civilisation.

Apart from a couple of brief, intriguing events, nothing has really shown up, which has prompted the Search for Extraterrestrial Intelligence (SETI) to shift more and more from "passive" to "active" mode.

We are already spewing out diffuse signals through TV and radio broadcasts that pass through the ionosphere.

With "active SETI", the idea is to use powerful radio astronomy transmitters to beam out to interesting stars or extrasolar planets in the hope of eventually hitting paydirt. The transmitters are operated by space agencies or institutions, which in some cases are paid for the service.

Projects have included a tiny 1,679-bit message beamed in 1974 to star M13, 25,000 light years away; two "Cosmic Calls" in 1999 and 2003; a 2006 TV show by the Franco-German channel ARTE which beamed messages from the public to the star Errai, 45 light years distant; and a "Message from Earth" to a planet orbiting the star Gliese 581, incorporating contributions from users of social networking site Bebo.

If they exist -- and if they are able to figure out the messages -- alien civilisations are in for a smorgasbord of human culture.

Messages range from the earnest and the philosophical to the cerebral, such as an "Interstellar Rosetta Stone" of symbols that give information about Earth and Homo sapiens.

The missives include the jokey: "Please send money. Any kind of money. Universal money is OK. Alien currency OK. Meteorites are good. Gold, Moon rocks, space junk also good. Send to: Maura, Planet Earth."

And there is the political: an image of George W. Bush as the personification of evil, juxtaposed against Barack Obama as the embodiment of good, sent out by "X-Files" actress Gillian Anderson.

Any life forms at Epsilon Eridani and Tau Ceti, meanwhile, will receive recordings of the vaginal contractions of ballerinas with the Boston Ballet, a renegade 1980s art project aimed at giving the galaxy an idea of human conception.

European Space Agency (ESA) astrophysicist Malcolm Fridlund says that in the absence of any evidence so far that extraterrestrial life exists of any kind, active SETI may well be a waste of time.

Even so, he urges caution about drawing attention to ourselves.

"I'm not lying awake at night worrying about the overlords of the galaxy or anything like that," he told AFP, "but when you don't know of anything that's out there, you should maybe be a little careful, you should know something about the (star) system first."

Those who share his concern include the British cosmologist Stephen Hawking, who suggests "we should keep our heads low," given any possibility of encountering a hostile, technologically superior civilisation.

"The risk posed by active SETI is real," the prestigious British journal Nature warned in 2006, in an editorial that unleashed divisions among enthusiasts as to who had the right to be ambassador of Earth.

"It is not obvious that all extraterrestrial civilizations will be benign -- or that contact with even a benign one would not have serious repercussions for people here on Earth."