The Voyager 1 and 2 probes carry this gold-plated disc containing sounds and images from Earth.© NASA

If an extra terrestrial wanted to send a field report describing all she had learned about Earth, she might be better off writing rather than phoning. A new analysis has concluded that a physical object would be a more efficient way to send a long message to the stars than a beam of radio waves.

So while we scour the heavens for radio broadcasts from other worlds, we should also search our planetary backyard for a parcel of alien information, says Christopher Rose, an electrical engineer at Rutgers University, Piscataway, New Jersey, who argues his case in this week's Nature¹.

"Communication by the transmission of matter once seemed ridiculous compared to using radiation, but it's not, and we should be looking for both," Rose says. He thinks that a stable orbit around Jupiter, or on the Moon or even the Earth could all be potential mailboxes - all locations occupied by the alien monoliths in Arthur C. Clarke's novel 2001.

There are plenty of places in the Solar System to hide, agrees Greg Wright, a physicist from Antiope Associates in Fair Haven, New Jersey, who worked with Rose on the calculations. "Only parts of the Moon, Mars and a few asteroids have been mapped carefully," he says. "We haven't launched a serious search for these sorts of things yet," adds Rose.

You've got mail

Rose calculated how much energy it would take to send a message to a star system 1,000 light years hence. The package, travelling at about one million kilometres per hour, would reach its destination after about a million years. The radio waves would be sent and received by giant radio telescopes.

For simple messages, a radio transmission would use the least energy, Rose found. But for a transmission of 100 terabits or more, it is easier to write.

<mediar rid='m1'/>A message of 100 terabits could contain all the books in the US Library of Congress five times over. Rose argues that any message worth sending would exceed this easily - perhaps being closer to the 40 million terabits contained in all the world's telephone calls in a year.

An inscribed object has the advantage of remaining legible no matter how far it travels, whereas even the narrowest beam of radio waves spreads out over interstellar distances, eventually becoming undetectable. For long messages over long distances, an alien civilisation is likely to send a package, says Rose. "Energy is kind of a currency: if something costs less, it's more likely to get done."

I hope that someone finds my...

We have sent several inscribed messages into space. The two Voyager probes each carry a long-playing record of "The Sounds of Earth", and both Pioneer craft, the first manmade objects to leave our Solar System, bear plaques charting their route, along with a picture of naked humans waving a greeting. A similar alien salutation could be waiting on Earth for us, says Rose.

The SETI Institute is already hunting the heavens for radio transmissions from other stars. Jill Tarter, Director of the Center for SETI Research in Mountain View, California, agrees with Rose to a point. "We should look for artifacts of all kinds in our local neighbourhood, including packages on our doorstep," she says.

But she adds that the most important contact is the first, which doesn't require a long, complicated message. "There's room for Rose and there's room for radio. For now I'm sticking with radio," she says. 

Rutgers University, Piscataway, New Jersey

SETI Institute, Mountain View, California

• References

  1. Rose C. & Wright G. Nature, 431. 47 - 49(2004). | Article | PubMed | ISI | ChemPort |