Science: Stars and Planets: July

NEXT TIME you phone from the train to say you'll be home in time for supper, just ponder this. Your call may be obliterating a radio signal that has been travelling through space for ten billion years. You may even wipe out that first call from ET...

The name of the game is radio-frequency interference, and this month astronomers from around the world are meeting in Vienna to draw up a plan to protect the interstellar communications channels.

The meeting will also address two other kinds of celestial pollution, where some progress has been made. The first is space debris: Nasa and the European Space Agency are now pushing all space agencies to reduce the amount of rubbish orbiting the Earth. Light pollution is the second. Amateur astronomers have been pushing the case that badly-designed outside lights spoil our view of the night sky and waste energy. The Institution of Lighting Engineers and the British Standards Institute are now backing the campaign.

But radio interference has a much lower profile. It doesn't endanger spacecraft, and you can't see it. Jim Cohen from Jodrell Bank is one of the organisers of the Vienna meeting. He says "most communications engineers have never even heard of radio astronomy, so they're not aware of any problem."

Radio telescopes, like the great Lovell Telescope at Jodrell Bank, listen for the faintest signals from the cosmos - but are also open to terrestrial radio broadcasts. Mobile phone masts are hot-spots of radio interference. The Vienna conference will call for "international radio quiet zones", where astronomers have a chance of picking out faint celestial radiation. Australia is one favoured place, while China has also offered a site.

However radio quiet its site on earth, a radio telescope is still vulnerable to radio pollution from satellites. For years, international agreements have safeguarded specific frequencies that are important in astronomy, such as that emitted by hydrogen. But the safeguards are being eroded. While transmitting its permitted frequencies, a satellite may leak other radiation. And however low the level, this can drown out signals from distant sources.

"Ten years ago, the major problem was radiation from the Russian Glonass navigation satellites," Cohen says. "Now it's the huge numbers of satellites being launched for mobile communications." The pioneering Iridium system uses 66 satellites, while other systems are adding hundreds more.

The ultimate radio quiet site is the far side of the moon, where telescopes would be permanently shielded from transmitters on and in low orbit around the Earth. Cohen sighs: "there are no firm plans for a lunar radio telescope - at the moment, it's just too expensive."

A committee of the International Astronomical Union has already picked out a suitable crater, named Saha, as a base for tuning into alien broadcasts. Expensive it may be, but short of curtailing the Earth's mushrooming telecommunications culture, a lunar Jodrell Bank may be our only hope of answering the ultimate question: is there anyone out there?