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Quiet please: the Arecibo telescope will have eight hours ‘peace’ a day after agreement. Credit: DAVID PARKER/SCIENCE PHOTO LIBRARY

Radioastronomers using the giant Arecibo telescope in Puerto Rico have won a promise from the owners of the Iridium satellite network to limit radio interference from the orbiting constellation for part of each day.

The agreement, which took five years to negotiate, guarantees eight hours of ‘quiet’ time from 22:00 to 6:00 Eastern time, during which time the Iridium system will not interfere with astronomical observations at the key frequency of 1612 MHz.

Arecibo officials hail the agreement as a good compromise. But other radio-astronomers worry that the interference issue is only getting worse.

The frequency band used for Iridium's ‘downlink’ to portable telephones on the ground is very near the emission frequency of the hydroxyl molecule, one of the most common interstellar molecules (see Nature 380, 569; 1996). Arecibo, the world's largest radio dish, is particularly suited to studying hydroxyl emissions, and some astronomers lobbied for unrestricted access to that band.

But, says Paul Goldsmith, director of Cornell University's National Astronomy and Ionosphere Center, which operates Arecibo, scientists got the best deal possible. “Some radioastronomers may have felt that they were entitled to 24 hours a day, but I'm happy that both sides could agree to eight,” he said.

The Iridium system is scheduled to begin operation this autumn, providing telephone services to users around the world. Other proposed networks will not downlink at the same frequency, and so do not pose as big a threat to radioastronomy.

Organizations that rarely observe hydroxyl emissions have found it easier to solve the Iridium problem. The US National Radio Astronomy Observatory (NRAO) operates the Very Large Array in New Mexico and the Green Bank telescopes in West Virginia, and needs only four hours of quiet time a day. It signed an agreement with Motorola, Iridium's owner, several years ago.

European radioastronomers are working out their own agreement with Iridium, and it may be even more difficult to negotiate than the Arecibo pact. The Nançay telescope in France, for example, spends about half its time studying hydroxyl emissions, and may need more than an eight-hour quiet period.

Tomas Gergely, who handles spectrum management issues for the US National Science Foundation, says each observatory has its own needs, which makes it difficult to work out a blanket agreement between satellite operators and the astronomy community as a whole.

In general, though, scientists face an uphill battle as more satellite systems like Iridium come online. “We must be very vigilant so that radioastronomy as a discipline survives,” says Gergely.