The agreement addresses harmful interference from unwanted emissions of the Iridium satellite system operating nearby at 1621.25 to 1626.5 MHz into the band 1610.6-1613.8 MHz allocated to radio astronomy on a primary basis. This is the frequency used by astronomers to study the distribution of the hydroxyl radical, one of the most common interstellar molecules, enabling them to investigate a wide range of issues including the evaporation of comets and the birth and death of stars.

Iridium will launch its global mobile telephone service this September. However, close to the Iridium operating frequency, radio astronomers want to detect signals with an intensity more than 109 times weaker than those from an Iridium satellite. Leakage from this neighbouring Iridium band, which will be used to link the satellites to individual mobile phones, must therefore be carefully controlled to ensure that the desired radio astronomical observations can continue to be conducted.

The agreement signed by the ESF, on behalf of its associated Committee on Radio Astronomy Frequencies (CRAF), and Iridium LLC is the result of six months of intense negotiations. Under its terms, Iridium guarantees Europe's radio astronomers 24 hours a day of 'unpolluted' observation time from 1 January 2006 (see notes for editors 3). Both parties are also committed to reaching a further agreement by 1 March 1999 on transitional arrangements, covering the number of hours each day, during which Iridium unwanted emissions are to be restricted and an agreed maximum interference level at other times, for the period 1 March 1999 to 31 December 2005. For the six months from Iridium's start-up in September 1998 until 1 March 1999, the satellite company has agreed to keep emission levels below harmful interference levels as requested by the radio astronomers. However, in practice, even these levels imply a concession by radio astronomers to satellite-enabled services as the sensitivity of current state-of-the art radio astronomy equipment would imply that they should be set considerably lower.

In addition, under the terms of the agreement, both parties will continue to work together to find adequate and technically practical solutions for reducing both the out-of-band emissions of the Iridium satellite system and the susceptibility of radio astronomy equipment to these emissions.

After signing the agreement, ESF Secretary General, Professor Enric Banda commented: "This is an important agreement for radio astronomy and provides welcome guarantees. Radio astronomy, as a passive service, is uniquely vulnerable to radio interference and CRAF's success, in representing the interests of Europe's different radio astronomy observatories during these negotiations, has once again demonstrated the value for Europe's scientific community of cooperation and of speaking with one voice."

He added: "The agreement also underlines the willingness of CRAF and Europe's radioastronomers to work constructively with the growing number of satellite-enabled companies to find sustainable technical solutions that will allow science and industry to continue to profitably coexist in space."

However, despite this agreement, interference from satellites remains an increasing threat to astronomy. "This is not an isolated problem," said Dr Jim Cohen, CRAF's Chairman. "The number of cases of interference to radio astronomy from satellites is growing steadily. Unless the protection of radio astronomy is taken into account early in the design of new satellite systems our science could face a difficult future."