A new agreement extends some protection to astronomers who use the Arecibo Radio Telescope in Puerto Rico and have been concerned about potential interference from the commercial satellite system IRIDIUM. The memorandum of understanding signed between the National Astronomy and Ionosphere Center, which operates Arecibo, and Motorola, Inc., which operates IRIDIUM, guarantees eight hours of observing time each day "unpolluted" by interference. The NAIC is headquartered at Cornell University and funded by the National Science Foundation.

The agreement, which took five years to hammer out, addresses IRIDIUM's potential interference with reception of faint radio signals at a key frequency for astronomers: 1612 megahertz. Hydroxyl, one of most common interstellar molecules, emits radiation at this frequency. This simple molecule can be produced in the atmospheres of old red giant stars; gas can blow off and eventually be swept up into new stars. Hydroxyl also appears in interstellar clouds, which are seed beds of young stars and solar systems. Tracing the path of this gas is one thread in reconstructing how our own galaxy evolved.

"This agreement is a good compromise in protecting astronomers' ability to observe at this frequency," said Paul Goldsmith, Cornell professor of astronomy and NAIC director. "Some radio astronomers may have felt that they were entitled to 24 hours a day, but I'm happy that both sides could agree to eight. The agreement should help radio astronomy and communications' use of the spectrum to coexist productively."

"We're very pleased that the agreement will help us fully exploit the newly upgraded Arecibo telescope," said Hugh Van Horn, NSF director of astronomy. "The telescope's ability to observe across a much greater range of frequencies, and its enhanced sensitivity will enable a vast new range of astronomical observations of sources from asteroids to distant galaxies. But careful protection of the radio spectrum is absolutely essential to use the telescope to its fullest potential."

The IRIDIUM satellite system, planned to become operational in the fall of 1998, will employ 66 satellites in low earth orbit to enable portable telephone communication anywhere in the world directly by satellite instead of by local cellular networks.

The protected time period of astronomical observation will span the period from 10 p.m. to 6 a.m., Eastern time. The agreement also specifies that radio astronomers may be allotted protected slots at other times of the day if special scientific opportunities arise.

Radio interference remains an ongoing and increasing threat to astronomy. "We must worry about interference with observations at other frequencies and at other radio telescopes as well," said Michael Davis, chair of the National Research Council's Committee on Radio Frequencies. "It's vital to protect access to these very faint whispers of natural radiation that tell us so much about the universe."