Nuclear-powered Soviet spy satellites capable of zeroing in on U.S. submarines have been detected emitting powerful gamma rays that interfere with space communication, scientists report.

The accidental discovery occurred last year but is reported in the current issue of the journal Science, which this week focuses on what before had been a highly classified government concern.Researchers from the University of California-Riverside pinpointed the three spy satellites by way of a telescope strapped to a high-flying space balloon during a mission aimed at getting a bird's-eye view of the stars.

The UC Riverside mission was continually interrupted by bursts of brilliant celestial light - gamma rays - more intense than the Crab Nebula, one of the brightest natural sources of gamma rays in the near reaches of the universe.

A gamma ray is electromagnetic energy that exists in a range higher than that of X-rays and is notable for the intensity of its light. Elsewhere in space, supernovas, neutron stars and quasars are sources of the little understood rays first detected in the deep regions of the heavens 22 years ago.

"What we had detected were gamma rays given off by a nuclear reactor," said UC Riverside physicist Terry O'Neill, referring to the power source inside the Soviet satellites. "We knew these weren't celestial events."

The university physicist described the light as too smooth and the bursts too long to have come from a natural source.

When O'Neill and his colleagues made an inquiry to the National Aeronautics and Space Administration about the problem, they were told last year the matter was classified.

"They wouldn't talk about it then," O'Neill said.

The physicist said his inquiries last summer led to a declassification of such information, some of which had been supressed for nearly a decade. O'Neill's and three other papers in Science this week mark the first scientific reports detailing observations of gamma ray radiation from Soviet satellites.

Physicist Joel Primack of the University of California-Santa Cruz writes in the same issue that the Soviet practice of powering spy satellites with nuclear reactors has been known since the late 1970s.

Cosmos 954, a Soviet satellite that flew out of control in 1978, re-entered Earth's atmosphere and crashed in the Arctic, scattering radioactive debris for miles.

However, the U.S. government never made public how many nuclear-powered Soviet spy satellites were aloft nor hinted at the targets they spied on.