SAN FRANCISCO (AP) _ The next time your cellular phone call drowns in static or your pager won't beep, blame the Van Allen Belts.

Once thought to be a slumbering cocoon of charged particles embracing the Earth, new research shows these radiation belts can become extremely powerful in a matter of seconds.

Such sudden changes pose far greater risks to orbiting telecommunications satellites _ and even spacewalking astronauts, scientists said Monday at a meeting of the American Geophysical Union.

``We had thought the radiation belts were a slow, lumbering feature of Earth, but in fact they can change on a knife's edge,'' said space physicist Daniel Baker of the University of Colorado.

Detected 40 years ago, the doughnut-shaped particles extending more than 20,000 miles around the planet were thought to be very stable, waxing and waning over a period of months.

New observations by an array of satellites show changes in the planet's own magnetic field can accelerate electrons in the belts to nearly the speed of light, transforming them into what some researchers describe as ``killer electrons.''

Under those conditions, the charged particles can pierce a sheet of aluminum a half-inch thick, possibly resulting in a catastrophic accumulation of particles in the sensitive electronics of hundreds of orbiting satellites.

``Many of the satellites up there now, and future spacecraft like the space station, have the potential to be severely impaired by light-speed electrons,'' Baker said.

Shielding people and hardware in space is expensive and heavy, and the discovery may compel space engineers to design orbiting systems differently.

Consumers already have experienced a taste of what can happen when the Van Allen Belts, in the words of one researcher, ``get whipping along.''

In early May, sensors on at least 10 science satellites started picking up indications that electrons were accelerating. On May 19, a heavily used telecommunications satellite, Galaxy 4, failed suddenly and 45 million customers lost pager service.

Scientists think the electron flow contributed to the outage.

Such fluxes also may have contributed to the failure of a Canadian telecommunications satellite, Anik E1, in 1996.

The risk to NASA's manned space program is less certain. Satellites orbit 22,000 miles above the planet in the midst of the most energetic fields of the Van Allen Belts.

The space shuttle and the space station orbit within about 250 miles of Earth. Scientists said that during intense periods the charged particles pulse down into the atmosphere.

Researchers said they cannot yet precisely forecast when the belts of highly charged electrons peak in intensity, but they are advising NASA when conditions appear to be changing so the agency can decide whether to delay space missions.

``If one of these events occurs during the space station assembly, do you have the astronauts hurry up and risk ripping a glove?'' said space physicist Terry Onsager of the National Oceanic and Atmospheric Administration. ``Or do you let the mission go as planned?''

NASA plans more than two dozen spacewalks a year during the assembly of the international space station. On Monday, astronauts successfully hooked up electrical connections between the first two pieces of the station.

Scientists expect the Van Allen Belts to become highly dynamic beginning in late 2000 during the Solar Max _ the peak of a cyclical period of violent storms on the sun that fill the solar system with charged particles.