A Soviet spy satellite carrying a nuclear power supply appears to be in trouble in space, has dropped into a dangerously low orbit and could reenter the Earth's atmosphere late this month unless the Soviets can remedy the problem, Pentagon officials said yesterday.

The officials said that radioactive debris from the satellite could pose a danger if it falls in populated areas.

If the satellite does plunge toward the Earth, it would come almost exactly five years after a similar Soviet spacecraft ran into trouble and disintegrated, spewing debris, including radioactive fragments of metal, across a remote section of northwestern Canada.

That failure on Jan. 24, 1978, caused great concern around the world. However, the 100 pounds of Uranium 235 fuel that powered the reactor apparently burned up during the fiery plunge through the atmosphere and did not cause any serious radiation hazard on Earth.

Pentagon officials said they did not know where the Soviet satellite would land or precisely when it would come down. The five-ton craft is designated Cosmos 1402, and was launched Aug. 30, 1982, into an orbit roughly 155 miles above the Earth.

The satellite is meant for ocean surveillance and carries a large radar, powered by the nuclear reactor, to track the movements of American and allied navies. The spacecraft's path is designed to give it the best view of the oceans, but it also carries the vehicle over all of the Earth's surface, which means it could come down anywhere.

Pentagon officials point out, however, that about three-fourths of the Earth's surface is water and that land areas also contain vast stretches that are not heavily populated. The reactor would cause "no consequential" problems if it landed in water, the officials said.

If the reactor and fuel survived the reentry and hit the Earth, officials said, there could be radioactivity problems, but the device would not explode like a nuclear bomb.

Officials of several government agencies met at the White House yesterday to discuss which agencies would do what if the satellite crashed in the United States. It was decided that the key role of informing the public of any danger would go to the Energy Department.

Normally, these satellites stay in their initial orbit for four to seven months. When they begin gradually to descend because of the slow tug of gravity, the Soviets activate a rocket on the craft that is supposed to heave the reactor portions deep into space, where they would remain for hundreds of years.

Officials said that the Soviets apparently have not been able to do that with this spacecraft, and there are indications of a malfunction aboard the craft late last month.

Officials said concern began to grow last week in Washington and in Canada, which participates with the United States in the North American Air Defense (NORAD) command network that monitors objects in orbit, after the Soviet satellite began to tumble and drop below its normal altitude. As of yesterday afternoon, NORAD was unable to provide the latest altitude of the craft in response to press inquiries.

Officials say it is hard to say when and where the craft would come down until about 12 hours before it begins to reenter the atmosphere.

They also said that the Soviets had not said anything about the satellite or issued any advisory, and that it was impossible for American officials to be certain whether the Soviets still have a chance to keep the vehicle in space.

The Pentagon, however, put out an official statement estimating that the craft, "which we believe contains a nuclear reactor as its power supply, will probably reenter the Earth's atmosphere in late January."

Defense sources also said that the administration intends to ask the Soviets about the situation and would make information available to other countries.

In recent years, the Soviets usually have had two or three such satellites in orbit at any one time to provide as complete ocean surveillance as possible. The worldwide protests in the aftermath of the disintegration of Cosmos 954 over Canada in 1978, however, apparently caused the Soviets to wait more than two years before resuming the flights.

After the 1978 incident, President Carter proposed a ban on satellites using nuclear reactors as a power source.

The United States has no nuclear-powered satellites in Earth orbit, officials said.

Geoffrey Perry, a leading private authority and tracker of satellites in Britain, was quoted by the Associated Press yesterday as agreeing with the Pentagon's assessment of the craft's problems and adding that "it could prove very dangerous" if the satellite crashes in a populated area.