The crippled, nuclear-powered Soviet space surveillance satellite is now losing altitude so fast that it could fall out of orbit, burn up in the atmosphere and spew radioactive debris anywhere over inhabited parts of Earth as early as this weekend.

Pentagon sources said that the descending perigee, the low point of an eccentric orbit, of Cosmos 1402 fell 4.5 miles on Tuesday and another 4.5 miles yesterday, bringing it 117 miles from Earth on its closest approach.

Satellites fall out of orbit and burn up in the Earth's atmosphere when their perigees fall to an altitude of about 105 miles, which is in the uppermost reaches of the atmosphere and close enough for the growing pull of gravity to draw it to Earth.

Launched from the Cosmodrome near Leninsk last Aug. 30, the satellite is powered by a small nuclear reactor fueled by 110 pounds of uranium isotope U-235, the fuel used in atomic submarines and to make atomic bombs.

Of most concern is not the uranium fuel but poisonously radioactive isotopes like strontium-90 and cesium-137, which are products of fissioning uranium and emit deadly gamma radiation for thousands of years. The uranium-fueled reactor was turned on the day Cosmos 1402 flew into orbit more than four months ago.

The Soviets turned off the reactor last Dec. 28, a day or two after radio signals beamed from the Earth caused the satellite to separate into two parts.

This signaled U.S. space watchers that the Soviets were about to fire an engine remotely on the section that houses the reactor in an attempt to propel it into an orbit so high that it would stay there for at least 1,000 years. The same maneuver has been performed on more than 20 other Soviet nuclear-powered satellites.

This time, the maneuver failed, either because the satellite's engine failed completely or because it fired so erratically that it caused the satellite to begin tumbling out of orbit instead of boosting it into a higher orbit.

That also happened to the Soviets' Cosmos 954, an identical surveillance satellite that burned up in the atmosphere five years ago and scattered radioactive debris across 20,000 square miles of Canada's Northwest Territories.

Nuclear physicists calculate that most of the radioactive fuel wastes should burn up in the atmosphere and be scattered over such a wide swath of Earth that they would pose no danger to human health. This is especially true if the Soviets fuel their space reactors with small beads of uranium instead of the uranium rods used in earthbound nuclear electric power plants, as U.S. experts have been told they do.

Of far more concern are the beryllium and stainless steel structural parts of the satellite that have been bombarded for the last four months and made radioactive by fissioning neutrons. Some pieces that fell after the burn-up of Cosmos 954 were so contaminated by radiation they could not be handled on the ground without special equipment.

By turning off the reactor of Cosmos 1402 last month, the Soviets greatly curtailed the amount of radioactive debris that will fall into the atmosphere when the satellite breaks up. One physicist calculated that if the reactor had remained active during the last month before falling to Earth it would be at least twice as radioactive as it is now.

The major fear is that the satellite will fall to Earth with its reactor core intact rather than burning up into thousands of small pieces dispersed over an enormous area.

If the reactor core and its 110 pounds of uranium fuel and nuclear wastes fall intact, said Dr. Richard Wagner, assistant secretary of defense, it "would be life-threatening" to anybody within a range of almost half a mile.

The size of [Cosmos 1402] is something of a mystery. Some Pentagon sources say it weighs 5,000 pounds; others say it is almost twice as heavy. It carries a huge radar dish, measuring up to 30 feet in diameter, that scans the sea for U.S. and NATO nations' warships.

Normally, such radar-bearing Soviet satellites fly in an orbit 150 miles high, on a northeasterly track that takes them over two-thirds of the Earth's surface and all the ocean surface between the Arctic and Antarctic icecaps. Cosmos 1402 could fall anywhere between 65 degrees N. and 65 degrees S. latitudes.

The nuclear reactor provides about 100 kilowatts of electricity to power the radar.