Two segments of the falling Soviet nuclear-powered satellite dropped another six miles yesterday, bringing them to within three or four miles of the Earth's atmosphere. The Pentagon said the first section would fall to Earth between 6:45 a.m. Sunday and 8:17 a.m. Monday.

Late yesterday afternoon the Soviet Union informed U.N. Secretary General Javier Perez de Cuellar that the satellite will plummet to Earth over the "region of the Arabian Sea" between 6 p.m. Sunday and 1 a.m. Monday, Washington time. A brief communique from the Soviet mission emphasized that the fragment was not part of the nuclear reactor.

Earlier yesterday the official Tass news agency in Moscow had ridiculed warnings about the falling satellite, saying the scare was being used by NATO strategists to cover a military buildup.

"It is not splinters of Soviet satellites that are falling on the heads of residents of western countries . . . . It is a stream of impudent lies and slander . . . to divert their attention from the unprecedented arms race launched by the United States," Tass said.

Pentagon officials say the larger segment of the Cosmos 1402 satellite is expected to come down first. It is believed to be the portion containing the radar surveillance equipment used to track the movements of U.S. Navy surface ships.

Pentagon officials say they believe it to be dangerously radioactive from four months of exposure to radiation from the nuclear reactor housed in the smaller satellite segment. Metal struts from this segment of Cosmos 954 that fell on Canada five years ago were so radioactive that they could have burned anyone handling them severely.

The smaller segment of Cosmos 1402 contains 110 pounds of highly enriched uranium (U-235), which has produced large quantities of poisonous fission products like strontium-90 and cesium-137. These emit deadly gamma radiation for thousands of years.

The Pentagon said yesterday that the smaller reactor core would probably not fall out of orbit before Feb. 10 because there is less drag on it. It is considered less dangerous to health and safety than the larger piece because it is expected to burn up in the atmosphere and be scattered over 20,000 square miles. Most of it should not even reach the Earth's surface unless rain or snow brings it down as radioactive fallout.

The re-entry of the reactor core could provide a spectacular sight as it falls toward Earth because 700 pounds of rocket fuel remain in the tanks of the segment. The fuel fed an engine that was supposed to shoot the reactor into a higher orbit more than three weeks ago so it would never pose a hazard to Earth. For some reason, the rocket engine never fired, which is why the reactor is falling to Earth with the segment containing the surveillance radar.

Even if the radioactivity never falls to Earth, it still poses such a hazard to aircraft that the Air Force and Federal Aviation Administration plan to warn aircraft away from the satellite's path once it is clear that the satellite's reactor core has begun to fall out of orbit.

The reactor's fission products would contaminate the aircraft with enough radioactive debris to make it hazardous for mechanics to work on these planes after they land. The FAA said it will dispatch inspectors with Geiger counters to most U.S. airports after the satellite comes down to monitor radiation levels on returning aircraft and order those contaminated to be flown to certain Air Force bases to undergo decontamination.

Meanwhile, emergency teams the world over stood ready to move to decontaminate whatever radioactive fragments they might find after they fall. Alerts were declared in Japan, Australia, Spain, West Germany, the Netherlands, Belgium and Scandinavia.

Cosmos 1402 follows a path that brings it over most of the inhabited Earth and the world's oceans south of the Arctic icecaps and north of the Antarctic icecaps.

Scientists give the radioactive debris a 70 percent chance of falling at sea, since 70 percent of the Earth that lies under the satellite's path is covered by oceans. The experts give the debris a 15 percent chance of falling on the Soviet Union, a 3 percent chance of raining on Canada and a 2 percent chance of descending on the United States.