IN January of 1978, the Soviet spy satellite Cosmos 954 dropped out of orbit and fell to earth. It did not completely disintegrate in the atmosphere. Instead, debris from the satellite fell across almost 40,000 square miles of northwest Canada. Metal from the sky is frightening enough, but this metal was radioactive.

The satellite had been powered by a nuclear reactor with more than 100 pounds of enriched uranium in its core. Emergency response crews searching the area found 3000 bits of enriched uranium and a few pounds of radioactive metal. If the satellite had fallen near any populated area, people could have died.

Cosmos 954 is not the only Soviet satellite to fall back to earth. In January 1983, Cosmos 1402 disintegrated upon returning to earth. Some of its radiation still remains in the atmosphere. And just a few months ago, Cosmos 1900, a satellite also containing more than 100 pounds of enriched uranium 235, burned up in the upper atmosphere. Luckily, the satellite jettisoned its nuclear reactor, which is still floating in space, at a higher, longer-lived orbit.

Although the U.S. doesn't fuel its spy satellites with nuclear reactors, we have sent up more than 20 spacecraft powered by plutonium. One of these had an accident in 1965 from which we received two-thirds of all the plutonium-238 in our atmosphere today. About 20 percent of all nuclearpowered spacecraft sent up by the U.S. and Soviet Union have broken down in some way.

There are approximately 40 radioactive Soviet satellites orbiting the earth today, all of which will ultimately fall back to earth. The prospect of what columnist Mary McGrory has called "flying Chernobyls" falling on our heads would frighten most of us, but the U.S. government isn't worried. Spurred on by Strategic Defense Initiative advocates, the government is planning to deploy its own earth-orbiting reactors, which would be hundreds of times more radioactive, and therefore many times more dangerous, than anything the Soviets have put up.

THE head of the "Star Wars" program, Lt. Gen. James Abrahamson has admitted that nuclear power in space is necessary for SDI to work Without it, he said, "that's going to be a long, long lightcord that goes down to the surface of the earth." Ten percent of the SDI budget goes towards developing satellites powered by nuclear bomb pulses and nuclear reactors; current plans call for up to 100 to be put into orbit.

In 1979 the Department of Energy contracted a study into the possible dangers of sending a nuclear reactor into orbit. Steven Aftergood, who heads the Committee to Bridge the Gap, a public interest group that concentrates on nuclear policy issues, knew that there was a joint DOE-SDI-NASA project to develop the SP-100, a space-based nuclear reactor. The SP-100 is in its final design stages, and a prototype is to be constructed in the next few years; deployment is tentatively scheduled for the mid- or late-1990s. Aftergood requested the DOE study through the Freedom of Information Act, but his request was turned down. However, he was able to get the study through other means and when he saw it, he realized why the DOE didn't want it made public.

The study, "Safety Assessment for Space Reactors," included a discussion of what would happen if a one-megawatt thermal nuclear reactor was to reenter the atmosphere immediately after 10 years of continuous output. The report read: "although not specifically calculated, the number of fatalities for reentry at time of shutdown or during operation would be 30,000-50,000." Translated from military-industrial- speak, this means that if the worst case scenario occurs, 30-50,000 people would be killed worldwide.

The report estimated that if the satellite fell back to earth five years after shutdown, 442 people would die. The satellite the U.S. is actually designing is almost twice as radioactive as the one discussed in the report.

AFTERGOOD naturally was curious about how likely the worst-case scenario was. He asked to see a different study, which would have answered his question but again was turned down. The DOE wrote back that release of this information "could reasonably be expected to cause damage to the national security." The DOE now has more detailed safety assessments, but these too are being withheld.

Luckily, Aftergood isn't the only person concerned about these dangers. In May of last year, a joint Soviet-American committee of scientists presented a proposal calling for a ban on nuclear reactors in earth orbit. The committee included Roald Sagdeev, a close adviser to Gorbachev and the leader of the U.S.S.R.'s space program, and a group from the Federation of American Scientists.

Besides recognizing the obvious threats to the environment and human life, the committee also noted that nuclear satellites would speed up the militarization of space. There seems to be little, if any, civilian use for nuclear power in orbit--it is too dangerous and too expensive.

Clearly, something must be done about these reactors. Last September, Congressman George Brown (D.-Calif.) introduced a bill that would address this problem. The bill would keep us from sending up any nuclear-powered satellites provided that the Soviets do the same. However, Congress, in an end-of-session rush, didn't consider it. Brown will introduce it again this year, but because of expected opposition from SDI advocates, the bill might have a hard time being signed into law.

The lack of attention this problem receives is emblematic of our response to environmental dangers. Environmentalists warned us for years about the possibilities of the greenhouse effect, but politicians and the media didn't talk about it until last summer's heat wave. New York Times columnist James Reston has said that "the networks will only cover the environment when you get a picture of a forest that died." Will we need a disaster the size of Chernobyl to start thinking about nuclear satellites?