Science / Medicine : Nuclear Debris : Sometime in the next several weeks, a disabled Soviet satellite with a highly charged nuclear reactor will drop toward Earth, possibly scattering radioactive debris between the Arctic and the Antarctic circles.

Sometime in the next few weeks, a disabled Soviet spy satellite carrying a highly charged nuclear reactor will drop out of its shallow orbit.

If it burns up in the upper atmosphere--as Moscow predicts--it will go largely unnoticed by the general public, just another of the hundreds of pieces of space junk that fall from the sky every year.

But if the satellite follows the predictions of some American space experts, it will plunge to Earth, scattering radioactive debris wherever it lands.

The 4.4-ton satellite, known as Cosmos 1900, could crash almost anywhere between the Arctic and the Antarctic circles.

Since nearly three-quarters of that area is ocean, the odds are good that the satellite will fall into the sea.

But if it doesn’t, U.S. scientists fear a major nuclear accident, the latest example of why reactor-powered satellites are a mounting concern for scientists in both the Soviet Union and the United States.

The satellite was launched in December, 1987. The Soviets lost radio contact with it in mid-April, preventing boost of the satellite to a higher, more stable orbit.

Just when it will descend is not clear. Soviet sources say late August. But Capt. Thomas Niemann of the U.S. Space Command in Colorado Springs says the many variables make predictions difficult. His best guess: sometime in September.

The dangers are equally unclear.

If the satellite comes down on hard ground, it will undoubtedly splatter radioactive debris across a wide area. In 1978, a similar Soviet nuclear-powered satellite contaminated nearly 40,000 square miles when it smashed into the tundra near the Great Slave Lake in northern Canada.

No less disturbing is the possibility that the satellite’s fuel core--110 pounds of enriched uranium 235--could land in large pieces and be subject to scavengers.

At the same time, experts say the hazard to human health could be minimal. “It could go anywhere from zero--nothing--to a few hundred fatal cancers in the worst case,” said Steven Aftergood, an electrical engineer and executive director of the Los Angeles-based Committee to Bridge the Gap, a public interest group that focuses on nuclear issues.

Since the launch of Sputnik in 1958, no one has ever been struck by metal falling from orbit, even though about three pieces fall from the sky each day. About 50 pieces a year are as big or bigger than Cosmos 1900.

“There’s been a lot of things to come down in the last 30 years and no one’s been hit,” said space expert Nicholas Johnson of Teledyne Brown Engineering in Colorado Springs. “So I wouldn’t go out and buy a hard hat just now.”

Nevertheless, in May, a committee of top scientists from the Soviet Union and the United States called for a ban on reactors in Earth orbit, including those being designed for the U.S. Strategic Defense Initiative, known as “Star Wars.”

At the same time, other scientists worry that highly toxic plutonium 238 used to power U.S. scientific probes into deep space could be widely dispersed in a launch pad accident.

Altogether, Aftergood estimates, the United States and the Soviet Union have launched about 62 spacecraft with nuclear-power systems.

For the most part Soviet spacecraft have carried reactors, while the United States has used a different form of nuclear energy: electric generators powered by heat from decaying radioisotopes.

The Soviet reactors have generally been used to power satellites that orbit the Earth, while the Americans have reserved nuclear power mainly for probes to other planets or to the moon.

Both sides have had their share of accidents.

The Soviet Union has had at least six failures out of 33 launched, mostly satellites similar to Cosmos 1900.

The satellites, called Rorsats (radar ocean reconnaissance satellites), are used to monitor American naval vessels.

The most famous crash, that of Cosmos 954, occurred in Canada in 1978. The government spent $14 million to clean up what one official called “a great shower” of tiny radioactive uranium balls, about 1 millimeter across. Eventually the Soviet Union picked up $3 million of the cost.

In 1983, another crippled Rorsat, Cosmos 1402, also went out of control. It jettisoned its reactor core on re-entry, dispersing about 80% of its radioactivity into the atmosphere.

According to a recent statement by the Soviet press agency Tass, Cosmos 1900 will do the same thing. “This system ensures a radiation situation within the limits recommended by the International Commission on Radiological Protection,” the statement said.

“I don’t buy that,” Aftergood said. “I think that it’s unacceptable to have any substantial release of radioactivity into the environment.”

Johnson, widely regarded as one of America’s leading experts on Soviet space efforts, does not buy the Tass statement for another reason: He doubts Cosmos 1900 will break up on re-entry.

He said the satellite’s tough, cylindrical housing will act as “a heat shield” for the reactor core while it plunges to Earth.

“It’s not on the magnitude of Skylab,” the U.S. space station that crashed into the Australian outback in 1979, Johnson said. “There won’t be nearly as many pieces. But the pieces that do survive re-entry are going to be radioactive.”

Coincidently, Tass first revealed the Cosmos 1900 situation in May while several prominent Soviet and American scientists were in Washington calling for a ban on orbiting nuclear reactors. They specifically cited Rorsats and nuclear-powered satellites being developed in the SDI program.

The scientists were led by Roald Sagdeev, director of the Space Research Institute, and Frank von Hippel, a Princeton University physicist who represented the Federation of American Scientists.

The group said it wants “to prevent both the radioactive contamination of the Earth’s surface and the extension of the arms race into space.”

The scientists are particularly unhappy with SP-100, a nuclear-powered space reactor being developed by General Electric for three federal programs: the Department of Energy, NASA and SDI.

The 100-kilowatt reactor could eventually cost more than $700 million and be used to power SDI weapons systems. It will not be tested until sometime in the mid-1990s.

If SP-100 is ever launched, it will be the second U.S. reactor ever put into space. In 1965, NASA orbited a reactor-powered experimental Snapshot satellite, which malfunctioned after 43 days. It remains in orbit 780 miles above the Earth.

The United States has launched 22 radioisotope thermoelectric generators (RTGs) and the Soviet Union has launched half a dozen.

Most of them have been on manned and unmanned missions to Mars or the moon.

Four of them continue to operate on the two Voyager and two Pioneer space probes pushing to the outer edges of the solar system that have amazed scientists with their longevity and steady stream of data.

Four others, however, have run into problems--no small matter since each contained large quantities of deadly plutonium 238.

In one case, a U.S. navigational satellite exploded when it failed to reach orbit, tripling the amount of plutonium isotopes in the atmosphere at the time.

Now the United States is planning two more scientific probes, both powered with one or more RTGs.

One of the probes, the Galileo, will go to Jupiter in October, 1989; the other, the Ulysses, will study the sun a year or two later.

Both are scheduled to go up on a shuttle.

If either mission were destroyed on launch, as much as 50 pounds of plutonium 238 could be thrown into the atmosphere.

Unlike the uranium in reactors, which is harmless until well after launch, the plutonium in an RTG is highly toxic from the start.

Scientists, however, are sharply divided on the dangers. Until the Challenger disaster, NASA had been convinced that both the Galileo and Ulysses would be safe. Now the agency is conducting what one official called “a rigorous review,” one that is still under way.

Meanwhile, the Soviet and American scientists specifically exempted deep-space probes from their proposed ban on reactors in space. The scientists said RTGs could be safe, even in Earth orbit. The key, they said, is to keep RTGs relatively small so they do not cause a “significant environmental impact” during an accident on launch or re-entry.

John Gofman, professor emeritus of medical physics at UC Berkeley, disagreed: “I don’t believe them.”

“They said Three Mile Island couldn’t happen. The Soviets said Chernobyl couldn’t happen. They said the Challenger couldn’t happen. None of these things can happen--until they do.”