Kosmos 1900 - the crippled Soviet nuclear-powered satellite - may drop any day now. The Soviet news agency Tass has reported that the satellite, with its nuclear reactor, could reenter Earth's atmosphere sometime between Oct. 4-8.
This is the third time this type of Soviet spy satellite has raised a radioactive alarm. Kosmos 954 scattered reactor debris across northern Canada in 1978. Kosmos 1402 came back in 1983. But it did eject its reactor, allowing it to burn up in the air.
The problem of orbiting reactors is part of the larger problem of the buildup of space debris in general.
Some 48,000 objects a centimeter across or larger now orbit our planet, according to a Massachusetts Institute of Technology study. That includes the 7,000 objects 10 centimeters across or larger that the North American Aerospace Defense Command routinely tracks.
The European Space Agency (ESA) has an even higher estimate, based on computer simulations of orbital collisions and debris spread. Its space debris working group figures that there are between 30,000 and 70,000 small fragments in addition to the thousands of larger objects that the United States and Soviet Union are tracking. In addition, ESA says, millions of bits of even tinier debris may be up there among the larger fragments.
Only 5 percent of these larger objects are operational spacecraft, according to the industry journal Aviation Week and Space Technology. Another 12 percent are debris such as discarded lens caps and packaging material from space operations or satellite delivery. Spent rocket casings account for 14 percent. Inactive payloads make up 20 percent. And those tens of thousands of fragments account for the remaining 49 percent.
Space junk threatens both manned and unmanned craft. A speeding fragment has pitted a shuttle window. Cosmonauts on the Soviet Mir space station have heard fragments hitting their orbital home. NASA's space station, Freedom, will have special meteor and debris shielding. Moreover, NASA estimates that, right now, a shuttle orbiting at 300 kilometers (186 miles) has a 3 percent probability of a debris collision per mission. If the debris continues to grow at its present rate, that could become a 10 percent probability by 2000 and a 25 percent probability by 2010.
Spacefaring nations are now considering how to minimize the junk accumulation. The National Aeronautics and Space Administration and the US Department of Defense, for example, are working out strategies to curtail the production of long-lived fragments.
Missions that make fragments, such as tests for the Strategic Defense Initiative, will be carried out in low enough orbits so that debris reenters the atmosphere rapidly and burns up.
However, deliberately leaving material such as old communications satellites or spent nuclear reactors in ``safe'' orbits will exacerbate the problem. Even though these orbits may be out of the way of current operations, there will be hundreds of the massive objects circling Earth in a decade or two.
Eventually, some way will have to be found to bring old satellites back or send them out of Earth orbit entirely.
Meanwhile, opponents of space nuclear reactors cite this as a second reason, besides safety, for banning the use of such power sources in Earth orbit.
The Soviets have launched about 30 such reactor-powered satellites, which are known as RORSATs (Radar Ocean Reconnaissance Satellite).
The US has preferred radioisotope generators that make electricity from the heat of decay of radioactive material for nuclear power in space. It did launch one experimental reactor system - SNAP 10A - in 1965. A nonnuclear malfunction in the parent satellite shut the reactor down after 45 days. It now follows a so-called ``graveyard'' orbit 1,287 kilometers (800 miles) high where it will remain for several thousand years.
However, NASA and the Departments of Energy and Defense have been working on a compact nuclear reactor for space for the past five years. They call it SP-100. With a reactor core about the size of an office wastebasket it will generate 100 kilowatts of electric power. The initial research ended in 1985. Now the project is preparing a prototype for testing by 1993.
SP-100 embodies a safety concept opposite to that followed by the Soviets. Both countries expect to boost their spent reactors into long-lasting orbits. But if something goes wrong, a Soviet reactor is designed to burn up and disperse its contents dilutely through the atmosphere. SP-100 reactors, on the other hand, will be built to reenter the atmosphere whole and bury themselves several feet in the ground. The idea is to keep their radioactive contents bottled up until a recovery team can remove them.
Skeptics in both the US and the Soviet Union dislike both plans. Last spring, a joint committee of the Federation of American Scientists and the Committee of Soviet Scientists Against the Nuclear Threat called for a ban on orbiting nuclear power sources. However, recognizing the need for such sources on missions far from the sun, they would allow nuclear power for interplanetary spacecraft.
Such a ban is embodied in legislation introduced in Congress Sept. 13 by Rep. George Brown Jr. (D) of California. It calls on the president to negotiate with the Soviet Union to stop using nuclear power in Earth orbit. It would ban such use by the United States if the Soviets agree to ban it also.
Tass had reported Sept. 12 that the Kosmos-1900 satellite would stay up until November. The earlier reentry is more in line with the European Space Agency's prediction of Oct. 9, with a one- to two-week margin of error.
How space junk proliferates When rockets and satellites explode or collide in space, as can happen for a variety of reasons, they create a horde of tiny fragments circling the globe. These fragments spread and cause further collisions, creating dangers for manned space flights, as this computer simulation shows. IMMEDIATELY AFTER EXPLOSION The fragments orbit near each other, but their speed and angle of orbit are slightly changed by the explosion itself. THREE MONTHS AFTER EXPLOSION The debris fragments drift farther and farther apart as they circle Earth, cluttering up a larger area. FOUR YEARS AFTER EXPLOSION Continuing to drift apart and perhaps colliding with other objects in space, the fragments orbit Earth over numerous locations. Source: European Space Agency