Boston — Today, more likely than not, at least one hunk of space trash will quietly slip into the Earth's atmosphere and burn up in the heat of reentry. No mess. No bother.
About 550 pieces of man-made junk - everything from spent rocket boosters to bits of broken solar panels - come down each year without fanfare.
But when this space-age litter is a nuclear-powered satellite, such as the Soviet's Cosmos 1402 now tumbling toward earth, it causes more concern. US officials estimate the radar satellite will come down sometime later this month.
While experts say any wreckage will probably fall into the ocean, the situation hints at a longer-term problem facing space planners: how to deal with a growing array of potentially dangerous space junk.
''It's something people haven't been concerned with in the past, but it will become a growing issue as we send more things up,'' says Donald Kessler, project scientist for the National Aeronautics and Space Administration's orbital debris studies.
In addition to falling satellites, it's possible orbiting junk could collide with working satellites or even smack into the space shuttle. In fact, during last July's shuttle flight, the crew received an ''orbital traffic'' report, warning that they were passing near a 3-by-12-foot chunk of Soviet rocket.
The United States keeps a careful tab on all pieces of space junk that can be picked up by radar from the headquarters of the North American Defense Command (NORAD) in Colorado. According to NORAD, there are more than 3,400 pieces of trackable debris floating around the Earth, along with 1,228 working payloads.
Says Mr. Kessler, ''It's possible we someday may be able to retrieve satellites before they break up, but that'll be a very long time in coming.'' A cheaper and more immediate solution, he says, would be to build satellites that could be signaled to come down at a predetermined spot - like the middle of the Pacific Ocean.
In the past, the Soviets have dealt with their nuclear-powered satellites by breaking them up, then boosting the portion holding the reactor into a higher orbit. Parked above 500 miles, a reactor - or anything else, for that matter - orbits for hundreds of years.
The Soviets say Cosmos 1402 has split into small enough pieces to burn on reentry and not pose a danger; but US officials are keeping a watchful eye on it. A similar satellite, Cosmos 954, fell to earth in 1978 and littered northwest Canada with thousands of radioactive fragments - one the size of a five-gallon barrel.
In the wake of that incident, former President Carter suggested a ban on putting reactors into earth orbit. The US currently uses nuclear generators only to power probes into deep space.
Soviet nuclear-powered satellites make for troublesome junk, observers say, because the generators are built into relatively sturdy packages. This means the reactor is less likely to fully disintegrate when it slips into the Earth's atmosphere.
For now, however, the US is pushing ahead with a plan to make at least some satellites retrievable. Most satellites are put together in a way that makes it impossible to grab them once they're in orbit.
''We don't have the proper technique yet for approaching a satellite that's not designed to be approached,'' says NASA space science spokesman Charles Redmond. Satellites are usually built of light materials, he adds, which tear apart if simply grappled.
Next year, the US plans to use the shuttle to fix the Solar Max satellite, which failed shortly after it was launched. The solar research satellite is designed with a special knob sticking out on one side - about the size of a clenched fist - just for being gripped by the shuttle's remote-control arm.
The satellite will be pulled inside the cargo bay, where astronauts will replace fuses on the mechanism that holds the satellite steady and repair a faulty solar camera.
Mr. Redmond says that someday, shuttle-based mechanics could also service the American Landsat IV satellite now in orbit. And the $1 billion space telescope, due to be launched from the shuttle in 1985, even has a schedule for five-year maintenance visits by the shuttle crew.
''We'll want to put better instruments on it every five years, if nothing else,'' Redmond says.
But experts point out that in the case of a falling Soviet satellite, it's highly unlikely an American shuttle would ever be permitted to pluck it up - even if that became technically possible.
In the meantime, it appears any country associated with marauding space trash can expect some sticky legal problems. There's a United Nations convention that says the nation launching the original payload will foot the bill for any damage it later causes.
For instance, the Soviets paid Canada $3 million for the cleanup of radioactive debris in 1978 - although Canada spent more than $6 million on the effort.
A similar liability question was raised when the Skylab came down in 1979. Although there was no physical damage, the US government received 42 claims from individuals around the world.
''They ranged from $2.47 from somebody who had a nightmare about Skylab and broke a lampshade to someone claiming mental anguish for $10 million,'' says Neil Hosenball, general counsel for NASA. Nobody received compensation for any of those claims.