Garbage in orbit: debris from 40 years of space exploration presents a thorny disposal problem.

A two-ton Chinese spy satellite the size of a small car is headed for a crash landing somewhere on Earth. Although most satellites quickly burn up when they reenter Earth's atmosphere, this one is prepared for the fiery ride, and is deemed sufficiently armored to survive it. Launched from the Gobi Desert in October 1993, the spacecraft malfunctioned 10 days later. Since then it has been losing altitude, and is expected to drop from the sky in early April. Most likely it will plunge into the ocean, but other possible landing sites include many of the Earth's major cities and all of the continental United States.

Since the launch of the Soviet "Sputnik I" in 1957, over 4,500 spacecraft have been hurled from the Earth's surface, nearly half of which remain in orbit. According to the new book Orbital Debris: A Technical Assessment, only about 10 percent of these craft are still functional; the rest simply constitute space junk, very expensive garbage. But that's just the intentional debris. There are also many tons of "mission-related" garbage littering the solar system.

Among the useless junk released into space during a craft's deployment and operation are spent rocket bodies, lens caps, bolts, aluminum oxide particles from rocket motor exhausts, paint chips and fragmentary objects generated by more than 120 spacecraft and rocket body breakups.

Large debris at relatively high orbital altitudes is so stable it can perform loops around the Earth for millions of years. All told, there are trillions of projectiles, ranging from particles less than a millimeter in size to larger objects like the Chinese satellite. The U.S. Space Station in Colorado Springs keeps busy tracking and cataloging 8,000 of these objects, leaving the positions of more than a few others in doubt. The debris remains in orbit until retarding forces cause it to disintegrate in flames into the atmosphere or, as sometimes happens, it plunges back to Earth.

Astronauts have reason to worry about entering such a potentially lethal environment. A paint chip one millimeter in diameter traveling at 10 kilometers per second could easily tear a hole in a space suit. If the astronaut survives the impact, the resulting pressure loss is still very dangerous.

There's not nearly as much danger to the Earthbound. Nicholas Johnson, senior scientist at the Space Environment and Orbital Debris Resource Center at the Kaman Sciences Corporation in Colorado Springs, says, "Nearly 17,000 objects have reentered the atmosphere. The majority are burned up, and of those that do make it, most splash harmlessly into the ocean." One famous example is the 100-ton Skylab, the first U.S. space station, which broke up and rained down on the Indian Ocean and remote parts of the Australian desert in 1979. According to Johnson, errant spacecraft have never caused a confirmed injury and apparently no property has ever been damaged.

Joe Loftus, assistant director for engineering at the National Aeronautics and Space Administration (NASA)'s Johnson Space Center in Houston, has studied orbital debris for the last 20 years. He says, "Because space is a commerce and the people who operate in space know each other, it's in everybody's self-interest to preclude contamination of the space environment." In the past, he says, weather satellites have routinely been sent into space for up to five years, after which parts would fail or become obsolete. They fixed that sending up fresh satellites. "The difficulty is they accumulate," says Loftus. "Now there are satellites and upper stages that will continue to go round and round for a thousand years." Today, NASA designs lighter rocket bodies and sends them into lower orbits to increase atmospheric drag and accelerate orbital decay.

The garbage that already litters space continues to be a problem. In the absence of the chemical, natural and artificial recycling processes used on Earth, objects senc t into orbit become fossil-projectiles. This leaves future space-faring generations to cope with dangerous technological relics.

According to the Technical Assessment, one disposal technique involves shifting space debris from current orbits into "disposal orbits," where they do not pose a threat to functional spacecraft. Another idea is to develop a space vehicle whose sole purpose is that of a patrolling garbage man. Unfortunately, some predictions put the price of such a project at over $15 million for each piece of junk removed.

Satellites never appear as more than tiny specks in the sky, but as long as debris is dragging through the atmosphere, in line for a free fall back to Earth, there's always a chance that some space junk will come home in a rather spectacular way.

CONTACTS: Space Environment and Orbital Debris Resource Center, Kaman Sciences Corporation, P.O. Box 7463, Colorado Springs, CO, 80933/[phone]; National Aeronautics and Space Administration, Johnson Space Center, 2101 NASA Road 1, Houston, TX 77058/[phone].