On the wall of Darren McKnight's office, in Reston, Virginia, is a display that would give pause to anyone who might be considering a trip into space: two gnarled pieces of metal shot through with shredded electrical wires and mangled rivets. They are fragments of a Navy satellite that was shattered during a test in a Tennessee bunker by a plum-sized aluminum ball traveling at a speed of about four miles per second. On McKnight's desk is a photograph showing the thousands of other pieces of debris generated by the test, which McKnight, a vice-president of a company called Titan Research and Technology, and other researchers carried out to simulate the collision of a satellite with a piece of orbiting space junk. From the photograph alone it's impossible to tell what the metal fragments might once have formed.
Everything that human beings launch high enough into space will ultimately end up like that shattered satellite. As long as an object is above the last traces of Earth's atmosphere, it will stay in orbit for thousands or even millions of years. Eventually, whether a month or a millennium after launch, it will hit one of the millions of other objects orbiting Earth. That collision will generate new fragments, like the ones in McKnight's picture, which will go whirling around the planet until they, too, are involved in collisions. Over time everything in Earth's orbit will be ground into celestial scrap.
The space shuttle Endeavor is scheduled to ferry the first U.S.-built component of the International Space Station into orbit several months from now. A steady stream of modules and structural elements will follow over the next five years. If the station remains in space, it will eventually collide with a piece of debris. Maybe by then the station will be empty, its human occupants having moved on to other pursuits. But maybe not. If not, here's what the astronauts can expect. If a piece of debris the size of McKnight's aluminum ball hits a pressurized module, it will rip a five-inch hole in the wall. Because of the tremendous speed at which objects in orbit move—typically about six miles per second—the collision will liquefy both the piece of debris and the wall of the module. Molten metal will splatter the inside of the module, accompanied by a flash of heat and blinding light. Air will begin streaming out the hole, leaving any surviving astronauts just a few minutes to escape. If the piece of debris is larger, the module may undergo what engineers call "unzipping": its exterior will peel away from the frame like that of a banana, spewing the contents of the module into space.
Today the risk of such a disaster for a satellite or a small craft like the shuttle is relatively low, though Mir, the Russian space station, launched in 1986, has been hit by objects large enough to dent the inner wall of the crew compartment. But the International Space Station, much larger than Mir, will be a plump target for debris. Each decade that it is in orbit, according to a recent study, the station will have about a 20 percent chance of undergoing a "critical penetration" that could kill a crew member or destroy the station—and the chances will increase as more objects are launched into space. In contrast, the chances of being in a commercial-airliner accident in the United States are about one in three million.
Venturing into space is inherently risky, and orbital debris is just one of many hazards that a space traveler faces. But the debris hazard is unique in being a product of our environmental negligence. After just forty years in space we have seriously polluted the final frontier. Valuable orbits are peppered with debris that threatens the operation of satellites and the lives of astronauts. A small group of orbital-debris experts have been concerned about this problem for years, and have slowly gained the attention of the government agencies and commercial enterprises that are now leading the way into space. Yet every four days, on average, another rocket that will make the problem worse is launched into space. According to Molly Macauley, a debris expert and a senior fellow at the nonprofit environmental organization Resources for the Future, "It's going to take a major catastrophic debris event, probably involving loss of life, before this issue gets widespread attention."
Space may seem remote, but it's really not that far away. If you could drive your car straight up, in just a few hours you'd reach the altitude at which the space shuttle flies. The popular orbits for satellites begin twice as far up—about 400 miles above our heads. The only satellites that are truly distant from Earth are the several hundred in geosynchronous orbit, a tenth of the way to the moon. There telecommunications and weather satellites orbit at the same rate that Earth rotates, allowing them to hover above a single spot on the Equator.
Since 1957 the United States and what is now the former Soviet Union have conducted about 4,000 space launches (the launches conducted by all other countries and international organizations combined account for just a few hundred additional forays into space). The leftovers from these launches—used-up satellites, the rockets that carried the satellites aloft, equipment from aborted scientific experiments—form a sort of orbital time capsule, a mausoleum of space technology. In 1963 the Air Force released 400 million tiny antennas about the size of needles into orbit in order to see if radio waves would bounce off them. Though communications satellites soon made the antennas obsolete, they still float in lethal clumps 1,500 miles overhead. In 1965 the astronaut Michael Collins lost his grip on a camera while on a space walk. Many spacecraft shed debris—bolts, lens caps, equipment covers, thermal blankets—the way children shed toys. A series of Soviet nuclear-powered spy satellites are leaking coolant into space that is congealing into balls about an inch in diameter. Even the paint on spacecraft has a tendency to erode in the harsh environment of space, creating a cosmic grit that now pelts everything in orbit.
Many of the objects released into space in the lowest orbits, like Collins's camera, have fallen back to Earth. The upper atmosphere, where the space shuttle flies, gradually slows objects down; they re-enter the atmosphere and burn up within a few months or years. But a few hundred miles higher the atmosphere is so thin that it is ineffective for cleanup. Spacecraft that are launched into orbits at this height will stay in space indefinitely. "We and the Russians have been putting stuff up there for more than thirty years, and it's just where we left it," says Nicholas Johnson, the head of the orbital-debris program at the National Aeronautics and Space Administration.
Today radars that were designed to scan the horizon for incoming Russian missiles track a silent armada of space junk instead. The U.S. Space Surveillance Network routinely follows more than 8,000 objects that are larger than four inches across, which is approximately the lower limit of detectability for current technologies. When the network determines that the shuttle will pass within about a mile of a piece of debris, the astronauts may decide to fire the shuttle's rockets to give the junk a wider berth. But Earth's orbit also contains perhaps 100,000 objects that measure from half an inch to four inches across—objects too small to see on radar but large enough to cause a spacecraft to fail. They are the land mines of space, undetected until something crosses their path.