Cosmos 2251 was an ordinary satellite designed to transmit signals across the vast Russian landmass. Launched in 1993, it would appear every 90 minutes or so over the northern skies, relay electronic blips of information among a network of satellites and ground stations like a hockey player passing the puck, and disappear over the southern horizon.
Iridium 33, launched for Motorola in 1997, did something similar, though it took a slightly different orbit that brought it closest to Earth during its pass over North America. For years the two satellites circled the planet, minding their own business, never coming within a thousand kilometers of one another.
Then something happened to Cosmos. It may have sprung a small leak; perhaps it struck a tiny asteroid or a piece of debris. Nobody knows for sure, but for one reason or another, Cosmos drifted off course. T. S. Kelso, an aeronautics expert at Analytical Graphics, which provides satellite-tracking services to NASA, noticed that the orbits of Cosmos and Iridium were bringing the two satellites closer to each other all the time. In February he issued a warning that they would pass within a kilometer of one another. He was right. On Feb. 10, Motorola lost track of Iridium's signal. Over the next few days, Kelso and others surmised that what many had feared for years had finally come to pass: two intact satellites had collided head on.
The consequences go far beyond merely the loss of two pieces of property. Each satellite weighed more than half a metric ton and was moving at 7.5 kilometers per second. The resulting explosion was catastrophic, generating a massive cloud of cosmic debris—perhaps 100,000 pieces of junk bigger than one centimeter in diameter, estimates David Wright, a space expert at the Union of Concerned Scientists. In one stroke, the accident increased by nearly a third the number of stray objects in the crucial 700-to-900-kilometer band known as low Earth orbit (LEO). The junk cloud will eventually disperse around the entire planet, like a shroud.
The event served as a wake-up call to space planners. Insurance rates for the $18 billion worth of active commercial satellites now in orbit have ticked upwards by 10 to 20 percent since the accident. Governments, too, have grown to rely on networks of satellites to gather intelligence, direct weapons systems, forecast climate and weather changes, monitor agriculture, and operate communications and navigation systems. Experts calculate that debris will now strike one of the 900 active satellites in LEO every two or three years. For the first time, junk is the single biggest risk factor to equipment in some orbits. Among the orbital threats are two former Soviet nuclear reactors. Even the International Space Station may one day be at risk, as debris slowly descends to its 350-kilometer orbit.
Many experts now believe that even if all space littering were to stop completely, the number of stray objects would continue to increase for centuries. The reason: debris is now so dense that objects will continue to crash into each other, creating even more objects, expanding the rubbish cloud geometrically. "We've been saying for years that these things are going to happen," says Nicholas Johnson, head of NASA's Orbital Debris Program Office. "Until they happen, it's hard to get people's interest."
NASA engineer Don Kessler predicted the current situation with uncanny accuracy back in 1978. At the time, rockets carrying astronauts or communications satellites would discard upper stages like empty beer cans, often without having completely burned up their fuel. Several rockets exploded spontaneously in orbit, with no immediate consequences except to add to the orbiting debris. Each time an astronaut lost a bolt or a wrench, the object would take its place in the debris cloud. The Soviet Union may have been the most egregious polluter. In the 1970s and '80s, it launched 32 radar satellites, designed to track the positions of U.S. Navy ships, each powered by its own nuclear reactor.
Kessler ran the calculations, and the results came as a surprise. When one object slams into another, he found, they splinter into hundreds of pieces, each moving like a projectile at high speed. "Everybody had had this concept, probably from science fiction, of things floating together in space," he says. "People just hadn't thought about it." By about 2000, he predicted, collisions between satellites would start to outpace other forms of space accidents.
To avert what came to be known in the trade as the Kessler Syndrome, NASA formed its Orbital Debris Program Office, made Kessler the head, and gave him a staff of 20 or so engineers and scientists to tackle the problem. The group, headquartered at the Johnson Space Center in Houston, led a quiet and successful effort to reform the more wasteful practices of spacefaring nations. Now, discarded rocket stages are routinely angled to disintegrate in the atmosphere, or at the very least they're left with empty fuel tanks.
As Kessler and his team worked against the clock to slow the accumulation of debris, the cloud continued to expand. The Soviets tried ejecting the liquid metal at the cores of its nuclear satellites in the hope that the radioactive droplets would burn up harmlessly upon reentering the atmosphere; instead the liquid hardened into 100,000 or so metal balls, each too small to detect but big enough to cause significant damage to other satellites. In 1991, Cosmos 1934 hit a piece of junk that had previously broken off Cosmos 296. In 1996, France's Cerise satellite struck a discarded Ariane rocket stage. Junk struck a U.S. weather satellite in 1997 and a Russian satellite in 2002. Discarded U.S. and Chinese rocket stages collided with each other in 2005. In 2007, in separate collisions, the Meteosat 8 weather satellite and NASA's UARS satellite were knocked out of their orbits. Even so, for a while the total number of objects in the sky seemed to be leveling off, appearing to undermine Kessler's forecasts, until the China incident.
China's medium-range missile took off from its Xichang space center without incident on Jan. 11, 2007. It climbed to about 850 kilometers, the typical altitude of U.S. intelligence satellites (which is probably not a coincidence). The missile's lower stages dropped away to burn up in the atmosphere, leaving the "kill vehicle" to continue on to its target: a defunct Feng Yun weather satellite.
The engineering was flawless. The missile blew the satellite to bits—2,500 of them, each larger than 10 centimeters, according to the experts who keep count. The explosion increased orbital debris in LEO by about 40 percent. What Beijing hoped would be an impressive display of military prowess instead made China the world's biggest space litterbug. In one move it undid a decade of diplomatic progress in slowing the buildup of debris.
Even if the opprobrium heaped on China is enough to deter more anti-satellite missile tests, the future seems destined now to conform to the Kessler Syndrome, as the Iridium-Cosmos incident suggests. At present 750,000 pieces of man-made junk greater than one centimeter in diameter—about the size of a marble—are thought to be orbiting the planet. (If you include smaller objects, which can still cause damage because of their great speeds, the figure climbs to millions.) Half these objects can be found in LEO, which also contains about half the world's active satellites.
The China debacle, followed by the Iridium-Cosmos crash, galvanized NASA, the European Space Agency, and the United Nations, which have since held meetings on what steps might be taken to curb collisions and protect satellites. Shielding a satellite's delicate electronics might fend off some objects smaller than one centimeter, but it won't work against bigger objects. A better option might be to give satellites the capability to steer, but that would require equipping them with additional fuel, making them a lot heavier and more expensive to launch. It would also require better tracking of space objects. The U.S. Space Surveillance Network currently uses a combination of radar and optical telescopes around the globe to keep tabs on objects greater than five to 10 centimeters, periodically updating the position of each one. Even so, it can manage only about 13,000 objects. And the dynamics of orbiting flotsam and jetsam are complicated; the calculations in predicting any collision are likely to be off by hundreds of meters. A satellite could use up a lot of fuel steering so wide a berth around a threatening hunk of junk.
Many engineers are beginning to think that the only way to reverse the Kessler Syndrome will be to start actively removing junk from orbit. There is no shortage of ideas for doing so. For small and medium-size objects, engineers are noodling the idea of building lasers with beams powerful enough to "push" objects into higher orbits, where they're less likely to collide with satellites. (Eventually they'd come drifting back down, but that would be a problem for future generations.) One method to remove bigger, more threatening objects might be to send up some kind of spaceship to capture them one at a time and cart them to a lower orbit, where they would burn up harmlessly in the atmosphere. Another idea is to extend a tether from a space ship, grab hold of a piece of junk, and yank it down out of orbit. Either way, chasing down enough objects to make a difference would call for an enormous expenditure of rocket power. "Gravity," says Kelso, "is the big challenge." Until somebody finds a way to overcome that fundamental force, it looks as though we're just going to have to put up with the accidents.