单选题 2. Space Junk
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, tie 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 l00,000 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 hand 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.