Last month, it was reported that a broken spy satellite was falling toward Earth and would crash in early March. Today, word leaked that the Bush administration has decided to shoot down the satellite before it makes impact. Anonymous Defense Department officials suggest that a specially modified missile will be shot from a Navy cruiser to destroy the satellite. In a January 2007 "Explainer" column, Daniel Engber explained how anti-satellite weapons take care of satellite space junk. The article is reprinted below.
China blasted one of its own satellites with a ballistic missile last week, proving it has at least some proficiency at space warfare. The only other countries to have developed anti-satellite weapons are the United States and the former Soviet Union. How do these weapons work?
The Chinese used a "kinetic kill vehicle." That kind of anti-satellite system doesn't use any explosives—instead, it relies on the enormous force of a high-speed collision. (Remember, these satellites orbit at about 17,000 miles per hour.) First, they waited until their old weather satellite was directly overhead—which happens twice per day—and then they fired off a medium-range ballistic missile fitted with some kind of homing device.
The United States experimented with a similarly ground-based missile system in the early years of the Cold War. The plan was to detonate a nuclear warhead in the general vicinity of an enemy satellite. But a single botched test—and an international treaty against the use of space nukes—convinced the military to abandon the idea.
The first operational anti-satellite (or "ASAT") weapon didn't show up until the Soviets started testing in the late 1960s. Instead of sending up a missile for a head-on space collision, they decided to build a "hunter-killer" satellite that used radar to close in on its target from behind. Once it got within about five miles, it would blow up and release damaging shrapnel. The system didn't always work: Fewer than half of its test-runs were successful.
The hunter-killer satellites had to be fired from the ground, which meant that the Soviets—like the Chinese—could attack targets only when they were directly overhead. In the 1980s, the United States developed a heat-guided, kinetic-type weapon that could be fired on the move from a modified F-15 fighter jet. On Sept. 13, 1985, the new system was tested on what the Air Force said was an aging and obsolete research satellite. Astrophysicists felt differently: "It's a definite loss, no doubt about it," said the chief of NASA's solar physics branch, according to Science magazine.
A successful hit from any of these ASAT systems blows its target to smithereens. That can be very dangerous, as the smithereens remain in orbit for years to come. Stray bits could collide with other satellites, which would create still more bits of space debris hurtling around the Earth at high speeds.
Secret research projects in the United States have focused on tidier alternatives, like a ground-based laser system that can disable a satellite without breaking it apart. Another idea focused on a three-stage rocket that would deploy a giant Mylar mitt to smack its target and sweep up the pieces.
Explainer thanks Laura Grego of the Union of Concerned Scientists and Theresa Hitchens of the Center for Defense Information.