How did the U.S. shoot down its spy satellite?

One could make the argument that the Cold War arms race between the United States and the Soviet Union was a good thing, at least in one case. If it weren't for the development of ballistic missiles, there would have been no need to develop anti-ballistic missiles. And without the latter, there was no way to tell where the two-ton (1,800 kg), highly classified spy satellite that the U.S. Navy shot down might land.

The Pentagon modified an existing missile system, and the Navy spent three weeks making modifications to the Block III, a Raytheon SM-3 antiballistic missile. The missile launched from the U.S.S. Lake Erie, a guided missile cruiser in the Pacific Ocean near Hawaii, hitting the satellite at 10:26 p.m. Eastern Standard Time (EST) Feb. 20. The Navy had a very short window to make the strike; the missile had to be fired sometime between 9 p.m. EST on Feb. 20 [source: ABC News] and 10:30 p.m. EST on Feb. 21, 2008 [source: Wired]. It was the first time the United States attempted to use a tactical missile to take out a spacecraft [Business Day].

The Raytheon SM-3 isn't designed to explode upon impact; rather, the missile destroys whatever it hits using brute force, like a bullet. It contains a heat-seeking component, which guided it to the wayward spy satellite. The Navy launched the Block III so that it traveled in the opposite direction of the USA 193, in order to produce a head-on (collinear) collision. The closing velocity of the impact -- which in this case is the sum of the two objects' velocities -- is estimated around 22,000 miles per hour (35,406 km/h) [source: Department of Defense].

The USA 193 spy satellite was about the size of a school bus and weighed 5,000 pounds (2,268 kg) -- a sizeable target -- if it had re-entered Earth's atmosphere intact, its course would have become erratic. The Union of Concerned Scientists said before the launch that the missile had "no better than a 50 percent chance of hitting its target" [source: The Washington Post], although the Block III is a highly accurate missile. As of November 2007, the system had hit 11 of its last 13 targets [source: Wired].

One of the greatest challenges of "the shot" was the brief window of time that the Navy had to take it. When China shot down its weather satellite in January 2007, the altitude (nearly 600 miles (966 km) above sea level) caused the debris created by the impact to hang around in space. While the space junk shouldn't enter our atmosphere -- and pose a threat to life on Earth -- it does present a problem for space travel. The debris can collide with functioning spacecraft, including those that bear human passengers.

To reduce the chance that the debris from the USA 193 will stay in space, the United States chose to shoot it down once it reaches an altitude of about 150 miles (241 km) [source: The Washington Post]. According to the Reuters news agency, the Block III missile hit USA 193 at an altitude of 153 miles (246 km) above the Earth. The window for a perfectly successful shot was narrow. The New York Times put it this way before Wednesday night's successful mission: "If they fire too late, the satellite will enter the atmosphere and start hurtling in unpredictable directions. If they fire too soon, space debris could spread and threaten the International Space Station and other satellites." The Navy had a back-up plan in the event of a missed target: It had two more ships ready to fire. "The worst is that we miss, and then we have a known situation, which is where we are today," said the Joint Chiefs' Gen. Cartwright [source: Department of Defense].

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