Shooting Down a Satellite: All in the Timing

Feb. 15, 2008 -- This week, as the military unveiled its unprecedented plan to shoot down an ailing spy satellite, an ammonia tank once used aboard the International Space Station plummeted to Earth. So did the second stage of a Delta rocket that put an Italian radar imager into orbit in December. By week's end, part of a Russian Molniya rocket that left Russia's Plesetsk Cosmodrome in 1996 should dive into the atmosphere as well.

The Aerospace Corp.'s Center for Orbital and Re-entry Debris Studies estimates there are some 70,000 objects about two centimeters (0.8 inches) in diameter orbiting between 525 and 625 miles above Earth.

Now the U.S. military wants to do away with one nagging piece of debris: a 2.5-ton satellite that failed shortly after reaching orbit in December 2006.

It won't be easy -- especially the timing -- but this piece of space debris poses a particular risk.

While the chances of being hit by falling space junk are minute -- the Aerospace Corp puts the risk at less than one in one trillion, compared with, for example, a one in 1.4 million chance of being hit by lightning in the United States -- the military says the dead satellite contains about 1,000 pounds of toxic propellant, making it potentially more dangerous should it fall on a populated area.

The plan, reviewed and approved by President George W. Bush, is to modify a missile designed to intercept short- and medium-range ballistic missiles, aim it at the wayward satellite just as it reaches the edge of Earth’s atmosphere, and shoot the fuel tank so it spills its toxic load before reaching the ground.

Without intervention, the tank and some of its frozen load of hydrazine, would survive the re-entry into the atmosphere, experts say, pointing to the fuel tanks recovered from the space shuttle Columbia wreckage.

"The analysis that we've done is as certain as any analysis of this type can be," said NASA administrator Michael Griffin. "The hydrazine tank will survive intact … the hydrazine in it is frozen solid. Not all of it will melt. So you will land on the ground with a tank full of slush hydrazine that would then later evaporate. The tank will have been breached -- not probably, but the tank will have been breached -- because the fuel lines will have been ripped out of the main spacecraft, and so that hydrazine will vent."

Straight shot

The missile would be launched from a Naval cruiser, likely from the Pacific Ocean, sometime between Wednesday, when the space shuttle Atlantis is due to land, and next Friday, a time period when the satellite is expected to be close to re-entering the atmosphere.

The challenging part of the operation is not the shot, which military officials say is very similar to what the missile is designed to do, namely intercept and destroy an incoming missile.