HOUSTON — Less than a month ago, White House officials said a falling spy satellite would likely pose little threat to humans — but on Thursday, the Pentagon said President Bush himself approved an unorthodox plan to destroy the satellite with a missile strike.
What could warrant such a change of heart? It's the realization that the spacecraft could be bringing a toxic iceberg back down to Earth.
Aboard the 2.5-ton derelict satellite, designated "USA-193," is a fuel tank containing half a ton of hydrazine. Since the satellite went dead within hours after launch 14 months ago, the fuel has not been depleted by normal rocket maneuvers.
Hydrazine is a nasty chemical that could poison the area where it is released. Until recently, U.S. officials were saying that the tank would be crushed as the satellite fell through the atmosphere, sometime in early March. If that were the case, the toxic hydrazine would almost certainly be burned off and safely dispersed during the fiery fall.
NASA Administrator Michael Griffin sketched out a different scenario, however, during Thursday's news conference with Gen. James Cartwright, the vice chairman of the Joint Chiefs of Staff. Griffin said NASA experts calculated that the hydrazine was frozen solid due to the satellite’s yearlong drift through the cold of space. The tank, with its half-ton ice core of hydrazine, would thus become one of the most perfect re-entry vehicles ever to fall back to Earth.
Griffin explained that the contents of the tank could turn to slush during the fall, but would very likely survive and leak toxic gas over the crash site. Another expert told msnbc.com privately that the solid ice would provide structural support against the 20 to 25 G’s of deceleration experienced by the satellite during re-entry.
Pentagon officials said it was that safety concern, rather than the intention to test a potential anti-satellite weapon, which led them to develop the plan for a missile intercept. They hope the impact of the warhead on a modified Standard Missile-3, or SM-3, will shatter the satellite — and particularly the spherical hydrazine tank. The first shot could occur as early as next week, after the space shuttle Atlantis' return from its mission to the international space station.
Video: U.S. to use missile to shoot down satellite Would a direct hit be required? Experts on space debris told msnbc.com that even a glancing blow would likely be enough. The force of a missile hitting an orbiting object is much more violent than the force of a bullet striking a target, or even an anti-aircraft missile hitting an airplane. In the space case, the tremendous speed of the impact carries so much kinetic energy that both vehicles literally explode due to the hypersonic shock waves sweeping through their structures.
If the missile strike leads to such a disintegration, sharp observers should be able to spot the ice fragments from the fuel tank. As the fragments evaporate in direct sunlight, they could create mini-comets visible from Earth’s surface, lasting for hours before dispersing.
Pentagon officials said the intercept would occur within range of military optical and radar sensors. Their goal would be to confirm the existence of dispersed hydrazine in the debris. If the sensors don't show the fuel dispersing, missile operators would target the fragment judged most likely to be the still-surviving fuel tank. A second shot could occur within a day or two of the first.
Giving the missiles a boost
Last week's orbital readings indicated that the satellite was circling Earth at an altitude between 160 and 168 miles (255 and 268 kilometers) and descending at an increasing rate, currently about six-tenths of a mile (1 kilometer) per day. Gen. Cartwright said the intercept would be attempted when the satellite descended to about 150 miles (240 kilometers).
The SM-3 has typically been used for testing the Pentagon's missile defense system, and reaches a nominal maximum altitude of just 100 miles (160 kilometers). For the satellite intercept, three missiles — one each on three different AEGIS-class Navy cruisers — will be modified to reach the higher altitude.
This isn't the first time hydrazine has posed a problem in space: The fuel freezes at temperatures below 36 degrees Fahrenheit (2 degrees Celsius), and satellites without active heating will drop to temperatures below that — as a case involving the Soviet Salyut 7 space station demonstrated dramatically in 1985.
After Salyut 7's power system failed, the water in its supply tanks froze solid, along with the hydrazine in its propulsion system. A pair of cosmonauts reached the icy station and were able to activate its electrical system, and then carefully thawed the frozen tanks.
NBC News space analyst James Oberg spent 22 years at NASA's Johnson Space Center as a Mission Control operator and an orbital designer. He is the author of several books on international space policy, including "Space Power Theory" and "Star-Crossed Orbits."