WASHINGTON — U.S. President George W. Bush approved a Defense Department plan to try and shoot down a crippled spy satellite after becoming convinced that the spacecraft's toxic hydrazine fuel posed an unacceptable risk to people on the ground, senior U.S. government officials said at a Pentagon press briefing Feb. 14.

Controllers lost contact with the classified satellite shortly after its Dec. 14, 2006, launch and U.S. government officials recently acknowledged its orbit is decaying and that it would re-enter the atmosphere sometime in March. In order to prevent the satellite's hydrazine fuel tank from coming down intact and possibly dispersing highly toxic fumes over an area roughly the size of two football fields, U.S. officials will take the extraordinary step of attempting to shoot it down just before it re-enters. A direct hit to the spherical tank, which measures about 40 inches (100 centimeters) across, would result in the hydrazine being dispersed in the atmosphere and posing no hazard on the ground, the officials said.

The intercept attempt, which will involve a Standard Missile 3 fired from a U.S. Navy Aegis ship, will take place during a window that opens after NASA's space shuttle Atlantis returns from its International Space Station-construction mission Feb. 20 and will remain open for several days. The window was selected based on the satellite's current re-entry trajectory and the U.S. government?s desire to minimize hazards in space, in the air and on the ground.

U.S. Marine Corps Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, said the intercept, if successful, will take place at a low enough altitude to minimize orbital debris. Cartwright, along with NASA Administrator Mike Griffin said well over 50 percent of the debris from a successful intercept will re-enter and burn up in the atmosphere within two orbits, or 10-15 hours, while the remainder would come down within a matter of weeks. Satellites typically operate at altitudes higher than the debris is expected to reach. The international space station orbits Earth at an altitude of around 199 miles (320 kilometers).

Griffin said the intercept is expected to occur at an altitude of about 149 miles (240 kilometers). He noted that the controversial anti-satellite test conducted by China last January occurred at an altitude of some 528 miles (850 kilometers) and much of the resulting debris will remain in orbit for 20 or more years.

Ambassador James Jeffrey, President Bush's deputy national security advisor, said another difference is that the United States is informing the international community well in advance of the attempted shoot-down. He emphasized that the motivation behind the test is to protect lives on the ground and that the action does not violate any U.S. treaty obligations.

Cartwright said he was confident in the intercept's chances of success. The Standard Missile 3 and Aegis system, developed for the sea-based component of the U.S. missile defense architecture, are both well understood. He said the software on the missile and the ship-based Aegis targeting system will have to be modified for the attempt.

Cartwright characterized the modifications as reversible actions that are being done strictly for what he characterized as a one-time-only event. The modifications, he said, are "not transferable to a fleet configuration."

The intercept's location will be chosen to maximize the chance that any debris that survives the kinetic impact of the missile and re-entry will land in the ocean, Cartwright said. The satellite and missile will close on one another at a velocity of about 22,783 miles (36,667 kilometers) per hour. Even if the kinetic interceptor only grazes the satellite it is likely to force it to re-enter the atmosphere sooner than otherwise would have been the case.

The operation will involve three ships and two backup missiles, Cartwright said. He outlined two possible scenarios in which the backup missiles would be used, one involving a failure of the primary interceptor to launch. If the first missile launches but misses the satellite, a second shot might be attempted depending on the likelihood that it can be pulled off in a location that likely would result in the debris falling harmlessly at sea.

After the first shot is attempted, the Pentagon will use its network of ground and space-based sensors to assess whether it scored a hit. In the case of a miss, a second shot, if attempted, could take place some two days after the first.

The satellite, a test craft owned by the secretive U.S. National Reconnaissance Office, weighs about 5,015 pounds (2,275 kilograms), Cartwright said. U.S. officials estimate that about half of the hardware would survive the fiery re-entry and plunge to Earth, he said.

Cartwright insisted there was no concern that the satellite?s sensitive, classified technology would survive re-entry and fall into the wrong hands, and said the decision to attempt the shoot-down was driven strictly by safety considerations. "This is all about trying to reduce the danger to human beings," he said.

Cartwright said satellites of this size — he said it is about the size of a bus — have re-entered throughout the space age, but few have done so with a full load of hydrazine.

The hydrazine tank aboard the failed satellite is about the same size as the one that was aboard the doomed space shuttle Columbia. The tank landed intact in an unpopulated wooded area in Texas after the orbiter broke up on re-entry in 2003.

Griffin said analyses have concluded that it is all but certain that the hydrazine tank aboard the crippled satellite will land intact if no action is taken. The tank will be breached and full of slushy hydrazine, which will evaporate and dissipate over the immediate area, he said. If this occurs in a populated area it could result in injury or even death, officials said.

The officials said the decision to attempt the intercept was based on a simple risk assessment. The outcome of a failed intercept will be no worse than if there was no attempt at all, they said. A successful intercept, they said, would result in reduced risk to people on the ground.