By Kristin Roberts
WASHINGTON (Reuters) - The Pentagon plans to shoot down a disabled U.S. spy satellite before it enters the atmosphere to prevent a potentially deadly leak of toxic gas from the vehicle's fuel tank, officials said on Thursday.
President George W. Bush decided to have the Navy shoot the 5,000-pound (2,270 kg) minivan-sized satellite with a modified tactical missile, after security advisers suggested its reentry could lead to a loss of life.
U.S. officials said they were not trying to protect classified information on the satellite or to demonstrate their capabilities to China, which downed one of its own satellites with a missile last year, drawing criticism from Washington.
But some experts disagreed and questioned the risks associated with shooting down the satellite.
Two defense officials also cited disagreement within the administration over the action and said the decision appears to have been strongly influenced by the White House.
The Pentagon hopes to strike the satellite just before it reaches the atmosphere and drive it into ocean waters. Officials would not estimate the likelihood of success, only calling it high.
Thousands of space objects fall to Earth each year, but they generally scatter over a huge area and there have never been any reported injuries.
What makes this different is the likelihood that the satellite could release much of the more than 1,000 pounds (454 kg) of hydrazine fuel as a toxic gas, according to James Jeffries, deputy national security adviser.
He said the satellite was unlikely to hit a populated area and described the danger from toxic gas as limited. But Jeffries added: "There was enough of a risk for the president to be quite concerned about human life."
SIGNAL TO CHINA?
Jeffries and other U.S. officials rejected suggestions that Bush opted to shoot down the satellite out of concern that classified material on board could survive reentry into Earth's atmosphere, and potentially land in the wrong hands.
They also said Washington was not shooting the satellite down in response to China's anti-satellite test last year, noting the United States had already demonstrated its capability to hit a space object with a missile in the 1980s.
China did not notify other countries before its test, marking a significant departure from U.S. efforts this week.
But some experts questioned Bush's decision and the calculation of the risks associated with shooting the missile down compared with allowing it to burn up upon re-entry.
Jonathan McDowell, astronomer with the Harvard-Smithsonian Center For Astrophysics, called the decision "regrettable."
"Clearly someone in the administration who has the instincts of a cowboy has decided this is the perfect excuse to rattle our sabers and show the Chinese that we have the same capabilities," he said.
SATELLITE NEVER WORKED
The satellite is a classified National Reconnaissance Office spacecraft launched from Vandenberg Air Force Base in California in 2006, according to four senior U.S. officials, who asked not to be named.
Defense officials said the satellite, known as L-21, was built by Lockheed Martin Corp. It cost hundreds of millions of dollars but the U.S. government will not say precisely how much, calling the figure classified.
Boeing Co played a role early in the project, but was relieved by Lockheed.
The satellite has been out of touch since shortly after reaching its low-Earth orbit. Since the satellite never became operational, it has toxic rocket fuel on board that would have been used to maneuver the satellite in space.
Officials said that without the shootdown, about half the satellite, including the fuel tank, would survive reentry into Earth's atmosphere and estimated a crash could spread toxic material across an area equal to two football fields.
"The tank will survive. It will be breached. The hydrazine will reach the ground and that's not an outcome we want to see," NASA administrator Michael Griffin said at the briefing.
The Pentagon has a window of about seven to eight days to fire the missile. That window opens in about three to four days, the officials said.
(Additional reporting by Andrea Shalal-Esa)
(Reporting by Kristin Roberts and David Morgan; Editing by Todd Eastham)
The launch of USA-193.
The United States military has announced their intention to use a missile to destroy USA-193, a spy satellite which failed immediately after launch in December 2006. Since launch, the satellite's orbit has decayed and it is approaching the point at which it will re-enter the atmosphere and potentially fall to Earth. The missile will be used as a kinetic anti-satellite weapon (ASAT), and will destroy the satellite through hitting it at high speed.

The satellite potentially has hazardous materials on board, including hydrazine. Ordinarily, the hydrazine would be used as fuel, but since the satellite failed immediately after launch, this was never used. It is expected that about half of the satellite would be capable of surviving re-entry, potentially including the fuel tank. Since hydrazine is toxic, the satellite could pose a severe danger if it hits in a populated area, however it has been considered likely that the tank would probably rupture and explode during re-entry. Destroying the satellite on the grounds of safety is the stated purpose of shooting down the satellite. Officials have denied claims that it is to prevent secrets that the satellite is carrying from falling into the wrong hands, or that it is a demonstration in response to a Chinese ASAT test in January 2007.

The missile which will be used to shoot down the satellite will be a Standard Missile 3 (SM-3), which will be launched from the USS Lake Erie in the North Pacific Ocean. The missile, which was designed to shoot down other missiles, will be modified slightly in order to allow it to recognise USA-193 as its target. 

If the attack is successful, the satellite will be broken into many small pieces. This will eliminate the hazard from the hydrazine and break into small enough pieces that nothing will survive reentry. Officials reported that the likelihood of success was "high". NASA Administrator Michael Griffin said that it was impossible to make the situation any worse. Missing the satellite would change nothing, while a slight blow would still result in the satellite returning to Earth, and a direct hit would destroy the satellite. It is believed that the missile will be aimed directly at the satellite's fuel tank.

While the debris will not pass near enough to the International Space Station to cause disruption, it is not clear whether it could affect spacecraft passing through the lower orbit in which USA-193 currently resides. For this reason, the satellite will not be destroyed until the Space Shuttle ''Atlantis'', which is currently conducting mission STS-122, has landed. The debris caused by the satellite's destruction is expected to re-enter the atmosphere within a few weeks.