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.”

A view of the Earth in an undated image. 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. REUTERS/NASA/File

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)