U.S. officials rejected suggestions that Bush opted to shoot down the satellite out of concern that classified material on board could survive re-entry 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 sabres 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.
Defence 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 manoeuvre the satellite in space.
Officials said that without the shootdown, about half the satellite, including the fuel tank, would survive re-entry into Earth's atmosphere and estimated a crash could spread toxic material across an area equal to two football fields.
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.
