Some experts theorized that the administration was influenced by concern that classified components on the intelligence satellite could fall into hostile hands. Denying that, Marine Gen. James E. Cartwright, vice chairman of the Joint Chiefs of Staff, said any sensitive instruments would burn on reentry.

"Once you go through the atmosphere and the heating and the burning, that would not be an issue in this case," Cartwright said at a news conference. "It would not justify using a missile to take it and break it up further."

However, the government has never resorted to shooting down a disabled spacecraft or satellite, despite dozens of crashes and reentries over decades of spaceflight. Administration officials said this instance is different because the satellite failed shortly after its launch in December 2006, leaving almost all of its 1,000 pounds of hydrazine rocket fuel frozen in the uncontrollable spacecraft.

Because of its size -- Cartwright compared it to a bus -- only half of the craft is likely to burn on reentry. That means the fuel tank could survive if it is not destroyed by the missile strike. Normally, aging satellites -- their onboard fuel mostly consumed -- are steered into the ocean at the end of their life. But with the spy satellite's power and communications inoperable, it is tumbling, unguided, to Earth.