Official sources say that such a move seems important to prevent the vehicle's fuel tank from leaking toxic gasses into the environment. Speaking on condition of anonymity, four senior US officials have revealed that the satellite is a classified National Reconnaissance Office spacecraft launched from Vandenberg Air Force Base in California in 2006. The sources have also revealed that President George W. Bush opted for a plan to shoot down the 2270 kg minivan-size satellite called USA-193 (also known as L-21) after being suggested by his security advisers that its re-entry might cause a loss of life.
Military officials hope to strike the satellite just before it reaches the atmosphere, and send it plunging into ocean waters. The satellite is said to have been out of touch since shortly after reaching its low-Earth orbit. Built by Lockheed Martin Corp at a cost of hundreds of millions of dollars, it has fallen more than 70 km to an orbit at around 280 km above the Earth.
According to officials, the satellite still contains toxic rocket fuel on board that would have been used to manoeuvre it in space, as it never became operational.
"Contact with hydrazine (a component in some rocket fuels) is hazardous. Direct contact with skin or eyes, ingestion or inhalations from hydrazine released from the tank upon impact could result in immediate danger. If this operation is successful, the hydrazine will then no longer pose a risk to humans," New Scientist magazine quoted a news release from the US Department of Defense as saying.
However, there are some experts within the administration who are concerned about the risks associated with shooting down the satellite, despite the fact that thousands of space objects that fall to Earth each year have never led to the reporting of any injuries.
"What makes this case a little bit different ... was the likelihood that the satellite upon descent to the Earth's surface could release much of its 1000-plus pounds (454 kg) of its hydrazine fuel as a toxic gas," New Scientist magazine quoted James Jeffries, assistant to the president and deputy national security advisor, as saying.
Jeffries was one of the experts who told a Pentagon briefing that the satellite was unlikely to hit a populated area and described the danger of toxic gas as limited.
He, however, added: "There was enough of a risk for the president to be quite concerned about human life."
US officials have denied that their plan is aimed either at protecting classified information on the satellite or at demonstrating their capabilities to China, which downed one of its own satellites with a missile in 2007, drawing criticism from Washington.
However, some experts still doubt the intentions of the Bush administration.
"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," said Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center For Astrophysics in Cambridge, US.