Washington - Taking a page from Hollywood science fiction, the Pentagon said Thursday it will try to shoot down a dying, bus-size US spy satellite loaded with toxic fuel on a collision course with the Earth.
The military hopes to smash the satellite as soon as next week, just before it enters Earth's atmosphere, with a single missile fired from a Navy cruiser in the northern Pacific Ocean.
The dramatic maneuver may well trigger international concerns, and US officials have begun notifying other countries of the plan, stressing that it does not signal the start of a new American anti-satellite weapons program.
Military and administration officials said the satellite is carrying fuel called hydrazine that could injure or even kill people who are near it when it hits the ground. That reason alone, they said, persuaded US President Bush to order the shoot-down.
"That is the only thing that breaks it out, that is worthy of taking extraordinary measures," said Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, during a Pentagon briefing.
He predicted a fairly high chance, as much as 80 percent, of hitting the satellite, which will be about 150 miles up when the shot is fired. The window of opportunity for taking the satellite down, Cartwright said, opens in three or four days and lasts for about seven or eight days.
"We'll take one shot and assess," he said. "This is the first time we've used a tactical missile to engage a spacecraft."
US Deputy National Security Adviser James Jeffrey dismissed suggestions that this was simply an attempt by the US to flex its muscles, and that officials were overstating the toxic fuel threat.
There might also be unstated military aims, some outside the administration suggested.
Similar spacecraft re-enter the atmosphere regularly and break up into pieces, said Ivan Oelrich, vice president for strategic security programs at the Federation of American Scientists. He said, "One could be forgiven for asking if this is just an excuse to test an anti-satellite weapon."
Left alone, the satellite would be expected to hit Earth during the first week of March. About half of the 5,000-pound spacecraft would be expected to survive its blazing descent through the atmosphere and would scatter debris over several hundred miles.
If the missile shot is successful, officials said, much of the debris would burn up as it fell. They said they could not estimate how much would make it through the atmosphere. They said the largest piece that would survive re-entry would be the spherical fuel tank, which is about 40 inches wide, assuming it is not hit directly by the missile.
The goal, however, is to hit the fuel tank in order to minimize the amount of fuel that returns to Earth, Cartwright said.
A Navy missile known as Standard Missile 3 would be fired at the spy satellite in an attempt to intercept it just before it re-enters Earth's atmosphere. It would be "next to impossible" to hit the satellite after that because of atmospheric disturbances, he said.
Known by its military designation US 193, the satellite was launched in December 2006. It lost power and its central computer failed almost immediately afterward, leaving it uncontrollable. It carried a sophisticated and secret imaging sensor.
Software associated with the Standard Missile 3 has been modified to enhance the chances of the missile's sensors recognizing that the satellite is its target. The missile's designed mission is to shoot down ballistic missiles, not satellites. Other officials said the missile's maximum range, while a classified figure, is not great enough to hit a satellite operating in normal orbits.
"It's a one-time deal," Cartwright said when asked whether the modified Standard Missile 3 should be considered a new US anti-satellite technology.
He said that if an initial shoot-down attempt fails, the military would have about two days to reassess and decide whether to take a second shot.
NASA Administrator Michael Griffin told reporters that analysis shows the hydrazine tank would survive a fall to Earth under normal circumstances, much as one did when the Space Shuttle Columbia crashed.
"The hydrazine which is in it is frozen solid, as it is now. Not all of it will melt," he said. If the tank hits the ground it will have been breached because the fuel lines will have broken off and hydrazine will vent out, he said.
Jeffrey said members of Congress were briefed on the plan earlier Thursday and that diplomatic notifications to other countries were being made by the end of the day.
"It should be understood by all, at home and abroad, that this is an exceptional circumstance and should not be perceived as the standard US policy for dealing with errant satellites," said House Armed Services Chairman Ike Skelton.