The Pentagon said yesterday that it will try to shoot down a malfunctioning satellite before it has a chance to return to Earth and spread hazardous materials over populated areas.
The Defense Department will attempt to blast the U.S.-made satellite with a missile launched from a Navy ship in coming weeks before the satellite re-enters Earth's atmosphere, officials said.
The high-altitude strike ordered by President Bush would represent the first time the United States has taken such a step to deactivate a wayward orbital device. China performed a similar mission last year, which the United States criticized.
Experts say the plan may be the best of several bad options, and noted that it would almost certainly leave more litter in space that could ultimately damage working satellites and space capsules.
"This is all about trying to reduce the danger to human beings," Deputy National Security Adviser James Jeffrey told reporters during a briefing at the Pentagon.
But some experts questioned the need for the blast - and the United States' motives.
"It's consistent with the administration's political interests in responding to the Chinese anti-satellite demonstration and taking an aggressive posture militarily," said Dr. Jonathan C. McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics. "This is really about an excuse to test their existing anti-missile system in a satellite-killing mode."
The satellite, known as USA-193, was built by Lockheed Martin Corp. and conked out shortly after launch in December 2006. It contains about 1,000 pounds of frozen hydrazine, a hazardous propellant stored in metal tanks, and government officials and experts say the tanks could survive re-entry into the atmosphere.
When China blasted one of its orbiting weather satellites out of the heavens Jan. 11, 2007, it prompted criticism and created an estimated 800 debris fragments - many of which have to be continually tracked.
Yesterday, Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, said that the "window of opportunity" for the American shoot down will open in the next three or four days and last for seven or eight days. He did not say whether the Pentagon has decided on an exact launch date.
"This is the first time we've used a tactical missile to engage a spacecraft," Cartwright said, but he would not say what the odds of success are.
He said a Navy missile known as Standard Missile 3, designed to hit ballistic missiles, would be fired in an attempt to intercept the satellite at an altitude of about 150 feet, just prior to it re-entering Earth's atmosphere. It would be "next to impossible" to hit the satellite after that because of atmospheric disturbances, Cartwright said.
A second goal, he said, is to directly hit the fuel tank in order to minimize the amount of fuel that returns to Earth.
Cartwright also said that if an initial shoot down attempt fails, a decision will be made whether to take a second shot.
Jeffrey said members of Congress were briefed on the plan earlier yesterday and that diplomatic notifications to other countries would be made before the end of the day.
While shooting down the missile would pose minor risks in the short term, McDowell, the Harvard-Smithsonian astronomer, pointed to the possibility that it would start or accelerate an arms race in space that could lead to more serious, and damaging, demonstrations by the United States and other countries.
Dr. Paul Fischbeck, professor of engineering and public policy at Carnegie Melon University, said shooting down the satellite would give the Pentagon a rare opportunity to test its capabilities.
"I think it's a good opportunity for touching up the system," Fischbeck said.
But Donald Kessler, who was director of NASA's orbital space debris monitoring program from 1980 to 1996, said it's unlikely the Pentagon is shooting the missile to test its capabilities. The military is modifying a short-range missile for the task in such a way that it can only be used to target that specific satellite, Kessler said.
The government announced in January that the satellite was falling from the sky, but initially said little else, fueling speculation that it was a failed spy probe. It was left to a community of hobbyists who track satellites to identify it - providing descriptions in Internet chatter that were later confirmed by government officials.
Space debris is a well-known and studied hazard, experts say. Every year, an estimated 200 pieces of debris are added to the inventory monitored by NASA and the military. NASA officials say they are currently tracking about 12,400 pieces of orbital debris.
The International Space Station, orbiting space observatories such as the Hubble Space Telescope and NASA's space shuttles are all designed to withstand being struck by smaller objects - that must travel at 7 kilometers a second to stay in orbit. An average of two windows have to be replaced after each shuttle mission because of debris damage.
But Kessler and others say there is usually little risk to people being hit by debris falling to Earth, since most of the planet is covered by oceans and the debris is minute.
John E. Pike, director of GlobalSecurity.org, which tracks military and space activities, said the mission seemed to make sense but questioned why the Pentagon did not announce it sooner.
"This is getting pretty down to the wire," he said.
He said he believes some officials may be concerned that other countries could obtain pieces of the satellite and study them.
"I suspect they may be concerned about pieces showing up on eBay and Russians and Chinese getting into a bidding war on potentially sensitive technology," Pike said. "I can guarantee you there's somebody in the government that has that concern."
The Associated Press contributed to this article.