WASHINGTON — The Pentagon counted down Wednesday toward a dramatic nighttime effort to shoot down a dying and potentially deadly U.S. spy satellite, using a souped-up missile fired from a ship in the Pacific. Foul weather threatened to delay the operation.

The timing was tricky. For the best chance to succeed, the military awaited a combination of favorable factors: steady seas around the Navy cruiser that would fire the missile, optimum positioning of the satellite as it passed in polar orbit and the readiness of an array of space- and ground-based sensors to help cue the missile and track the results.

The operation was so extraordinary, with such intense international publicity and political ramifications, that Defense Secretary Robert Gates _ not a military commander _ was to make the final decision to pull the trigger.

The government organized hazardous materials teams, under the code name "Burnt Frost," to be flown to the site of any dangerous or otherwise sensitive debris that might land in the United States or elsewhere.

High seas in the north Pacific posed the first obstacle as the USS Lake Erie prepared to launch a three-stage missile. Beyond a certain point, rough seas can interfere with the cruiser's launch procedures.

The plan was for the SM-3 to soar 130 miles to just beyond the edge of the Earth's atmosphere in an attempt to speed its non-explosive warhead directly into the satellite.

Early in the day, a senior military officer said it didn't look as if the weather would be good enough. That was shortly after the space shuttle Atlantis landed at 9:07 a.m. EST, removing the last safety issue for the military to begin determining the best moment for launch.

Another officer said hours later the weather was improving and might permit a launch by Wednesday night. Or the military could try again on Thursday or any day until about Feb. 29, when the satellite is expected to have re-entered the Earth's atmosphere.

The aim is not just to hit the bus-sized satellite _ which would burn up upon re-entering the atmosphere anyway _ but to obliterate a tank onboard that is carrying 1,000 pounds of hydrazine, a toxic fuel. The fuel, unused because the satellite died shortly after reaching orbit in December 2006 _ could be hazardous if it landed in a populated area.

The Centers for Disease Control and Prevention issued a health bulletin saying that the health risk from satellite debris was considered to be low. "However, CDC is encouraging health officials and clinicians to review information about the health effects related to hydrazine to prepare in case their communities are affected by satellite debris."

In a routine precaution, notifications have been issued worldwide to mariners and aviators to stay clear of an area in the Pacific where the satellite debris might fall. The military has calculated that the risk to aviation is so low that U.S. and international aviation officials have decided they are probably not going to reroute air traffic, a senior military officer said Wednesday.

The officer briefed reporters at the Pentagon on technical and logistical matters related to the effort. Under ground rules set by the Pentagon, the officer could not be identified by name.

The attempted shootdown, already approved by President Bush, is seen by some as blurring the lines between defending against a hostile long-range missile and targeting satellites in orbit.

Much of the equipment used in the satellite shootdown is part of the Pentagon's missile defense system, a far-flung network of interceptors, radars and communications systems designed primarily to hit an incoming hostile ballistic missile fired at the United States by North Korea. The equipment, including the Navy missile, has never been used against a satellite or other such target.

The three-stage Navy missile, the SM-3, has chalked up a high rate of success in tests since 2002 _ in each case targeting a short- or medium-range missile. A hurry-up program to adapt the missile for this anti-satellite mission was completed in a matter of weeks; Navy officials say the changes will be reversed once this satellite is down.

Some people are skeptical.

"The potential political cost of shooting down this satellite is high," said Laura Grego, an astrophysicist with the Union of Concerned Scientists. "Whatever the motivation for it, demonstrating an anti-satellite weapon is counterproductive to U.S. long-term interests, given that the United States has the most to gain from an international space weapons ban. Instead, it should be taking the lead in negotiating a treaty."

Defense Secretary Gates is being advised directly by Air Force Gen. Kevin Chilton, commander of U.S. Strategic Command. Gates was traveling to Hawaii on Wednesday to kick off a nine-day trip. Officials said his stop at U.S. Pacific Command was scheduled before it was known that the satellite shootdown could happen while he was there.

The military has hours each day to monitor a long checklist of technical factors and conditions before deciding whether to proceed with the missile launch. But there is a very narrow window _ described by the senior military officers as "tens of seconds" _ in which the missile must be launched in order to have the best chance of having the satellite debris land mainly in the Pacific.

Officials will know within minutes whether the missile has hit the satellite, but it will take a day or two to know whether the fuel tank has been destroyed, officials said.

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.

___

Associated Press writer Pauline Jelinek contributed to this report.