WASHINGTON -- The Pentagon says bad weather in the north Pacific makes it unlikely that a Navy ship will launch a missile Wednesday in an attempt to shoot down a wayward U.S. spy satellite in space.

A Pentagon official who briefed reporters on technical aspects of the decision on when to make the shootdown attempt said high seas currently are an obstacle for the Navy ship that would fire the missile.

The military will be making decisions each day on whether to proceed with an attempt or not -- and criteria could change several times each day, he said.

The opportunity to attempt a shot will be available only seconds each day, the official said.

The attempted shootdown was approved by President Bush out of concern that toxic fuel on board the satellite could crash to earth, the Defense Department has said.

Officials will know nearly immediately 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.

The military has readied a three-stage Navy missile, designated the SM-3, which has chalked up a high rate of success in a series of missile defense tests since 2002.

In each case it targeted a short- or medium-range ballistic missile, never a satellite. 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.

The government issued notices to aviators and mariners to remain clear of a section of the Pacific Ocean beginning at 10:30 p.m. EST Wednesday, indicating the first window of opportunity to launch an SM-3 missile from a Navy cruiser, the USS Lake Erie, in an effort to hit the wayward satellite.

Having lost power shortly after it reached orbit in late 2006, the satellite is out of control and well below the altitude of a normal satellite. The Pentagon wants to hit it with an SM-3 missile just before it re-enters Earth's atmosphere, in that way minimizing the amount of debris that would remain in space.

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.

Adding to the difficulty of the shootdown mission, the missile will have to do better than just hit the bus-sized satellite, a Navy official said Tuesday.

It needs to strike the relatively small fuel tank aboard the spacecraft in order to accomplish the main goal, which is to eliminate the toxic fuel that could injure or even kill people if it reached Earth. The Navy official described technical aspects of the missile's capabilities on condition that he not be identified.

Also complicating the effort will be the fact that the satellite has no heat-generating propulsion system on board. That makes it more difficult for the Navy missile's heat-seeking system to work, although the official said software changes had been made to compensate for the lack of heat.

The Pentagon press secretary, Geoff Morrell, said Defense Secretary Robert Gates was briefed on the shootdown plan Tuesday by the two officers who will advise him on exactly when to launch the missile -- Gen. Kevin Chilton, head of Strategic Command, and Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, who held Chilton's post until last summer.

China and Russia have expressed concern at the planned shootdown, saying it could harm security in outer space. At the State Department on Tuesday, spokesman Sean McCormack told reporters that the U.S. action is meant to protect people from the hazardous fuel and is not a weapons test.