Within days, a Pearl Harbor-based Navy ship may be called upon to perform a first: try to shoot down a failing minivan-sized military satellite while it's still in space.

The cruiser USS Lake Erie is expected to fire one or more modified SM-3 missiles to punch a hole in the 5,000-pound spy satellite over the Pacific. Two other Navy ships will provide trajectory information and backup.

The window to accomplish the mission will open in three to four days, and remain open for about a week after that, officials said. But the Pentagon said the Navy will not fire until after the shuttle Atlantic mission ends next Wednesday.

President Bush ordered the shoot-down attempt after security advisers said its re-entry posed potential danger to civilian populations.

James Jeffrey, a deputy national security adviser, said with descent, the satellite could release more than 1,000 pounds of hydrazine fuel. The fuel could spread across an area equal to two football fields. Hydrazine is similar to chlorine or ammonia and affects the lungs and breathing.

"The likelihood of the satellite falling in a populated area is small, and the extent and duration of toxic hydrazine in the atmosphere would be limited," Jeffrey said at a Pentagon news conference today. "Nevertheless, if the satellite did fall in a populated area, there was the possibility of death or injury to human beings beyond that associated with the fall of satellites and other space debris."

Marine Gen. James E. Cartwright, vice chairman of the Joint Chiefs of Staff, would not say exactly where the ships would fire from, saying only it will be from the northern hemisphere and the Pacific Ocean.

The goal is to hit the satellite just before it enters earth's orbit so that the hydrazine tank explodes. The satellite belongs to the National Reconnaissance Office and was launched on Dec. 14, 2006. Intercepting the satellite at about 130 nautical miles altitude will reduce the risk of debris in space, officials said.

If the satellite is hit, officials hope 50 percent of the debris will fall to earth in the first two orbits and the rest shortly thereafter.

The Pentagon also wants to intercept the satellite at a point just above the atmosphere so there would be a high likelihood of bringing it down in an unpopulated area.

In January, the U.S. government notified other nations that the satellite was unresponsive and would make an uncontrolled re-entry in late February or early March.