"The window will open when the shuttle is on the ground," said Army Lt. Gen. Carter Ham, Joint Staff director of operations at the Pentagon.

"All those who have a vested interest in this will then apply their best judgment as to when the best opportunity is to intercept the satellite."

Lt. Gen. Ham, acknowledged the difficulties involved in accurately targeting the satellite.

"It's a bit of an imprecise science at this point," he said.

The satellite is carrying 1,000 pounds of poisonous hydrazine fuel in a tank expected to survive re-entry, as well as a fuel tank liner made from toxic beryllium.

If the satellite is not shot down, the hydrazine could disperse into the atmosphere and severely damage lung tissue or cause death.

The Bush administration is now trying to convince foreign countries that the mission is not a secret weapons test.

If the missile is successful, there are concerns that it could be used as an offensive weapon against enemy orbiting communications and intelligence systems.

The State Department has instructed US diplomats around the world to inform their host governments that the mission is solely about protecting people.

"Our role is to reassure nations around the world as to the nature of what we are tying to do," said Sean McCormack, Assistant Secretary of State.

"It's an attempt to try to protect populations on the ground."

"This particular action is different than any actions that, for example, the Chinese may have taken in testing an anti-satellite weapon."

"The missions are quite different and the technical aspects of the missions are quite different."