The U.S. navy shot down a defective spy satellite over the Pacific Ocean on Wednesday night, but it was too soon to say whether the tank of toxic fuel on board was destroyed.

The navy fired a missile at the American satellite, striking it as it travelled 247 kilometres above the Pacific Ocean at a speed of 27,000 km/h, the U.S. Defence Department said in a news release.

One of the main goals of the U.S. mission was to rupture the satellite's fuel tank, which contained 453 kilograms of hydrazine, a hazardous material that could pose a danger to humans. The navy wanted the fuel to dissipate before entering the Earth's atmosphere.

"Confirmation that the fuel tank has been fragmented should be available within 24 hours," the Defence Department said in its release.

The SM-3 missile was launched from the USS Lake Erie, a warship that was designed to launch missiles at incoming enemy missiles.

Because the satellite was relatively close to the Earth's surface when it was hit, debris was expected to begin entering the Earth's atmosphere almost immediately, the Defence Department said. It did not say where the debris was expected to fall.

"Nearly all of the debris will burn up on re-entry within 24-48 hours, and the remaining debris should re-enter within 40 days," the department said.

The U.S. government has a hazardous material team, known by the code name Burnt Frost, that will fly to any site where dangerous or sensitive debris might land, even if it's outside the United States.

If the satellite, which is the size of a bus, had not been shot down, it could have landed on Earth in early March, military officials had said. More than 1,000 kilograms of junk could have spread over hundreds of square kilometres.

The RCMP had feared the debris, including the tank containing toxic fuel, could have landed on Canadian soil.

"There is a 25 per cent chance the satellite will impact on Canadian soil," the RCMP said in an internal memo obtained by CTV prior to the satellite being shot down. "If it does hit Canadian soil, the debris field could be up to 600 miles radius. If it does not hit Canadian soil, we still may receive some debris."

The satellite, launched in December 2006, recently lost power and went out of control after the central computer failed. It carried a sophisticated and secret imaging sensor.

The Defence Department announced plans Feb. 14 to destroy the satellite just outside the atmosphere to minimize potential harm from its uncontrolled descent to Earth. The department wanted to limit the amount of debris remaining in space, ensuring only small pieces plunged toward Earth.

The operation was considered so extraordinary that Defence Secretary Robert Gates was to make the final decision to pull the trigger on the missile. A military commander would usually make such a decision.

Military officials initially feared that bad weather and high seas on Wednesday would postpone their missile attack. Had they not launched the missile Wednesday, they could have tried again, but only until Feb. 29, given the position of the satellite.

While Russia and China have expressed concern about the project, saying it could contribute to the militarization of space, Pentagon press secretary Geoff Morrell denied that the mission is designed to test the U.S. ability to shoot down satellites.

"We did that in 1985. Been there, done that," he said Tuesday.