Washington, Jan. 29 (Reuters): A disabled US spy satellite is likely to break into small pieces when it falls to Earth within weeks, posing little danger to humans, US government officials and space experts said today.

Most, if any, debris that survives the intense heat of re-entry would likely fall into the oceans, which cover more than 70 per cent of the Earth, White House National Security Council spokesman Gordon Johndroe said.

But he added that the US government was monitoring the satellite’s descent from orbit and examining different options to “mitigate any damage”.

The US military could potentially use a missile to destroy the minivan-sized satellite in space, but one senior US defence official said that was unlikely for several reasons, including concern about creating space debris as China did when it shot down one of its satellites last year.

“Given that 75 per cent of the Earth is covered in water and much of the land is uninhabited, the likely percentage of this satellite or any debris falling into a populated area is very small,” Johndroe said.

Pentagon spokesman Bryan Whitman said more than 17,000 man-made objects re-entered the Earth’s atmosphere over the past 50 years without major incident.

“We are monitoring it ... we take our obligations seriously with respect to the use of space,” Whitman said, noting the satellite was expected to return to earth “over the next several weeks ... late February, early March”.

The satellite is a classified National Reconnaissance Office spacecraft launched from Vandenberg Air Force Base in California in 2006, according to four senior US officials, who asked not to be named.

The satellite, known as L-21, has been out of touch since shortly after reaching its low-Earth orbit. Built by Lockheed Martin Corp at a cost of hundreds of millions of dollars, the satellite has fallen more than 70 km to an orbit at around 280 km above the Earth. US and European astronomers estimate it is dropping at an accelerating rate of some 8 km a day.

Because the satellite never became operational, it has toxic rocket fuel on board that would have been used to manoeuvre the satellite in space. It could pose a danger if the fuel tank does not explode upon re-entry. Thousands of space objects fall to Earth each year, but they generally scatter over a huge area and there have never been any reported injuries, two US officials said.

Occasionally, bigger objects survive, including a 255-kg stainless steel fuel tank from a Delta II rocket that landed 50 yards from a farmer’s home in Texas.