The US is preparing to shoot down one of its own satellites that is falling to Earth.

The 10-ton spy satellite was launched in December 2006 but failed properly to deploy and has been slowly falling back to Earth ever since.

US forces are planning to shoot it down to avoid danger to the human population as it is still loaded with toxic hydrazine rocket fuel.

The Pentagon has apparently spent the past three weeks modifying an existing missile to allow it to reach and destroy the satellite. The agency said in a statement that the mission has a "reasonably high opportunity for success".

Most satellites are intentionally crashed into the sea at the end of their useful life, but this model has never been under human control since reaching orbit and there is no way to ensure that it will not hit a population centre.

There are also suggestions that the military want to blow up the system to avoid the possibility of sensitive equipment landing on enemy soil.

Another suggested reason is to show China, which recently used a surface-to-space missile to shoot down a satellite in orbit, that the US has orbital warfare capabilities.