A FOUR-TONNE chunk of a SpaceX rocket is on a collision course with the Moon, according to online space junk trackers.

The out-of-control booster was launched from Florida in February 2015 as part of the rocket firm's first interplanetary mission.

ArsTechnica reports that the Falcon 9 second stage completed a long burn of its engines before deploying the NOAA's Deep Space Climate Observatory on a journey to a point more than 1 million km from Earth.

It did not have enough fuel left to take it back into Earth's atmosphere, leaving it on a chaotic orbit around our planet.