Donald Trump proposes a 'Space Force'

Donald Trump is seriously considering turning one of his jokes into reality.

Earlier this week he was speaking to a crowd of marines in San Diego, when he called outer space a "war-fighting domain, just like the land, air, and sea".

"I said 'maybe we need a new force. We'll call it the Space Force.' Not really serious," he told the crowd, who laughed.

"And then I said, 'What a great idea. Maybe we'll have to do that.' That could happen. That could be the big breaking story."

He called the proposed Space Force "vital to ensuring America continues to lead the way into the stars".

With Mr Trump's reputation for flip-flopping on issues, it would probably come as no surprise that just last year, his administration nixed plans for the Air Force to form a spinoff 'Space Corps'.

"I oppose the creation of a new military service and additional organisational layers at a time when we are focused on reducing overhead and integrating joint warfighting efforts," Air Force and Secretary of Defense James Mattis said at the time.

The 1967 Outer Space Treaty, which the US and Russia (as the Soviet Union) both signed, bans most military activities in space - including using and testing weapons of mass destruction and establishing military bases.

Russia operates its Space Forces as part of the Russian Air Force. Its publicly stated focus is defence against incoming missiles.