Donald Trump mused to an audience of Marines about launching an extraterrestrial branch of the military.

“Space is a warfighting domain, just like the land, air and sea”, Mr Trump told an audience at Miramar Air Station, near San Diego. “We may even have a space force - develop another one”.

“I was saying it the other day, because we’re doing a tremendous amount of work in space - maybe we need a new force, we’ll call it the Space Force”, Mr Trump added. “I was not really serious, and then I said ‘what a great idea, maybe we’ll have to do that. That could happen’”.

In viewing space through a national security lens, Mr Trump was echoing his administration’s past justifications for bolstering American space exploration. Last year the President signed a directive seeking to revive the country’s moribund space programme.

The stated rationale for those efforts is in part a military one. Vice President Mike Pence cited America’s “prosperity, security and identity” in describing the initiative last year, noting that Russia was eclipsing America beyond Earth’s atmosphere and cautioning that both Russia and China were formulating technology that could interfere with US satellites.

Reaching Mars has also been a recurring focus for Mr Trump. In signing the space policy directive, he spoke of establishing a foundation “for an eventual mission to Mars and perhaps, someday, to many worlds beyond”.

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In his speech to the Marines, Mr Trump projected more certainty about reaching Mars - and worked in a dig at former Democratic nominee Hillary Clinton.

Huge ice sheets on Mars could allow humans to live on the Red Planet, Nasa spacecraft shows