The Air Force X-37B Orbital Test Vehicle at NASA's Kennedy Space Center Shuttle Landing Facility in 2017 (Picture: US Air Force).
The White House has announced ambitious plans to create the US Space Force as a sixth, separate military warfighting service by 2020.
US president Donald Trump tweeted his approval for the multi-billion-dollar scheme, saying: "Space Force all the way!"
The proposal requires congressional approval and has been met with scepticism from military leaders and experts who question the wisdom of launching an expensive, bureaucratic new service branch.
Vice President Mike Pence announced the new force during a Pentagon speech, fleshing out an idea that President Donald Trump has flagged in recent months as he vowed to ensure American dominance in space.
Mr Pence described space as a domain that was once peaceful and uncontested but has now become crowded and adversarial, saying:
"Now the time has come to write the next great chapter in the history of our armed forces, to prepare for the next battlefield where America's best and bravest will be called to deter and defeat a new generation of threats to our people, to our nation."
Mr Pence portrayed the change as a response to foes' potential aggression rather than any offensive US military effort.
"As their actions make clear, our adversaries have transformed space into a warfighting domain already, and the United States will not shrink from this challenge," he said.
In June, the president directed the Pentagon to create a "separate but equal" space force, a complicated and expensive move that could take years to gain Congress' approval and become operational.
On Thursday, Mr Pence said the administration will work with Congress on the plan and will outline a budget next year.
The last time the US created a new uniformed military service was in 1947, when the Air Force was launched after the Second World War. It joined the Army, Navy, Marine Corps and Coast Guard.
Defence secretary Jim Mattis has endorsed steps to reorganise the military's space warfighting forces and create a new command but has previously opposed launching an expensive new service.
A new branch of the military would require layers of bureaucracy, military and civilian leaders, uniforms, equipment and an expansive support structure.
Asked about the cost, deputy defence secretary Patrick Shanahan told reporters the Pentagon does not have a number yet, but will when the legislative proposal is finished by the end of the year.
"I would assume it's billions," he said.
Deborah James, who served as Air Force secretary for the final three years of the Obama administration, estimated it would be five to 10 years before a separate service would be fully formed.
"Eventually, it'll settle out, but you will go through years of thrashing. And is that thrashing going to slow your momentum or is it going to help you achieve your goals and address the real challenges that we have on our plate?" she said at Brookings Institution last week.
"I don't think so. I don't. I wouldn't vote in favour of it."