WASHINGTON — Creating a Space Force as a separate military service, as proposed by President Donald Trump, would cost an estimated $12.9 billion in its first five years, according to a detailed Air Force plan for how to go about it.

This is the first publicly available cost estimate. When the White House announced plans to establish a Space Force in August, Deputy Defense Secretary Patrick Shanahan declined to give a figure but said it would be in “the billions.”

The Air Force’s estimate is contained in a Sept. 14 memo from Air Force Secretary Heather Wilson, who proposed that the Pentagon ask Congress for the authority and money to establish a Space Force headquarters in 2020.

“The President has clearly communicated his desire for a military department for space,” she wrote. “Strategic competition with Russia and China is the focus of our approach.”

Creation of Space Force as a separate military service will require congressional action. The administration is expected to submit proposed legislation early next year authorizing the establishment of a Space Force.

A copy of the Air Force memo was obtained Monday by The Associated Press.

The memo says the first-year cost of a Space Force would be $3.3 billion, and the cost over five years would be an estimated $12.9 billion.