NATIONAL HARBOR, Md. – President Donald Trump's proposed Space Force is likely years away, and the military has begun breaking down the numbers for what setting up a sixth branch might cost.

But in the meantime, the military isn't going to wait for a Space Force to develop in order to defend space.

In a Wednesday interview with CNBC's Morgan Brennan, Secretary Heather Wilson, the Air Force's top civilian, called her office's proposal for the new branch "a bold idea to dominate in space." Yet the U.S. is already the top dog in the great beyond.

"The Air Force has a lot of authorities now to continue to accelerate defendable space so that we dominate and win," Wilson said. "We're the best in the world at space. Our adversaries know it, and they're developing the capabilities to deny us the use of space in crisis or war and we are determined that that not be allowed to happen."

Various outlets obtained a memo from Wilson – sent to Deputy Secretary of Defense Patrick Shanahan's office on Friday – which pegged the costs of setting up a Space Force headquarters and operations at about $13 billion for the first five years.

"You've got to have a secretary and all the budget people, you have to have people recruit, and all of the things that an independent department does," Wilson said on "Squawk Alley."

Shanahan, who is currently organizing the Pentagon's efforts on implementing a Space Force, requested relevant offices within the Defense Department to propose concepts for the newest branch. Wilson's memo emerged in the wake of Shanahan's request and was in some elements out of step with the Pentagon's plan to stand up the space-focused branch by 2020.

The Pentagon has yet to release a cost estimate for Trump's proposed Space Force.

Read more: What will Trump's Space Force cost? The Pentagon hasn't figured that out yet

"The process that we are going through is to put together a plan that we can carry forward for legislative proposal and what I would tell you is, there is no group think in the Pentagon," Shanahan said Wednesday at the Air Force Association's annual Air, Space and Cyber conference. "We are really wrestling with the how."

Defense Secretary James Mattis said in August that the Pentagon's cost estimate process is "underway now," speaking at his first press conference since Trump tasked the Pentagon to create the new sister service branch. Mattis said the U.S. cannot afford "complacency in any domain" and recognized cyberspace and outer space as war-fighting domains on par with land, air and sea.

"We have worked with Congress and the White House to define the evolving space problem that we confront. Now we are implementing the National Defense Authorization Act and its provision for a unified space command in line with the president's vision for a needed Space Force," Mattis said.

He added that the Pentagon looks to Congress for the necessary legislation in creating the sixth branch.

Read more: Trump's space force won't work, a former Air Force secretary says

This new sister service branch would stand alongside the Army, Air Force, Navy and Marine Corps, with a force of about 13,000 people. Wilson expects a debate over the details of establishing Space Force to take "over the next six to eight months."

Pushback continues to build against the idea of a Space Force, with the Air Force Association, or AFA, being the latest to disparage the proposal.

"To split up the well-integrated set of air and space capabilities that have been organized to seamlessly contribute to America's military capabilities would result in more harm than good," the AFA said in a statement on Monday.

Wilson, meanwhile, told reporters on Tuesday that the $13 billion estimate "is a conservative number."

"The proposal that the Air Force circulated to other elements of the Defense Department on Friday was actually tasked to us by the group that is trying to develop concepts to implement the President's vision," Wilson said.

But Wilson also said the Air Force, which executes "about 90 percent" of U.S. military operations related to space, does not "have to wait" to be the strongest force in outer space.