President Trump’s vision of a new Space Force military service went from off-the-cuff public remarks to Pentagon policy in just five months.

The president has charged the military with carrying out one of the most profound changes in its organizational structure since the Air Force was created in 1947. But ultimately Congress will decide whether Trump’s plan achieves lift off or comes crashing back to Earth.

That political debate is looming in 2019 and could hinge on money. The price tag for a Space Force is so far unknown. The Air Force estimates it will cost $13 billion.

“We're saying that the cost associated with standing up the additional structure, we'll probably know by the end of the year. We haven't done that cost estimation yet,” Deputy Defense Secretary Pat Shanahan said.

Once calculated, the tab will be handed to the House and Senate early next year as part of a detailed Pentagon request for legislation needed to create the sixth service branch.

Top supporters in the House, including the leaders of the Armed Services Committee, are already anticipating a budget fight.

“One of the things I can tell you I’m focused like a laser on, as [are Reps.] Mac Thornberry, Adam Smith and Jim Cooper, is we are not going to let them gold-plate this because that is one of the ways that you try to kill things is to make it too expensive to do, and we’re not going to let them slow-roll it,” said Rep. Mike Rogers, R-Ala., the House Armed Services subcommittee chairman who has spearheaded a space service for nearly two years with Cooper.

Rogers called any opposition over cost a red herring and said Space Force will not require much additional funding.

“There are going to be some costs for the initial transition just because it’s disruptive,” he said.

Trump’s new space service is envisioned as a way to pull together the Pentagon’s space operations, which are mostly handled now by the Air Force, into an independent service to better protect the constellation of military and commercial satellites that provide critical services. The satellites that provide GPS services and missile detection are increasingly under threat from Russia and China.

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“The Department of Defense is moving forward with initial steps to strengthen American security in space, and we will continue to work with both parties in Congress to provide the necessary authorities and funding to stand up a new branch of our armed forces, and the United States Department of the Space Force will be a reality by the year 2020,” Pence told an audience at NASA’s Johnson Space Center in August.

Pence’s ambitious timeline would likely require Congress to create and fully fund the new service in a single budget cycle. Standing up the Space Force in a single year would be “unprecedented,” said Gen. Paul Selva, the vice chairman of the Joint Chiefs of Staff.

It could also mean the Pentagon will be requesting billions in new spending just as the defense budget falls back into uncertainty. Congress’ two-year deal to raise Budget Control Act spending caps will expire and the law is set to slash $71 billion from this year’s $647 billion defense cap.

The creation of the Space Force next year is “entirely possible” just as the Air Force was created by Congress in 1947 by a single piece of legislation, said Todd Harrison, the director of defense budget analysis and the director of the Aerospace Security Project at the Center for Strategic and International Studies.

“The vast majority of the money for the Space Force comes from within the budget, so really all you’re doing since it’s a reorganization, you are moving people, organizations and their budgets from wherever they are today in the services, you just move those line items into the new Space Force,” Harrison said.

The Pentagon will likely need additional funding for a Space Force headquarters staff and a secretariat office, he said.

“I think that that should be pretty minimal,” said Harrison, although the Pentagon could exaggerate the costs if it wants to scuttle the plan.

Defense Secretary Jim Mattis and Air Force leaders originally opposed a new space service, saying it would add unneeded bureaucracy, but have backed the plan since Trump started publicly touting the idea during a March speech in San Diego.

Key members of the Senate Armed Services Committee have also been skeptical of the need for an entire service branch and helped scuttle a push last year by the House for what was then called Space Corps. The military has about 18,000 active-duty troops who focus on space operations, less than half of the size of the active-duty Coast Guard.

The Senate committee has authority over the Pentagon’s Space Force plans and will delve into the cost as part of the upcoming budget process, said Sen. Jim Inhofe, R-Okla., who is the senior Armed Services majority member.

“We’ll be holding hearings on how we can be most effective and most cost-effective at the same time,” Inhofe said. “I don’t know why it would cost that much more. If you can perform the same thing without having the additional bureaucracy we might want to do that. That’s what hearings are all about.”

Inhofe was an opponent of the space service, making a rare break with Trump, yet warmed to it after meeting with Mattis. But other members have remained cool to the idea, making its future uncertain.

“Where I’ve been focused is, hey, let’s get the five services we currently have back to readiness levels that the American people think we should have and that we’re not at. Nobody thinks we’re at the readiness levels that we should be,” said Sen. Dan Sullivan, R-Alaska, a committee member. “Then once we get there, then we can talk about the Space Force.”