President Trump’s Space Force, a proposed military department dedicated to fighting war in space, would cost the Pentagon $1.5 billion to $2.7 billion in additional money over five years, according to a study released this week by the Center for Strategic and International Studies, a Washington think tank.

That estimate is far below the $13 billion price tag that Air Force Secretary Heather Wilson recently reported to Pentagon leaders and is certain to fuel the debate over the cost and necessity of what would become the first new military service branch since the Air Force was created in 1947.

While the White House has pushed aggressively for the establishment of the Space Force, which Trump has championed in rallies, a new military department would need to be approved by Congress. It is unclear whether there is enough support for it to pass.

Many in the Pentagon, including Defense Secretary Jim Mattis, initially said they were against the idea. Leaders of the Air Force, which now runs most of the Pentagon’s space operations and would have the most to lose if a new service were created, also came out against the proposal.

Speaking last week at a conference held by DefenseOne, Wilson said the $13 billion estimate “was the cost of a fully fledged, stand-alone department and also a unified combatant command.” But she added that the level of funding would ultimately be dictated by the legislation proposed by the White House. “The costs will be really based on what are the elements in the model in that proposal,” she said.

Last week, Deputy Defense Secretary Patrick Shanahan told reporters that the cost to create a Space Force would be much lower than that, possibly as little as $5 billion, which is closer to the think tank’s estimate.

In presenting the report Monday, Todd Harrison, a defense analyst at the Center for Strategic and International Studies, said the Space Force would mostly pluck people from across various departments in the Pentagon who work on space issues and gather them under a new department.

“Most of this is a simple matter of reorganization,” he said. “I don’t think cost should have a big factor in [Congress’s] decisions. I think a bigger factor is whether or not you think it is needed.”

Harrison estimated that the annual budget of a Space Force would be about $21.5 billion, which is more than the Coast Guard’s $11.7 billion budget but less than that of the Marine Corps, $29.2 billion.

Its total workforce would be fewer than 50,000 — or about the size of the Coast Guard.