The Trump administration's proposed Space Force, which faces an uncertain fate in Congress, would be the smallest military service — by far.

Details of the Pentagon's proposal released Friday show the new service would have about 15,000 personnel, including an unspecified number of civilians, but would begin in 2020 as only a headquarters of about 200. The proposal was submitted Wednesday to Congress, which must authorize the new service.

Space Force would be the first new military service since an independent Air Force was established in 1947 as part of a broad reorganization of the government's military and intelligence agencies.

Space Force would reside within the Department of the Air Force, similar to how the Marine Corps exists within the Department of the Navy. It would have its own chief of staff, a four-star general who would answer to the secretary of the Air Force, currently Heather Wilson.

Currently the smallest branch of the armed forces is the Coast Guard, which has about 40,000 active-duty members in uniform and is part of the Department of Homeland Security. The second-smallest service, the Marine Corps, has 186,000. The biggest military service is the Army, with 487,500 active-duty members.

President Donald Trump has pushed the Pentagon to create a Space Force that is "separate but equal to" the other military services. His first defense secretary, Jim Mattis, initially was cool to the idea, arguing against adding expensive new layers of bureaucracy.

Critics have questioned the need to create a Space Force as a separate military service, noting that there are relatively small numbers of people required to carry out space-related missions. The Pentagon argues that a separate service will give space issues a stronger voice within the military and create a special space culture and expertise.

The plan submitted to Congress calls for phasing in a Space Force over five years beginning with the 2020 budget year. This would add an estimated $2 billion in costs beyond what is spent on existing military space activities. Many details of the new service have yet to be worked out, including whether it would have its own boot camp for recruits and whether it would have its own uniform design.

Not all space-related government activities would fall under the Space Force. It would not include the Pentagon's Missile Defense Agency, the National Aeronautics and Space Administration, the National Oceanic and Atmospheric Administration or the National Reconnaissance Office, which builds and maintains intelligence satellites in space.

Acting Defense Secretary Patrick Shanahan issued a written statement Friday calling the proposal "an historic moment for our nation," ensuring the U.S. can "compete, deter and, if needed, win in a complex domain." He credited Trump with a "bold vision for space."

While many in Congress support organizational changes to improve U.S. space defense capabilities, some key members question the wisdom of creating a separate military service. Sen. James Inhofe, the Oklahoma Republican and chairman of the Senate Armed Services Committee, has expressed doubts, although on Friday he issued a statement welcoming the Pentagon's proposal.

Sen. Jack Reed of Rhode Island, the ranking Democrat on the Armed Services Committee, has questioned the need for a separate space service, as has the new Democratic chairman of the House Armed Services Committee, Rep. Adam Smith of Washington state.