In June, President Donald Trump called for the establishment of the sixth armed service, to join the Air Force, Army, Marine Corps, Navy and Coast Guard.
"We have not done cost estimation yet", Shanahan told reporters at the Pentagon, adding that he expects he won't have a more concrete number until November.
Faced with growing competition and threats from Russian Federation and China, the White House on Thursday said it will create the U.S. Space Force as a sixth, separate military service by 2020.Vice President Mike Pence told a Pentagon audience that the plan fulfills President Donald Trump's vow to ensure America's dominance in space - a domain that was once peaceful and uncontested that has now become crowded and adversarial.
They include establishing civilian oversight for the Space Force and creating a United States Space Command to "improve, evolve and plan space war-fighting".
The White House says that the Space Force will be created by 2020.
Shanahan - who led the new Pentagon report that details what the department can do to stand up the new service sans Congress - said he supports the Space Force's creation.
Democratic Senator Brian Schatz said on Twitter that Space Force "won't happen".
It's 2018 and Space Force is, somehow, a real thing.
"This report is a step in a multi-year process that we think will result in a safer, stronger America". This will be an "elite group of joint war fighters specializing in the domain of space", and it will draw its ranks from men and women from across the military, according to Pence.
Several lawmakers have already hit back at the move as being costly and unnecessary. "This report reviews the national security space activities within the Department of Defense, and it identifies concrete steps that our administration will take to lay the foundation for a new department of the Space Force".
The creation of the Space Development Agency.
"It's not enough to have an American presence in space", Pence said Thursday in a speech at the Pentagon.
One of the key functions of the new branch would be to aid USA forces on the ground and the air force, giving them combat superiority. Rather, she says, the billions of dollars could be better spent in so many other areas.
"China and Russian Federation have been conducting highly sophisticated on-orbit activities that could enable them to maneuver their satellites into close proximity of ours, posing unprecedented new dangers to our space systems", he said.
The force would require congressional approval.
"What was once peaceful and uncontested is now crowded and adversarial, today other nations are seeking to disrupt our space based systems and challenge American supremacy in space", Pence said.
Mattis had previously opposed the creation of a separate military branch focused on space, writing to Rep. Mike Turner of OH in July of 2017 that he did "not wish to add a separate service that would likely present a narrower and even parochial approach to space operations".