Deputy Defense Secretary Patrick Shanahan on Thursday said Pentagon officials do not yet know how much it will cost to establish President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE’s desired "Space Force" but assume it would cost "billions."

“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.

Trump in March first announced his desire to create a Space Force, and in June directed the Pentagon to begin creating the new military branch.

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Vice President Pence followed up by announcing Thursday at the Pentagon that the administration will seek to create the Space Force as the sixth branch of the military, to be established by 2020.

“Our adversaries have transformed space into a war-fighting domain already and the United States will not shrink from this challenge,” Pence said.

He did not say the total cost estimated to stand up the new military arm, but called for Congress to initially authorize $8 billion for the effort in next year’s defense spending bill.

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.

He also argued that Defense Secretary James Mattis’s initial reluctance to embrace such an effort last year was due to the uncertain defense budget and a desire to avoid bureaucracy and more overhead costs.

Mattis last year opposed a House effort to establish a space corps within the Air Force, even sending a letter Congress stating his position.

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“At a time when we are trying to integrate the Department’s joint warfighting functions, I do not wish to add a separate service that would likely present a narrower and even parochial approach to space operations,” Mattis wrote in a July 2017 letter to Rep. Michael Turner (R-Ohio).

On Thursday Mattis appeared to change his tune, and said space was “becoming a contested war-fighting domain and we have got to adapt to that reality.”

Several lawmakers have already hit back at the move as being costly and unnecessary.

“The VP just announced a new military branch - a 'Space Force,' because no [Republican] is willing to tell POTUS it’s a dumb idea,” Sen. Brian Schatz (D-Hawaii), wrote on Twitter. “Although 'Space Force' won’t happen, it’s dangerous to have a leader who cannot be talked out of crazy ideas. Elect D’s to check POTUS power.”

Rep. Tulsi Gabbard Tulsi GabbardRepublicans call on DOJ to investigate Netflix over 'Cuties' film Hispanic Caucus campaign arm endorses slate of non-Hispanic candidates Gabbard says she 'was not invited to participate in any way' in Democratic convention MORE (D-Hawaii) wrote on Twitter that the Space Force will mean “more money for defense contractors” while “Flint still does not have clean water. Puerto Rico is still struggling to recover from Hurricane Maria. Infrastructure across the country is failing. People are going homeless because they can't afford rent & more. Priorities?”

And former NASA astronaut Mark Kelly said Trump's plan for a new military space branch is “redundant” and “wasteful.”

Other lawmakers, however, expressed their support for a Space Force, calling it a “much-needed” advancement of U.S. defense policy.

“We have been warning for years of the need to protect our space assets and to develop more capable space systems," House Armed Services Committee members Mike Rogers (R-Ala.) and Jim Cooper (D-Tenn.) said in a statement.

Rogers and Cooper last year tried and failed to lead the effort to establish a separate space corps.