Retired NASA astronaut Mark Kelly slammed President Trump’s directive to create a “Space Force” — calling it a “dumb idea” as a sixth branch of the military.

“This is a dumb idea. The Air Force does this already. That is their job,” the former Navy pilot, combat vet and four-time space flier tweeted.

“What’s next, we move submarines to the 7th branch and call it the ‘under-the-sea force?’” added Kelly, husband of former Democratic Congresswoman Gabby Giffords of Arizona.

He was referring to the Colorado-based Air Force Space Command, whose responsibilities include supporting military use of satellites, rocket launches and cyberwarfare operations.

The group — also known as the US Space Command — also helps keep track of space junk orbiting the planet as it poses a threat to spacecraft.

“AFSPC’s mission is to provide resilient and affordable space and cyberspace capabilities for the Joint Force and the Nation,” according to the website of the command, which has more than 35,000 people assigned to 134 sites across the globe.

In July 2016, Space Command announced the creation of a Space Mission Force, which military brass likened to an expeditionary force aimed at responding to any outer-space attacks from adversaries, Business Insider reported.

“Despite world interest in avoiding militarization of space, potential adversaries have identified the use of space as an advantage for US military forces, and are actively fielding systems to deny our use of space in a conflict,” Gen. John E. Hyten wrote in a 2016 white paper when he led the Space Command.

On Monday, Trump announced that he was directing the Pentagon to create the out-of-this-world force to ensure the safety of American spacecraft and astronauts as traffic in the cosmos increases.

“We are going to have the Air Force, and we are going to have the Space Force — separate but equal,” Trump told a meeting of the National Space Council. “It is going to be something — so important.”

The president called it “a giant step toward inspiring future generations and toward reclaiming America’s proud destiny in space.”

Trump first broached the subject of a space-based branch of the military in March during a speech before a crowd of Marines at the Miramar Air Station in San Diego.

Some members of Congress have poured cold water on Trump’s call for a new military branch.

“The president told a US general to create a new Space Force as 6th branch of military today, which generals tell me they don’t want,” Democratic Sen. Bill Nelson of Florida tweeted Monday.

“Thankfully the president can’t do it without Congress because now is NOT the time to rip the Air Force apart. Too many important missions at stake.”