Elon Musk attacked Sean Duffy, the US secretary of transportation, on Tuesday in a series of posts on X, accusing him of trying to “kill NASA”, suggesting he should be fired and calling him “Sean Dummy”. The posts intensified a long-running feud between Duffy, who is also the acting head of Nasa, and Musk, whose company SpaceX is central to the US space program.

Musk’s tirade against Duffy followed a statement from the transportation secretary on Tuesday that Nasa would reopen contracts for the agency’s Artemis mission to land humans on the moon, which SpaceX had previously secured. Duffy said that SpaceX had fallen behind on its timelines. Duffy suggested the contract might go to another billionaire’s rocket company, Jeff Bezos’s Blue Origin.

Duffy’s remarks and Musk’s backlash are part of a broader fight over the direction of US space travel and a months-long battle over who will take over leadership of Nasa. The clash was a central part of Musk’s dramatic exit from the White House earlier this year, after the Trump administration rejected his preferred Nasa pick, Jared Isaacman, and selected Duffy as the space agency’s acting chief.

The debate over who should lead Nasa has been revived in recent weeks, according to the Wall Street Journal, with Isaacman and Duffy both vying for the job. Trump denied Isaacman the position earlier this year, citing his donations to the Democratic party, calling him a “blue blooded Democrat, who had never contributed to a Republican before”. Duffy has lobbied for Nasa to be placed under the authority of the transportation department, the Journal reported.

Musk, who is no longer part of the administration, but is in the process of a detente with Trump following their public blowup, has used his platform to criticize Duffy and his reported proposals to fold Nasa into the transportation department.

“The person responsible for America’s space program can’t have a 2 digit IQ,” Musk posted on Tuesday on X, the social media platform which he owns.

“Sean Dummy is trying to kill NASA!” Musk said in another post.

The SpaceX CEO also posted a poll referencing Duffy’s past as a champion lumberjack speed-climber, asking: “Should someone whose biggest claim to fame is climbing trees be running America’s space program?”

Musk has long favored Isaacman, his ally and a billionaire investor in SpaceX, to lead the US space agency. Musk’s company is Nasa’s largest private contractor, and the agency has been critical to its growth, providing SpaceX with billions of dollars in funding. SpaceX won a $2.9bn contract in 2021 to provide the lunar lander for the Artemis 3 mission.

The Artemis program, which dates back to Trump’s first term, aims to return astronauts to the moon for the first time since 1972. Its initial goal was to land on the Moon’s south pole in 2024 and establish a human presence by 2028 with the eventual intention of reaching Mars, one of Musk’s long-held ambitions. Nasa has repeatedly delayed Artemis, however, and its next step to orbit four astronauts around the moon is now set for no earlier than 2026.