"Count me in for $50M," he wrote in the posting.
Billionaire tech entrepreneur Jared Isaacman, who bought the flight with SpaceX and announced the mission would raise funds for St. Jude’s, donated $100 million to the cause.
With Musk’s contribution and the $60 million donated by the public, $210 million was raised for children’s cancer research.
Hayley Arceneaux, the flight’s medical officer and a physician assistant at St. Jude’s, said Musk’s pledge "brings tears to my eyes."
"Thank you @elonmusk for this generous donation toward our $200 million dollar fundraising goal for @StJude!!!," she said on Twitter.
The Inspiration4’s mission took off from Kennedy Space Center in Florida atop a SpaceX Falcon 9 missile last Wednesday.
The SpaceX Crew Dragon capsule carried Isaacman, Arceneaux, a childhood cancer survivor, Sian Proctor, a geologist, and Chris Sembroski, a Lockheed Martin employee.
The crew remained in orbit for three days and reached an altitude of 367 miles.
The mission marked the first time a flight went into orbit with an all civilian crew.
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