The explorer had filed a lawsuit last year after Mr. Musk accused him of being a “pedo guy.” Mr. Musk wanted the case thrown out.
A federal judge delivered a pair of legal setbacks to the tech billionaire Elon Musk this week, rejecting Mr. Musk’s attempt to throw out a defamation lawsuit brought against him by a British cave explorer whom Mr. Musk had accused on Twitter of being a “pedo guy.”
The judge, Stephen V. Wilson in United States District Court in Los Angeles, also ruled that the explorer, Vernon Unsworth, was not a public figure — meaning the bar will be lower for him to prove defamation.
Judge Wilson ordered Monday that a jury trial begin on Dec. 3. He denied Mr. Musk’s argument that the case should be tossed because his statement, a shortened version of the word pedophile, was a throwaway insult not to be construed as fact.
Mr. Musk had also argued that Mr. Unsworth was a public figure because of his participation in a high-profile rescue of a youth soccer team trapped in a cave in Thailand last year. Public figures need to meet a high legal bar called “actual malice” — essentially knowing a statement is false when the statement is made and making it anyway — to prove defamation.
But in an 18-page order, Judge Wilson said that Mr. Unsworth was a private figure, who needed only to meet a lower bar — that Mr. Musk acted negligently — to recover some damages.
“This case creates the ‘perfect storm,’ where a jury is going to tell us what they think about this kind of conduct on social media,” said L. Lin Wood, a lawyer for Mr. Unsworth. “If they do what I believe they must under the evidence, the message is going to be strong: ‘Don’t do this,’ and to Musk: ‘Don’t do it again.’”
In response to questions about Judge Wilson’s ruling, a lawyer for Mr. Musk, Alex Spiro, said, “We look forward to the trial.”
Mr. Musk had sent a team of engineers from the companies he leads — Tesla, SpaceX and the Boring Company — to help retrieve the children. The engineers produced three miniature submarines that Mr. Musk thought could have helped with the rescue, according to Judge Wilson, but the head of the search operation rejected the idea as impractical.
In an interview with CNN in July 2018, Mr. Unsworth called the submarine idea a “P.R. stunt.”
“He can stick his submarine where it hurts,” Mr. Unsworth said. “It just had absolutely no chance of working.”
Mr. Musk lashed out at Mr. Unsworth on Twitter in a series of posts and called him a “pedo guy.” Mr. Musk later deleted and apologized for those messages.
In an email to a BuzzFeed reporter about Mr. Musk’s statements, Mr. Musk urged the reporter to “stop defending child rapists” and suggested that Mr. Unsworth had had a child bride. Mr. Unsworth has denied all of Mr. Musk’s accusations.
Both the email and the tweets are part of Mr. Unsworth’s defamation claim. He is seeking damages in excess of $75,000.
Laura Prather, a First Amendment lawyer with the law firm Haynes and Boone who is not involved in the case, said the determination that Mr. Unsworth was a private figure made it much easier for him to succeed in trial.
“With actual malice, you have to basically have known that what you were saying was false,” she said. “With negligence, it’s just whether or not you exercised due care.”
Erwin Chemerinsky, dean of the University of California, Berkeley, school of law, said that Mr. Musk had faced an uphill battle to get the suit thrown out. Mr. Unsworth is not a well-known public figure and an accusation of being a pedophile is “very damning.”
Mr. Chemerinsky, who teaches First Amendment law, said the facts of the case were so distinctive that it would be difficult to predict what could happen going forward.
“What you have here is, the plaintiff being involved in a tragedy, Elon Musk accusing him of being a pedophile, the plaintiff not really being a public figure and Elon Musk being a huge public figure,” he said. “I’d be very cautious about generalizing because it’s such a unique set of facts.”