According to Business Insider, billionaire Elon Musk announced on Saturday that he’s writing a book about Tesla and SpaceX, his two biggest companies.

“Time to tell the story of Tesla & SpaceX,” Musk tweeted. “Of Earth and Mars,” he said in his second tweet. “Lessons learned,” he said in his third.

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Many of his 46 million followers began to speculate what he meant, and in response to one user asking if he was writing a book, Musk replied: “Yes.”

Another user suggested Musk should release a regular podcast, similar to the Clubhouse interview he conducted on January 31 with Robinhood’s CEO Vlad Tenev.

Musk, who said Tuesday he was going off Twitter only to return two days later, tweeted that a podcast is “not that easy.” He said a podcast would need him to review his “two decades of intense work,” and go through old notes, emails, and texts or context.

According to the business news website, the last time Musk spoke about writing a book was in 2019, when he said that the story of his early career “may be worth a book one day,” adding that this would take him away from his work at SpaceX, Tesla, and Neuralink, his neural tech company.

As the CEO of SpaceX and Tesla, founder of The Boring Company, and co-founder of both OpenAI and Neuralink, it’s unclear how Musk will make time to write a book. According to the news website, in Tesla’s Q4 earnings call on January 27, Musk said he expected to stay Tesla CEO for the next few years, but that it would be nice to have “a bit more free time.”

Musk’s biography wouldn’t be the first book about his business journey. Journalist Ashlee Vance published a book about him in 2015 titled “Elon Musk: Tesla, SpaceX, and the Quest for a Fantastic Future.” Vance shadowed Musk for a year to get a taste of how Musk lives his life.

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