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Jeremy Grantham says the odds of a SpaceX stock crash are high.

The GMO cofounder raised concerns about SpaceX's valuation and the company's ambitious goals.

SpaceX's planned projects are "utterly inconceivable" to most "serious physicists," Grantham said.

Count Jeremy Grantham among the SpaceX skeptics.

The investing titan and GMO cofounder said it is highly likely that shares of SpaceX will plunge one day. Speaking to Morningstar this week, Grantham shares a number of concerns about Elon Musk's rocket company, including those related to itys valuation, its lofty space exploration goals, and general doubts about the productivity benefits of AI.

SpaceX shares have cooled after their initial post-IPO surge, hovering close to levels around $150, which is where the stock began trading on June 12 after its IPO. The stock joined the Nasdaq 100 index on Tuesday.

"If its price is justified, we live in a strange world, and we'll be lucky not to be bossed around by our automaton friends," Grantham said, referring to Tesla's humanoid robots, which SpaceX CEO Musk has said he plans to send to Mars on a SpaceX spacecraft.

"And if it's much more likely to have a crash — both ways it will be historically notable — and I would bet at least 90% on the second," the billionaire investor said.

Questions about SpaceX's valuation have been swirling since before it went public. SpaceX is the seventh most valuable public company in the world, despite losing nearly $6 billion in 2025.

In its S-1 filing, the company also estimated it had a total addressable market of $28.5 trillion, 90% of which it said was driven by AI.

Grantham said valuation figures were "just amazing" when looking at the firm's total addressable market projection, considering that its AI products were a "third-rate" offering when compared to products from Anthropic and OpenAI.

Second, Grantham took issue with SpaceX's space exploration plans. In its S-1 filing, the firm outlined several of its ambitions, including its goals to mine asteroids, build data centers to orbit the earth, and establish a colony on Mars.

Most of those goals are considered to be "utterly inconceivable" by "serious physicists," Grantham said.

Finally, he raised concerns about how the expected productivity gains from AI in general might be unrealistic, hobbling the AI component of SpaceX's growth story.

"It will be amazing, by the way, if it doesn't collapse, because it will need such massive developments on AI that our entire lives are totally different," he said. "And here we are promising—I mean, some people are saying it'll be 10%, 20% a year productivity—I mean, they have no idea what they're talking about."

Grantham is known as a permabear on Wall Street, frequently making dire predictions for markets. In recent years, he's repeatedly sounded the alarm for a stock bubble and a steep stock crash, even as equities posted back-to-back years of double-digit gains.

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