The SpaceX IPO has been a windfall for investors — or most of them, at any rate.

But there’s a fear at the back of their minds that they just can’t shake off. What if a significant number of buyers turn tail and become sellers, dumping their SpaceX stock and leaving the most loyal Elon Musk fans holding the bag?

It hasn’t happened yet, since the vast majority of investors for the time being are locked into their shares. Nonetheless, top venture capitalists feel then need to assure that there won’t be a mass selloff when selling opens, betraying a persistent anxiety beneath the bonanza.

“That is the bet now facing the market,” writes Edward Ludlow in Bloomberg’s Tech In Depth newsletter. “Not whether investors want to own SpaceX, but whether the people who already own it will keep owning it.” 

With the mountains of cash that the IPO has already generated, the temptation to sell is formidable. It made Musk into the world’s first trillionaire and catapulted SpaceX into one of the most valuable companies in the world, briefly edging out Amazon’s vast empire. Major backers like Andreessen Horowitz now own stakes north of $10 billion. If they sell early, it’ll guarantee massive profits.

Musk loyalists are convinced they’ll hold fast, however. “Many of them are not going to be selling,” Christian Garrett, an investing partner at 137 Ventures, an OG SpaceX investor that hasn’t sold a single share since 2011, told the Bloomberg newsletter.

Shaun Maguire of Sequoia Capital, a major tech VC and another longtime SpaceX investor, was especially bullish.

“I am personally going to hold my shares in this company forever, quite literally,” he said Tuesday in an interview on Bloomberg Television. “This is the biggest vision, the biggest mission of any company in history.”

The structure of the IPO is designed to shield against a premature wave of selling. IPOs typically have a 180 day lock-up period, during which shareholders are unable to sell. This provides stability and prevents insiders, like employees who own small shares, from instantly dumping their stock. SpaceX’s IPO uses a more complex lock-up scheme, with staggered release dates at predetermined times, some of them based on hitting certain milestones. Some of the largest shareholders have committed not to sell until mid-2027.

Arguably, the “long-term belief is embedded in the IPO itself,” according to the Bloomberg newsletter. 

But it’s not bulletproof. There could be a surge in sales once these lock-up periods start to open up, which is when SpaceX stock will really be put to the test.

“The supply of SpaceX shares available for public trading is set to roughly double in late August, grow about sixfold by the end of September, and reach roughly a third of the company by Halloween,” Chan Ahn, CEO of private equity firm Tessera, told Business Insider, with 58 percent of the company being tradable by the 180-day mark in December.

Before that milestone, there’ll be a concentration of unlocked stock between September and November, leaving quite a bit of room for market volatility, according to Ahn.

“I don’t think people have gotten their head around its significance,” he added.

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