Virgin Galactic Holdings (SPCE -5.11%) chairman Chamath Palihapitiya has sold a significant portion of his stake in the company, a move that is likely to further pressure shares of the space tourism start-up.

According to a regulatory filing Palihapitiya sold 6.2 million shares at an average price of $34.32 per share, raising $213 million. He still owns 15.8 million shares, or about 6.2% of the company, along with investment partner Ian Osborne.

Palihapitiya and Virgin Galactic helped kick off the wave of start-ups going public via special purpose acquisition companies, or SPACs. The company went public in October 2019 via a merger with Palihapitiya's Social Capital Hedosophia, and until recently had been a top performer.

The Virgin Galactic Unity spacecraft in flight.

Image source: Virgin Galactic.

Virgin Galactic has a roster of hundreds of clients signed up to pay $250,000 for a brief flight to the edge of space, but the company is behind schedule in getting them there. A combination of pandemic-related delays and some testing setbacks forced Virgin Galactic to miss its 2020 goal to begin service. The company expects to launch service this year, but has experienced some delays in its testing schedule.

Until recently, none of those setbacks mattered for the stock. Virgin Galactic shares climbed nearly 150% in the first six weeks of 2021, but the momentum has faded in recent weeks. The shares are down 49% since Feb. 11 heading into Friday morning trading, and are down another 5% in premarket action.

Virgin Galactic on Thursday also announced the departure of chief space officer George Whitsides, who is leaving to pursue potential opportunities in public service. Whitesides, a former Chief of Staff at NASA, was CEO of the company through much of its development.