Virgin Galactic customers have begun asking for refunds after the crash that killed a co-pilot during testing.
It is the first time any of the 800 people who signed up for the space flights have pulled out, despite 10 years of delays.
George Whitesides, the chief executive of Virgin Galactic, said it was "unsurprising" that some were now cautious about the project.
He insisted his resolve was "unshaken" and claimed that allowing wider access to space was still a "deeply worthy goal".
Before the accident, Virgin founder Sir Richard Branson said the first space tourism flights would begin in [the northern] spring next year.
Whitesides predicted yesterday the company's new spacecraft, SpaceShipTwo serial 2, would begin test flights within six months.
After that, he claimed it would be "months not years" until the first commercial flights began, meaning Virgin Galactic could still make its first launch next year.
About 24 passengers have already pulled out and had the 157,000 ($320,000) ticket price refunded.
Although some may have only paid deposits, the company stands to lose more than 3million.
High-profile ticketholders include Angelina Jolie, Kate Winslet and Professor Stephen Hawking.
The investigation into the accident is likely to take more than a year, which could hold up the project.
The co-pilot, Michael Alsbury, 39, died when the aircraft crashed in the Mojave Desert in California last Saturday.
The pilot, Peter Siebold, 43, is still recovering in hospital.
The investigation by the National Transportation Safety Board is focusing on whether Alsbury unlocked the ship's movable tail section early, before aerodynamic forces were right to keep the structure held in place as designed.