SpaceShipTwo, a six-passenger, two-pilot suborbital spaceship owned by Virgin Galactic, an offshoot of Richard Branson’s Virgin Group, fired up its rocket engine for the first time on Monday to successfully complete a 16-second, supersonic test flight Branson called “critical.”

Under the watchful eyes of Branson, now-retired spaceship designer Burt Rutan, Virgin Galactic executives and workers at California’s Mojave Air and Space Port, manufacturer Scaled Composites rolled out the spaceship and its custom carrier jet WhiteKnightTwo early Monday in preparation for a 7 a.m. takeoff.

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“It looked stunning,” Branson later told Discovery News. “The pilots themselves were tempted to say, ‘Let’s get farther. Let’s just go straight into space today,’ but it’s good sense they decided not to do that and save it up for later on in the year.”

WhiteKnightTwo soared into the clear blue sky and climbed to an altitude of 46,000 feet before the three-man crew, including Virgin lead pilot David Mackay, released the gleaming white spaceship for its debut powered flight.

Scaled pilots Mark Stucky and Mike Alsbury, riding aboard SpaceShipTwo, then ignited their ship’s rocket motor, which pushed them forward and up another 10,000 feet, breaking the sound barrier in the process.

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“They said they were surprised by how smooth it was. The only thing that Mark commented on was that when the rocket was meant to have finished burning, it sounded like it was continuing to burn, but actually that was the boom from breaking the sound barrier,” said Branson, who plans to experience the ride for himself, along with his two grown children, within about a year.

It will take about that long for Scaled to complete the last phase of an extensive flight test program, similar but more involved than what the company did more a decade ago leading up the first flights beyond the atmosphere of the SpaceShipOne prototype. The three-person reusable craft clinched the $10 million Ansari X Prize in October 2004 for making the first privately funded, human spaceflights.

Branson, impressed by SpaceShipOne, hired Scaled to design and build a commercial follow-on. Virgin Galactic already has taken deposits for the $200,000 rides from more than 500 people.

"We are one step closer to achieving safe, routine and cost-effective access to space,” former astronaut Michael Lopez-Alegria, who oversees the Commercial Spaceflight Federation, wrote in an email to Discovery News after SpaceShipTwo's test flight.

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“There are millions of people who want to go into space. We’re really not long away now,” Branson said.

"We feel that we've pretty well ticked all the boxes. Now, it's just the final part of the test program before we we send people up. I think that the big, difficult milestones are all behind us," Branson said.

Virgin Galactic also is developing a low-cost satellite launch system that makes use of the WhiteKnight carrier aircraft.