MarsScientific.com / Clay Center Observatory Cold oxidizer streams from the back of SpaceShipTwo's engine during an unpowered test flight on April 12. Richard Branson, the billionaire founder of Virgin Galactic, says the rocket plane could go supersonic when its engine is lit up for the first time in flight, as early as Monday.





Virgin Galactic's billionaire founder, Richard Branson, says his company is planning to fire up SpaceShipTwo's rocket engine for the first time in flight on Monday — a "historic" blast that is expected to send the space plane supersonic.

"We're hoping to break the sound barrier," Branson told the Las Vegas Sun. "That's planned Monday. It will be a historic day. This is going to be Virgin Galactic's year. We'll break the sound barrier Monday, and from there, we build up through the rest of the year, finally going into space near the end of the year. I'll be on the first official flight, which we look to have in the first quarter of next year. We're doing a number of test flights into space first."

Branson made his comments on Monday during a visit to kick off Virgin America's airline service to Las Vegas. In just one paragraph, the British entrepreneur and adventurer capped off weeks of rumors and laid out a new timeline for starting up passenger flights to outer space.

SpaceShipTwo builds on the heritage of SpaceShipOne, which powered its pilots beyond the 100-kilometer (62-mile) altitude mark in 2004 to win the $10 million Ansari X Prize for private-sector spaceflight. SpaceShipOne is now hanging in the Smithsonian, but SpaceShipTwo has been under development for years at Scaled Composites in Mojave, Calif. Scaled has been conducting a series of unpowered tests at the Mojave Air and Space Port. During an April 12 "cold flow" flight test, Scaled's pilots rehearsed every step for a powered flight, short of lighting up the hybrid rocket engine.

So far, SpaceShipTwo has been attached to the belly of its WhiteKnightTwo mothership, carried up to altitudes of around 50,000 feet and then dropped into the air to make a glider-like landing. During powered tests, SpaceShipTwo's engine will be lit up after the drop. The rocket plane will make a spectacular blast into the heavens and then glide back to the runway.

"We’ve experienced about a year’s worth of vertical flight tests and captive-carry flight tests by a number of tenants, and now we’re entering the phase of manned research flights," Stuart Witt, CEO of the Mojave Air and Space Port, told NBC News. "We’re excited about that: The industry has been waiting for this for a long time – since 2004."

Virgin Galactic's CEO and president, George Whitesides, stressed that the test schedule was dependent on several factors, including the weather. He said the first rocket-powered test could easily slip to a later time.

Like Branson, Whitesides said the first powered flight, known as PF01, would be merely the first step in the next phase of testing. "PF01 will be the start of a series of increasingly longer-duration burns (PF02, PF03, etc.) that should lead us to space altitude before the end of the year and commercial ops start soon after that," he said in an email to NBC News.

Virgin Galactic plans to conduct commercial spaceflights from Spaceport America in New Mexico, with passengers charged $200,000 for a ride. More than 500 customers have already put money down for flights. The passengers would experience breathtaking views of the curving Earth beneath the black sky of space, go weightless for a few minutes and then strap themselves back in for a powerful plunge back to the runway.

The first supersonic flight of a new spaceship isn't always auspicious: When SpaceShipOne came in for a landing in Mojave after breaking the sound barrier for the first time on Dec. 17, 2003, its left landing gear collapsed and the plane ran off the runway. Fortunately, no major damage was done, and pilot Brian Binnie was unhurt. Binnie went on to fly SpaceShipOne into space on Oct. 4, 2004, to win the $10 million prize.

Blue Origin, a rocket venture backed by Amazon.com billionaire Jeff Bezos, put its prototype spacecraft through its first unmanned supersonic flight at a Texas rocket range in August 2011 — but the company said the craft had to be destroyed when it experienced a "flight instability" at an altitude of 45,000 feet. Blue Origin recovered from that setback and is continuing to work on suborbital as well as orbital spacecraft.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email inbox every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.