April 30--With a sonic boom that resounded above the Mojave Desert, a rocket plane belonging to British billionaire Richard Branson's commercial space venture Virgin Galactic got one step closer to carrying tourists into space.

On Monday the company's SpaceShipTwo ignited its rocket motor in mid-flight for the first time and sped to Mach 1.2, faster than sound, reaching about 56,000 feet in altitude.

The test flight is the biggest milestone in Virgin Galactic's 8 1/2-year endeavor to be the world's first commercial space liner, which would make several trips a day carrying scores of paying customers into space for a brief journey.

"We never thought it would take this long, but it was worth the wait," Branson said in an interview. "Now that we have accomplished supersonic flight, we feel ready to take the next step. There are an awful lot of exciting things to come."

Virgin Galactic, founded by Branson, hopes to reach space in test flight this year and make its first passenger flight sometime in 2014 from Spaceport America in New Mexico, where the company hopes to eventually offer the frequent tourist trips.

Branson first hoped he would blast tourists into space by 2007, but the date has repeatedly slipped. Space experts wonder whether even 2014 is too ambitious. Virgin Galactic still needs to clear regulatory hurdles, particularly satisfying safety concerns with the Federal Aviation Administration.

Marco A. Caceres, space analyst for the aerospace research firm Teal Group Corp. of Fairfax, Va., said that he wouldn't be surprised if there was a delay beyond 2014, but that the key is not one single launch. He said Virgin Galactic needs to get into a routine of launching multiple times a year.

"What they're doing isn't easy by any stretch," Caceres said. "This is a pioneering business. They can't afford to have a pilot and tourist killed on a flight. That would set the program back years, so they need to make sure they get it right."

The test flight Monday took place at 7:02 a.m. Pacific time beginning on the desert runway at Mojave Air and Space Port, about 100 miles northeast of Los Angeles. There had been some media buzz about the flight, and locals and space aficionados amassed at the spaceport to catch a glimpse.

During the test, SpaceShipTwo was taken to about 47,000 feet by a carrier aircraft, and approximately 45 minutes into the flight, it was dropped like a bomb.

After a short free fall, pilot Mark Stucky and co-pilot Mike Alsbury engaged the hybrid rocket motor -- powered by nitrous oxide and a rubber compound -- for 16 seconds, at which point SpaceShipTwo's speed reached Mach 1.2.

The entire flight test lasted a little more than 10 minutes, ending in a smooth landing in Mojave around 8 a.m.

The idea of Virgin Galactic routinely taking passengers to space this way was developed by retired maverick aerospace engineer Burt Rutan and his Mojave company Scaled Composites. Rutan, who lives in the lakeside community of Coeur d'Alene, Idaho, was on hand to see Monday's test flight.

Until now, astronauts have reached space packed tight in a capsule or shuttle attached to a high-powered rocket.

Instead, Virgin Galactic will use a WhiteKnightTwo carrier aircraft that will fly with the reusable SpaceShipTwo rocket plane under its wing to 50,000 feet, where the spaceship will separate and blast off.

When the rocket motor engages, it will power the spaceship to nearly 2,500 miles per hour and take the pilots -- and up to six passengers -- to the edge of space, more than 60 miles above the Earth's surface.

Once they reach that suborbital altitude, passengers will experience weightlessness and see the curvature of the Earth. Then they will reenter the atmosphere and glide back to the runway.

The price for the experience: $200,000.

Virgin Galactic said it has accepted more than $70 million in deposits from about 580 reservations made by people who are interested in the ride.

The WhiteKnightTwo carrier aircraft, which resembles a flying catamaran because it has two fuselages, and SpaceShipTwo are still in the midst of a test-flight program that will continue in Mojave until Virgin Galactic believes it can begin commercial operations.

Virgin Galactic's commercial space launch system is based on Rutan's SpaceShipOne, the world's first private manned spaceship, which flew a test pilot to space and back three times in 2004 to win a $10-million X-Prize purse.

The prizewinning spacecraft caught the eye of Branson, who wanted to work with Rutan on a much bigger rocket ship that could send not only a pilot into space but also fare-paying passengers.

The enterprise was shrouded in secrecy for years. Then in 2007, during a test of the spaceship's propulsion system, an explosion killed three workers and injured three others. The blast exposed the secret project and reminded the public of the risks of rocketry, which had long been the domain of powerful governments rather than small business.

The project endured and has run into several delays along the way. But Branson has since built a 68,000-square-foot facility at the space port for a joint venture, called Spaceship Co., to mass-produce its rocket ship and carrier aircraft. It was one of the first aircraft assembly plants to be built in the region in decades.

Today, Mojave spaceport is bustling with commercial space test activity. New spacecraft are tested daily by companies at the 3,300-acre site with a 2-mile-long runway.

One of those companies, Xcor Aerospace Inc., hopes to join Virgin Galactic in the space tourism business. It plans to have a test flight of its space plane in the year ahead.

"We applaud their exceptional teams on taking the next step in the greatest journey the human race has ever known," the company said. "We look forward to meeting them there, above the clouds, where the adventure has only just begun."

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