The pilots, who have not yet been identified, were flying the plane for Virgin Galactic, the space tourism company created by the entrepreneur Richard Branson, and Scaled Composites, the company that designed and built the plane.
One pilot was able to parachute from the plane and was taken to a hospital with “moderate to major injuries,” said Ray Pruitt, the public information officer for the Kern County sheriff’s office in California.
The test was the first time SpaceShipTwo had flown using a new, plastic-based rocket fuel.
It was the second major accident in a week for the commercial space industry, which has been widely promoted in recent years as an alternative to costly government programs. On Tuesday, an unmanned rocket launched by Orbital Sciences Corporation of Dulles, Va., which was carrying cargo to the International Space Station, exploded 15 seconds after launching.
SpaceShipTwo and Orbital’s rocket are very different in design and purpose, but both are part of an effort to bring private investment into the space business, until now largely the realm of government agencies like NASA and the military.
Virgin Galactic, which hoped to begin tourist flights next spring, already has more than 700 reservations, initially sold for $200,000 a seat before rising to $250,000 last year.
The list of would-be astronauts includes celebrities like Leonardo DiCaprio, Justin Bieber and Angelina Jolie.
Experts said it was too soon to tell when the effort would resume. “Virgin was out ahead of everyone else for space tourism,” said Michael Blades, the aerospace and defense industry senior analyst at Frost & Sullivan, a market research and consulting firm. “It will still happen, but it has been pushed way to the right.
“It is just like any kind of other new technology, especially when it comes to flight,” he continued. “You have your tests and you have your failures.”
Friday’s accident took place tens of thousands of feet above the desert. As planned, SpaceShipTwo was carried aloft by a larger plane, WhiteKnightTwo, then dropped at about 50,000 feet. In a tourist flight, SpaceShipTwo’s rocket engines would take it to the 62-mile-high boundary defined as the edge of space.
After the smaller plane was released, its motor ignited. The accident appeared to happen 60 to 90 seconds later, said Stuart Witt, the chief executive of Mojave Air and Space Port, where WhiteKnightTwo took off at 9:18 a.m.
“I knew something was wrong,” he said. “I didn’t know what. It wasn’t obvious at first.”
A radio call reported an anomaly. “And we waited,” he said.
The sheriff’s office received a call after 10 a.m. that an aircraft had gone down about 20 miles northeast of the city of Mojave. “We have located a debris field,” Mr. Pruitt said.
WhiteKnightTwo landed safely.
Via Twitter, Mr. Branson said, “I’m flying to Mojave immediately to be with the team.”
Virgin Galactic grew out of the success of the Ansari X Prize contest in 2004, for the first privately built and financed craft that could rise above the 62-mile boundary of space. Scaled Composites won the $10 million prize with a smaller version of SpaceShipTwo, an effort financed by Paul Allen, the co-founder of Microsoft.
Immediately after the X Prize, Mr. Branson announced his plans for a spaceship that would carry two pilots and six passengers on suborbital flights — in which the plane does not go into orbit but offers a few minutes of weightlessness at the top of an arcing trajectory.
Over the years, Mr. Branson has repeatedly said he hoped commercial flights would begin soon. This is the second time tragedy has struck the spaceport in connection with Scaled Composites; in July 2007, a rocket system test went awry, killing three people.
Most recently, he said that with the resumption of powered test flights, he hoped the first commercial flights would take off next spring. He and his family plan to be the passengers on the first operational flight.
Although SpaceShipTwo had flown 54 previous test flights, all but three were unpowered tests in which it glided to the ground. This was the fourth time its motor was ignited.
In May, Virgin Galactic announced it was switching to the plastic-based fuel from the rubber-based one it had used. Friday’s flight was the first powered by the new motor.
The previous version was problematic, causing strong vibrations in the spacecraft. In an article published last month in Popular Mechanics, Brian Binnie, a former test pilot for Scaled, said, “We had start-up instabilities and we had end-of-burn instabilities.”
While troubleshooting the problem, “we did everything but break down and pray to God to show us the light of day,” said Mr. Binnie, who has now moved to XCOR Aerospace, another company at Mojave that is building a suborbital space plane for tourists.
Marco Caceres, director of space studies at the Teal Group, a consulting firm, said that “in an age where it is very expensive to fly these vehicles, the pressure is to do the minimal amount of test flying.”
“So that may be something we have to take a look at,” he continued. “Everyone seems to be in need of more money to conduct more flights, so the pressure is to start operational flight too soon. Maybe we are being unreasonable here.”
Patricia Hynes, director of the New Mexico Space Grant Consortium, who organizes an annual symposium for people in the commercial space industry, said the accident “helps people understand why it’s never been done before.”
“This is a tough business,” she said.
Dr. Hynes has bought a Virgin Galactic ticket, and the news did not change her intention to fly. “No, absolutely not,” she said.
A second SpaceShipTwo plane is currently under construction.