Virgin Galactic's fledgling space tourism rocket was taken aloft over the California desert on an inaugural test flight attached to the wing of its mothership, the company said.
The captive-carry test marked the start of a test programme for SpaceShipTwo that will progress to free flights as a glider and then under rocket power, Sir Richard Branson's Virgin Galactic said in a statement.
Throughout the two-hour, 54-minute test, the spaceship remained slung beneath the middle of the wing of its unique twin-fuselage carrier aircraft, the WhiteKnightTwo.
The pair achieved an altitude of 45,000 feet before returning to the Mojave Air and Space Port, which is 70 miles north of Los Angeles, the company said.
"This is a momentous day," designer Burt Rutan said in the statement.
The rocket and mothership are the second-generation of the Rutan-designed system that sent the first privately developed, manned rocket into space in June 2004.
SpaceShipOne went on that year to make two more suborbital flights, winning the 10 million US dollar Ansari X Prize.
Sir Richard, the billionaire founder of Virgin Group Ltd, is in a deal with the Rutan-founded Scaled Composites LLC of Mojave, California, to develop passenger-carrying spacecraft and launchers.
The new SpaceShipTwo has been dubbed Virgin Spaceship Enterprise and the four-engine carrier jet is called Virgin Mothership Eve, after Sir Richard's mother.
"Seeing the finished spaceship in December was a major day for us but watching VSS Enterprise fly for the first time really brings home what beautiful, groundbreaking vehicles Burt and his team have developed for us," Sir Richard said.
Virgin Galactic said the flight test programme will run through 2011 before commercial operations begin.
SpaceShipTwo will be carried to an altitude of about 50,000 feet and then released by the mothership.
Powered by a single rocket motor, the spaceship will be flown by a crew of two and carry six passengers on a Mach 3 thrill ride through the edge of the atmosphere for a brief zero-gravity experience and views of the Earth far below before gliding to a landing.
More than 300 people have made deposits on the 200,000 US dollar tickets.
Such suborbital flights are similar to the 1961 flights of Mercury astronauts Alan Shepard and Virgil Grissom - up into space and then back down without circling the Earth.
But by launching SpaceShipTwo from a high-flying carrier aircraft, there's no need for the massive rockets that propelled the Mercury capsules.
Motherships have been used for decades by Nasa and the military for captive-carry tests and launches of such craft as the X-15 rocket planes.
The novel part of Rutan's design is his "carefree re-entry" system.
SpaceShipTwo has tailbooms extending rearward from its wings. In space, the booms pivot upward to a 65-degree angle. The position causes drag and slows the descent in the upper atmosphere before the booms pivot back to horizontal.
Commercial development was slower than expected. When Virgin Group licensed the technology from Microsoft co-founder Paul Allen, who funded about 26 million US dollars for SpaceShipOne, Sir Richard envisioned operating flights by 2007.
A major setback was an explosion during a 2007 ground test involving the flow of a rocket oxidiser. Three workers were killed and three others suffered serious injuries.