SpaceShipOne, the first contender to attempt the $10m Ansari X-Prize, has been successfully launched from the Mojave Airport in California.
After a delay the White Knight craft, which carries it part of the way before its rocket fires, took off as the Sun rose just after 0711 PDT (1411 GMT).
SpaceShipOne, piloted by Mike Melvill, became the first private, manned craft to go above 100km in June.
To claim the prize, it now must repeat its feat twice inside two weeks.
As test pilot Brian Binnie taxied the White Knight craft carrying SpaceShipOne down the runway, Mr Melvill waved to cheering crowds.
Both craft were designed by aviation legend Burt Rutan and backed by Microsoft co-founder Paul Allen.
SpaceShipOne is taken to 13.8km (46,000ft) slung under the White Knight aircraft.
When released, it glides for a few seconds before its pilot lights the rocket and points the vehicle straight up to break through the Earth's atmosphere, an altitude officially recognised as 100km (62 miles).
At the top of its flight, SpaceShipOne adjusts its wings to give the craft a different shape, known as the high-drag configuration.
This ensures that during its fall back to Earth, the vehicle's descent speed is controlled and heating of the airframe is minimised.
The whole journey should take about 90 minutes.
Qualifying flight
Mr Melvill, 62, became the first non-government-funded pilot to fly a spaceship out of Earth's atmosphere during June's test flight.
On both qualifying flights, SpaceShipOne needs to fly with a pilot and at least the ballast equivalent of two other people.
Guest of honour on-board Wednesday's flight is Terence, a teddy bear from Cumbria, who has travelled around the world to raise cash for charity
Mr Rutan said he aims be one of the first human passengers, possibly on the second X-Prize flight in October.
He admitted that June's record-breaking test was not perfect; some "anomalies" had occurred. Mr Melvill spoke of a loud bang during the record-breaking mission.
On the ground, he pointed out a section at the back of the craft where a part covering the nozzle had buckled, suggesting it may have caused the odd noise.
This prompted a tweak for Wednesday's attempt: the nozzle was reinforced and painted white to limit its temperature.
Focused effort
The X-Prize was established to galvanise the commercial spaceflight business. Already, one millionaire is looking to the future by offering a $50m reward for the first private orbiting spacecraft.
By focussing the efforts of private entrepreneurs, the X-Prize aims to break the monopoly of government organisations and jumpstart space tourism.
It appears the prize may have succeeded in this goal, with news on Monday that Virgin boss Sir Richard Branson would be offering commercial space flights in about three years using the technology employed by Rutan on SpaceShipOne.
Rutan says he plans to make the two qualifying flights within four to seven days of each other. High winds are a major concern and could lead to delays though.
A rival team, the Toronto-based GoldenPalace.com Space Program, which was formerly called the daVinci project, has put back its stab at the prize. The team had been scheduled for a first launch on 2 October.
Team leader Brian Feeney said the delay was necessary to allow more time to work with a pressure vessel for the Wild Fire spacecraft, as well a few other minor components.
http://news.bbc.co.uk/1/hi/sci/tech/3697852.stm