By Martin Redfern
BBC Radio science unit
We are about to witness a revolution in human spaceflight.
Many companies are expected to enter the market for sub-orbital journeys
Launching people into space has until now been the almost exclusive preserve of superpower governments. But, according to industry experts and entrepreneurs, the commercial exploitation of space is about to open a new frontier for mass tourism.
For one of the pioneers of this revolution, Burt Rutan of the Californian company Scaled Composites, it reminds him of the imaginings of his youth.
"Everyone dreamed of travelling in space during the Sixties. When we saw how much progress was made in 10 or 11 years, we all not only dreamed but knew that we would be flying in space," he told The X Factor programme on BBC Radio 4.
For most, the dream faded. It seemed clear that only military test pilots would ever get to space, and even they would have very few missions.
The US space shuttle, America's only route for humans into space in the last 25 years, was so ambitious and complex that it became prohibitively expensive and, with two fatal accidents, unreliable.
Nasa and the US Air Force tried several projects to produce space planes, from the first X-15 to the latest X-43. All eventually ran out of money.
Looking to longevity
The spirit of the X-planes, however, lived on in private enterprise. "X" in this case stands for experimental, the unknown.
It led a young entrepreneur called Peter Diamandis to propose the X Prize in the hope that it might do for spaceflight in the 21st Century what the Orteig Prize did for aviation in the 20th. That stimulated Charles Lindbergh's first solo, non-stop crossing of the Atlantic in an airplane.
Lindbergh actually crossed the Atlantic to win a $25,000 prize. Nineteen competitors spent $400,000 attempting to win this prize and Orteig didn't pay a cent to anyone but the winner.
"That's an amazing, high-leverage mechanism," said Peter Diamandis, president of the X Prize Foundation.
Adventurers such as Greg Olsen have had to part with millions to buy a space ticket
Erik Lindbergh is the grandson of Charles and an advocate of private spaceflight.
He commented: "Before my grandfather made his flight, pilots were known as barn-stormers, daredevils, flying fools, and they didn't have a very long life expectancy.
"After he made his flight, people who flew in airplanes were known as pilots and passengers. That's the shift in perspective, what the X Prize is trying to change today in relation to spaceflight."
The X Prize offered $10m to the first privately funded spacecraft to take the mass equivalent of three people on a sub-orbital flight over a 100km high, twice within a fortnight.
The money was made possible by telecommunications millionaires Amir and Anousheh Ansari. It seemed a very difficult challenge; but not for Burt Rutan.
He had already designed dozens of different innovative aircraft, including the Voyager plane that was the first to fly non-stop around the world without refuelling. His first spacecraft was called, logically, SpaceShipOne.
"To do SpaceShipOne I had to go five times as fast and five times as high as anything I had done before, so it was a big jump; but I didn't look at it that way," Burt Rutan told the BBC.
"I looked at it from the viewpoint of how strong does it have to be. And I didn't think of altitude as being a big deal, I just thought of the loads on the ship and I worked out the physics and thought that this might be hard, it may turn out to be dangerous, but it is worth doing," he recalled.
SpaceShipOne had several innovations. It is made of carbon fibre composite which makes it light, heat resistant and prevented thermal expansion.
It is launched from the air by another innovative craft called White Knight which carries it to nearly 50,000ft (15,200m) before releasing it, avoiding many of the risks of a rocket launch from the ground.
SpaceShipOne tucks its wings for a controlled re-entry
Once released, SpaceShipOne is powered by a hybrid rocket consisting of solid rubber fuel and a liquid oxidiser, giving the simplicity of solid fuel, like a firework, but the controllability of a liquid which can be turned on and off.
After a few minutes in space, during which the pilot can experience weightlessness under a black sky, the final innovation comes into play.
The twin tail structure lifts to make the craft behave rather like a shuttlecock, causing high drag to slow it down in the thin atmosphere and keep it the right way up regardless of the angle of re-entry.
Thanks to these innovations, the vehicle became the first privately funded spacecraft in June 2004 and went on to perform the double flight needed to win the X Prize later that year.
Rush to space
Burt Rutan's vision had been funded by Microsoft cofounder Paul Allen who is one of a new breed of space entrepreneur nicknamed "thrillionaires". These are mostly people who have made their fortune through software or the internet and are looking for exciting if risky investment opportunities.
Another in this league is Sir Richard Branson, who has now teamed up with Burt Rutan to order five spacecraft, much bigger versions of SpaceShipOne called, inevitably, SpaceShipTwo. President of his new company, Virgin Galactic, is Will Whitehorn.
"SpaceShipOne was designed to prove a concept, SpaceShipTwo is designed to prove the safety and commercial viability of air-launched spacecraft in the future," Whitehorn told the BBC's X Factor.
"It is going to be much bigger, six passengers and two pilot seats, about the size of an executive jet. Its carrier aircraft, White Knight Two, will be about the size of a 757. It will lift SpaceShipTwo to 55,000ft to launch it almost outside the atmosphere.
Early Nasa experiments failed to open up space as expected
"That means a short rocket burn with much less environmental impact than anything that has been done before. It will be the foundation of a completely different approach to spaceflight which I think will take off over the next couple of decades."
By late 2008, Virgin Galactic hope to be flying fare-paying passengers into space from a new spaceport under construction in New Mexico.
Tickets have initially been priced at $200,000 each, and already several million dollars have been taken in deposits.
Sir Richard Branson believes that the price will fall in a few years to around $50,000, a cost not far above that of adventure holidays such as Everest expeditions or trips to Antarctica.
He is confident of a huge demand, and expects to have five spaceships in service, performing two or three flights a day.
He may not have the market to himself. Several well-funded teams that came too late to win the X Prize have not been put off and claim to have a variety of rockets and space planes nearing readiness for test flights.
Some are thinking beyond sub-orbital flights that only give you a few minutes in space. But to reach full Earth orbit requires speeds at least six times higher and consequently greater problems during re-entry, problems which even the space shuttle has not overcome fully.
One other strategy is to use the old, tried and tested technology of Russia.
The X Factor presenter Jeff Hoffman underneath SpaceShipOne and Bell X-1
The company Space Adventures is the only one to have sent fare-paying passengers into space already. Dennis Tito, Mark Shuttleworth and Greg Olsen each paid around $20m to fly on a Russian Soyuz rocket to the International Space Station for a whole week in orbit.
Space Adventures recently announced plans to use Russian technology for sub-orbital flights from spaceports in Dubai and Singapore.
At the top end of the market, they are considering a space expedition that would include an orbit round the back of the Moon in a Russian rocket with an additional upper stage. For that, the recommended retail price is $100m.
The sky would seem not to be the limit for space tourism. This does seem to be a market set for considerable growth in the years ahead.
"I think people should get ready for a lot of dreaming and a lot of adventure and excitement in space," said Peter Diamandis.
"Kids out there, it's going to be an amazing 30 or 40 years in spaceflight. Get ready to have some fun."
The X Factor is being broadcast on BBC Radio Four. It is presented by veteran astronaut Jeff Hoffman, who is now professor of astronautics at the Massachusetts Institute of Technology. The second part of X Factor can be heard this Wednesday 22 March at 2100 GMT. It will also be available to listen again on the BBC Radio Player, along with the first part of the programme