How safe can we really make space for future tourists?
The Virgin Galactic tragedy high above California's Mojave Desert has reawakened the question of how safe we can ever truly make space, and whether we will be able to make it as routine as taking a flight from New York to London.
With the launch of the world's first satellite, Sputnik in 1957, space effectively became the preserve of governments and their agencies.
The much-vaunted space race was itself a child of the Cold War between the USSR and US, with astronauts and cosmonauts, all imbued with "the right stuff" striving for the high ground of space.
But with the ending of the Cold War and dramatic cuts in government space budgets, the past 20 years has seen a new breed of space entrepreneurs entering the arena.
The rhetoric of international rivalry has been replaced by a reworking of the pioneering ideology of 19th Century America.
Enthusiasts argue that space is humankind's manifest destiny - colonies on the Moon and Mars, and mining the Solar System's asteroid belt are but steps along the way to the stars themselves.
It is easy to get seduced by the vision.
Down on planet Earth, the hard facts remain - getting into orbit, or even just a quick up-and-down sub-orbital space tourist flight, remains expensive and technically difficult, and key questions remain about the safety of space travel.
In the wake of the crash the US National Transportation Safety Board said that the feathering mechanism designed to slow the craft on its re-entry was deployed too early and the craft then broke apart.
The NTSB says it will be looking at SpaceShipTwo's design as well as safety culture at Virgin Galactic, and cautioned its investigation would take time.
"We are a long way from finding (the) cause," said the NTSB's Christopher Hart.
Before last week Virgin had collected some $90m (£56m) in deposits from 700 people who had paid up to $250,000 a ticket for a ride to the edge of space and back.
Up to 3% have now asked for their money back, says the company.
"Safety is our guiding principle and the North Star for all programmatic decisions. Our culture is one of prioritising safety as the most important factor in every element of our work," Virgin Galactic said in a statement.
Scaled Composites, which designed the craft for Virgin, said: "Our team is hurting, but we are dedicated to getting back to the important work we do here."
Space accidents: Key disasters
- 1967: Apollo 1 - three astronauts die in fire during a ground test of the capsule
- 1967: Soyuz 1 - one cosmonaut dies in a crash on re-entry after a parachute malfunctions
- 1971: Soyuz 11 - three cosmonauts suffocate because of an air leak in their spacecraft after undocking from Salyut 1 space station
- 1986: Space Shuttle Challenger - seven astronauts die when shuttle's fuel tank explodes after liftoff because of faulty seals on the booster rockets (pictured above)
- 2003: Space Shuttle Columbia - seven astronauts die when shuttle breaks up on re-entry due to damaged heat shielding
Source: Nasa and Spaceflightnow.com
In 1960 - the year before Yuri Gagarin became the first man in space and also the beginning of the commercial jet age - the annual accident rate for commercial aviation was 36 every million departures.
In other words, for every million take-offs, 36 flights would be involved in accidents - fatal or otherwise. By 2013, commercial flying had got a lot safer. Accidents per million departures had fallen to 2.8.
But when it comes to space, the risks are orders of magnitude higher.
The US Space Shuttle flew 135 times with two fatal accidents before it was retired in 2011, while the Russian Soyuz rocket has flown 123 times so far, again with two fatal accidents.
So for human spaceflight, there's about a one in 65 chance of something going badly wrong with a rocket.
This may be acceptable for astronauts and cosmonauts who are often ex-test pilots used to risks, but not for rich thrill-seekers.
True, Virgin Galactic is not going into orbit, but the difference between space and flying serves to show the challenges that safety designers face.
Yet, there are niche companies who have been running successful operations where buying their services comes with a one-in-100 chance of dying accidentally.
The companies aren't selling tickets to space, but tickets to Everest, and the growth of commercial climbing on the world's highest mountain is perhaps one way of illustrating paying customers' willingness to accept risk.
Ever since Nepal opened up Everest to commerce in the 1990s, an industry has grown up allowing predominantly rich westerners to try to climb it. It will cost you about $65,000 to join a commercial expedition - and you may not come back.
According to the British Medical Journal, the long-term death rate among climbers on Everest is about 1.3%.
The figures suggest that this small but significant risk of death is not putting people off. For instance, in 2013 some 658 climbers made it to the top, while eight died.
In the aftermath of the avalanche this April that killed 16 Sherpas, it remains to be seen how commercial climbing fares in the present climate in Nepal, but selling tickets with this kind of risk does not seem to have been a bar to business success.
The 'control illusion'
Yet, as a counterpoint to customers' willingness to pay to climb Everest, it seems that enough people were wary of paying to fly in Concorde after its fatal crash in Paris in 2000.
It was retired three years later, and both Air France and British Airways cited low passenger numbers in the wake of the crash as one of the reasons for its grounding.
This perhaps comes down to how we perceive risk, or what psychologists call the 'control illusion'. If we feel we have more control over our fate - such as climbing a mountain - then we are likely to perceive risks as being less than they actually are.
Whereas if we are a passenger in a plane we know we have no control over our immediate fate, so we may be tempted to over-estimate the risks.
The accident will add further delays to Sir Richard Branson's space tourism project, where the Virgin Group is now paying Virgin Galactic's daily running costs, and raises questions about its long-term funding.
Commercial space race
Virgin Galactic is not alone. The increasing reliance of NASA on private firms, has tempted many tech entrepreneurs to fund commercial ventures.
For instance, Elon Musk co-founded SpaceX, Jeff Bezos started Blue Origin, Paul Allen has Stratolaunch Systems, while Jeff Greason founded XCOR, which also hopes to take tourists on suborbital flights with its Lynx craft.
While there are entrepreneurs out there with the money and the desire to reach for the stars - or at least get into orbit - it seems very unlikely that whatever the eventual outcome of Friday's fatal crash, the commercial space launch business will halt.
As XCOR's Jeff Greason said: "I'm in the private space business because I don't feel like waiting for something to happen."