How safe can spaceflight ever be?
In July 2005, Nasa was on the verge of launching its first shuttle mission since the Columbia disaster, in which seven astronauts were killed.
Columbia broke-up as it re-entered the Earth's atmosphere in February 2003.
The effort to return the shuttle to flight had been long and painful for Nasa. The Columbia accident investigation report had concluded that the agency's culture was at fault and that it had not learned the lessons of the Challenger disaster in 1986.
Nasa's administrator Mike Griffin, who joined the agency in the aftermath of Columbia, spoke plainly when he addressed journalists gathered at Kennedy Space Center to cover the launch.
"There is no recovery from the mistakes we've made, whether it's going back to the Apollo fire, the loss of Challenger, or the loss of Columbia, going back through 100 years of flight, the lessons that we who fly have learned... are written in other people's blood," he said.
A safe flight was crucial, Dr Griffin told reporters, but spaceflight was a dangerous business and it always would be.
This view is echoed by Dr Simon Prince, an aeronautical engineer at City University in London: "We've learned a lot. In terms of our experience it is safer. But it is still not safe. Spaceflight is never going to be safe."
"It's arguably the most extreme environment that humans will ever travel in."
In the early days, Dr Prince explains, space was "a great unknown in terms of the medical implications on the human body."
Before daring to launch humans, US and Soviet scientists carried out numerous test flights using animals. Experiments showed that it was possible to survive the journey, but they could only provide so much information.
While biosensors could monitor the pulse, breath depth and respiration rate of a monkey or dog, animals could not describe how they felt during the flight.
"If you are Yuri Gagarin or Alan Shepard, you have absolutely no idea what you're in for," says Dr Prince. "Even if you survive the rocket flight into space, what are you going to feel like? Are you going to become ill through radiation exposure? Can you react to stimuli and control your flight? Will you even be conscious?
"Then during re-entry, you have an enormous accelerative load on you due to the atmospheric drag - even more so than during launch. Would the human body take it?"
Some psychologists even thought that after too much time spent in orbit, humans might become detached from reality. Because of uncertainties about the effects of weightlessness, the first man in space - Yuri Gagarin - was not allowed to pilot his capsule. Instead, Vostok 1 was controlled from the ground during its 108-minute orbit of Earth.
The early days of manned spaceflight were packed with close calls. Gagarin's historic flight skirted close to disaster when cables linking two parts of his spacecraft failed to separate as planned during re-entry.
The capsule entered a wild spin and temperatures inside rose dangerously: "I was in a cloud of fire rushing toward Earth," Gagarin later recalled. It was 10 minutes before the cables burned through.
Launch pad disaster
When the US Liberty Bell spacecraft carrying Virgil "Gus" Grissom splashed down in the Atlantic Ocean after a 15-minute sub-orbital flight in 1961, the escape hatch blew away while the astronaut was waiting to be picked up by a Navy ship. The capsule rapidly began to take on water, but Grissom had already un-strapped himself and was able to swim free before it sank.
And during the first-ever spacewalk in March 1965, the suit worn by Alexey Leonov inflated once he was outside, preventing the Soviet spaceman from getting back inside the ship. Leonov had to manually bleed air out of the suit through a valve and was barely able to squeeze back inside the airlock.
But the loss of three Nasa astronauts during a launch pad test for the Apollo 1 mission was a shocking moment for the public. A fire broke out in the cabin of the Apollo Command Capsule on 27 January 1967, killing crew members Gus Grissom, Ed White and Roger Chaffee.
The ferocious blaze was blamed in large part on the pure oxygen atmosphere and high pressure in the cabin.
The Soviet Union had concealed the death in 1961 of cosmonaut Valentin Bondarenko, who was engulfed in flames during training in a high-oxygen, low-pressure isolation chamber. Details of the fatal accident were only made public in the late 1980s.
Whether having this information at the time would have swayed Nasa is unknown. But according to Cathleen Lewis, of the National Air and Space Museum in Washington, there was at least one attempt by a Soviet scientist to warn about the dangers of a pure oxygen atmosphere. But "it was not clear or full-throated enough to have prevented death," she says.
Simon Prince adds: "The Soviets learned their lessons several years before the Americans. But because the two countries didn't talk, those lessons had to be re-learned by the Americans."
In recent years, China has become the third country to develop a capability to launch humans into space. But China too has a reputation for secrecy on matters of space. So could there be any parallels here with the situation during the Cold War?
Simon Prince doesn't think so: "I don't think the Chinese programme is as secret as the Russian programme was... In terms of learning past lessons, it is a relatively mature area, so I don't think you can compare the two."
Dr Roger Launius, from the space history division at the National Air and Space Museum, replies that there "is always a danger". But he points out that failures of communication between participants in the same space programme played a central role in both the Challenger and Columbia shuttle accidents.
"People knew that the O-rings had a tendency to fail when it got too cold. But [in the case of Challenger] that information did not get to the decision makers in a way that enabled them to understand the problem," Dr Launius says.
"In the case of the Columbia accident, there were those who analysed some of the data and firmly believed there was a problem with the RCC panels and that a piece of the bipod ramp had dinged the thing and poked a hole in it and that could cause the loss of the wing, which is exactly what happened.
"But that information did not get to where a decision maker could say 'we've got a problem'. It's not that people are trying to obfuscate, but how do you pick out the really important data from all the noise?"
While some commentators have praised the safety record of the Soyuz programme, Dr Launius argues the Russian/Soviet launch system has proven less reliable than the US shuttle across the same number of flights.
The Soyuz programme has had the same number of fatal accidents as the shuttle (two, in 1967 and 1971). But, unlike the shuttle, there have also been several launch aborts, in which an escape tower has been required to pull the crew capsule clear of the rocket due to a malfunction on launch.
While those involved in the American or Soviet/Russian government programmes have always been well aware of the risks, the emergence of private space travel poses new questions, such as: how should the dangers be communicated to future space "tourists" paying for sub-orbital flights?
Dr Launius says: "I'm sure they'll have to sign all kinds of waivers. That tends to focus your attention on that particular issue.
"If anything happens on these test flights, that will be the end of it for a long time.
He added: "There will be a huge reassessment... and that will take years."
In an article in the Wall Street Journal published in 2007, Peter Diamandis, who has been a key figure in the development of private spaceflight, commented: "We must remember that the first people flying on these sub-orbital spaceships are really explorers in their own right.
"They will (must) understand the risk. They will be taking these flights because of their own passion. These early flights are not joy rides."
Ultimately, says Dr Launius, "you can refine thesystem as best you can, you can ensure that there are clear lines of communication and you will probably minimise the risk some.
"But you'll never eliminate it."