Space-industry watchers expressed concern that the emerging space tourism sector is not being candid enough about the safety risks for travellers.
They voiced some concerns during a conference of the International Association for the Advancement of Space Safety, which held three days of meetings in Montreal.
Tommaso Sgobba, an aeronautical engineer and the agency's outgoing president, called suborbital flight safety "a serious matter that should be pursued with openness and transparency."
"We have no clue what they are having as a policy," Sgobba said, speaking of the industry during an interview.
Virgin Galactic, one of the industry pioneers, held the first powered flight of its SpaceShipTwo last month, moving the company closer to its goal of flying paying passengers.
Sir Richard Branson initially predicted that suborbital flights by his space tourism company would begin in 2007, but a deadly explosion during ground testing and some delays during test flights have pushed the deadline back.
Sgobba conceded that space tourism companies might argue that issues related to ownership or design could be exploited and used against them.
But, in his view, there is too much resistance to discussing safety.
'I believe that they are shrouding this in to a level of secrecy that is not good for the industry itself.' —Tommaso Sgobba, aeronautical engineer
"I believe that they are shrouding this into a level of secrecy that is not good for the industry itself," said Sgobba, who was responsible for flight safety at the European Space Agency until last year.
"They should be more open and communicate what they do."
Members of the non-profit IAASS, which was established in 2004 in the Netherlands, include universities, institutions, corporations and professional associations.
Alexander Saltman, the executive director of the Washington-based Commercial Spaceflight Federation, cautioned that suborbital trips by space tourists will be dangerous.
"There are going to be dangers that we don't know about when we start flying," he said.
"There will be incidents and at some point somebody will lose their life in this industry... At this point, putting a number on it is going to be impossible, because the unknowns outweigh the knowns."
Ships designed like planes
Saltman added that space-tourism firms are designing their vehicles much like planes and are taking lessons learned through decades of aircraft operations.
"Obviously it's somewhat different for space flight, but there are many lessons that can be learned," he said.
Saltman's organization represents over 40 businesses and organizations that are working to make commercial human space flight a reality. Some big space-industry players, which include SpaceX, Virgin Galactic, XCOR Aerospace and Space Adventures, are among its members.
Space Adventures organized the 2009 visit to the International Space Station by Cirque du soleil founder Guy Laliberte. He reportedly paid $35 million to become the first Canadian space tourist.
Saltman compared the early space-tourism flights to adventure travel and sports, like mountain-climbing, where there is a risk involved.
"The question is: Is the experience worth the risk to you?" he said. "You are the only one who could make that decision."
Joram Verstraeten, an engineer with the Amsterdam-based NLR Air Transport Safety Institute, said he wasn't aware of what safety assessments had been carried out by suborbital space companies.
"They don't share that," he said in an interview. "They should do that more."
Verstraeten has calculated the risk of a fatal accident involving suborbital space flights at one in 1,000.
"What can happen is there are a couple of flights and nothing happens," he said. "That's a real possibility because we are talking probabilities here, so it might go well, but it might also go wrong."
Verstraeten has some advice to anyone who is thinking of becoming a space tourist: "Wait a couple of years."
But he has high hopes.
Suborbital space trips are also expected to lead to speedy inter-continental voyages. In a suborbital flight, a spaceship goes up into space but does not complete an orbit of the Earth.
2 hour flights from Paris to Tokyo
Such a flight could mean a trip from, say, Paris to Tokyo in about two hours.
Transcontinental flights are what most commercial space developers have in mind, said the European Aviation Safety Agency's Jean-Bruno Marciacq. The agency was established by the European Union to regulate civil aviation.
He said there would probably be a role for the International Civil Aviation Organization, the Montreal-based UN body that regulates global aviation.
"If flights involve international passengers, ICAO would be involved," Marciacq said.
Discussions have already been held with the international body, he added.