Aviation magnate Branson announced that his new Virgin Galactic venture aimed to send 3,000 new astronauts into space within the next five years.

However Branson's ambition to usher in an age of mass space travel was unlikely to be realised for now, said Andre Balogh, professor of space physics at London University's Imperial College.

"It is something that is technically possible, but space is a very expensive business, and space tourism is likely to remain an expensive business for a very long time," he told AFP.

"The idea is great, I like the idea, but I am very aware that even people like NASA find it a challenge. Eventually it will come. Whether it will come in Richard Branson's time, and in his way, remains to be seen," he said.

"I take it as a declaration of intent, to look into it, rather than to take bookings straight away."

The issue of safety was paramount and would have to be tackled before any flights took off, Balogh noted.

"I obviously don't think there is anything else other than the safety issue which will dominate the technical agenda," he said.

"Accidents are bad for publicity and bad for business. So to ensure it is technically feasible, the bottom line is that they have to ensure safety and minimise the risks -- which is quite a job."

Branson aims to use vessels based on the technology of SpaceShipOne, which in June became the first private manned craft to travel into space.

SpaceShipOne only made it to the fringes of space -- around 100 kilometres (62 miles) above earth -- and not in full orbit, but Balogh predicted that would satisfy most customers.

"I think most people would accept it being called space travel. You would be able to see the Earth being round, and the horizon. You would have a view that is very spectacular, you would take in a very large bit of the earth," he said.

The space tourists would not be completely weightless, he added.

"You can't have an orbit at that altitude, so you could not be totally weightless. It would be probably fairly close to it, but it is not an orbit, it is still within the upper atmosphere."

Branson's would-be civilian astronauts would have to pay fees starting at 115,000 pounds (170,000 euros, 207,000 dollars) and receive three days of flight training before taking the real trip.

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