The crash of a Virgin Galactic craft is going to make people think twice about going on tourist trips to the edge of space, a former astronaut has told Sky News.
Hundreds of people have already reserved seats and paid a deposit on the $250,000 (£156,000) ticket price for a minutes-long suborbital flight on SpaceShipTwo, which can carry six passengers.
During its 35th test flight, it exploded over California's Mojave Desert shortly after being released from its mother ship WhiteKnightTwo, which had taken it to an altitude of 50,000ft.
The death of a pilot marks a major blow for the project and its billionaire founder Sir Richard Branson, who immediately headed to the scene by plane.
Retired astronaut Jose Hernandez told Sky News: "It is a risky business and we need to make sure it's safe for humans to travel into space."
He said of the crash: "It's certainly going to make people think twice before getting onto a ship and trying a zero g flight into space.
"But I think this is natural progression of what happens when you're going from having zero experience to trying to send humans into space.
"It is a steep learning curve. Sometimes it is an expensive learning curve."
Another pilot was badly injured in what has been described as a "catastrophic failure".
The company said in a tweet that the craft had suffered a serious "anomaly", without going into details.
The plan was for SpaceShipTwo to fire its rocket engine and carry out a 30-second "burn" test.
It was the first flight testing its engine for nine months and experts were also assessing a new type of fuel mixture which is plastic-based rather than a rubber-based compound.
It was hoped the new formulation would boost the engine's performance.
During test flights, the craft had not yet reached the edge of outer space, about 62 miles above Earth.
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Gallery: Images Showing Wreckage Of SpaceshipTwo In The Mojave Desert
SpaceShipTwo and its mother ship WhiteKnightTwo are pictured before the test flight. Pic: Virgin Galactic/Scaled Composites/Jason DiVenere
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Part of the wreckage from the Virgin Galactic SpaceshipTwo lies in California's Mojave Desert after it crashed
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Photographer Ken Brown said the craft was released from the plane that carries it to high altitude, ignited its rocket motor and then exploded
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Two pilots were on board. California authorities said one died and the other was badly hurt
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A witness said the space tourism craft exploded during a test flight over the desert
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The aim of such flights was to assess SpaceShipTwo in preparation for suborbital trips to the edge of space about 62 miles above the Earth
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Hundreds of people have already reserved seats and paid a deposit on the $250,000 (£156,000) ticket price for the flights. Pic: Virgin Galactic
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After several delays, Sir Richard Branson's company had hoped to start taking passengers to the edge of space in 2015
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But space expert Marco Caceres said: "You are not going to see any commercial space tourism flight next year or probably several years after that."
But the company had said the first trip there could have taken place in the next few months, depending on how testing went.
The launch date of the project has been put back repeatedly from the original 2007 date, but Sir Richard claims he and his son will be on the inaugural flight by next spring.
He previously described the project as the "beginning of a whole new era of space travel" and even suggested his business could "start doing trips to Mars".
Space journalist Sarah Cruddas told Sky News that things had "been looking really good for Virgin Galactic".
She said there had been "problems" with the engine in the past.
SpaceShipTwo is the commercial version of SpaceShipOne, the first private spacecraft to reach the edge of space in 2004, and which is now on display at the National Air and Space Museum in Washington.
