(CNN) -- It’s an unforgiving place, 45,000 feet above the Earth. It’s brutally cold up there, as low as 59 degrees below zero, and there’s so little air to breathe, it takes just seconds to pass out.
It’s at least 10,000 feet above the typical cruising altitude for a passenger jet, a full 3 miles above the peak of Mount Everest, and a staggering 6 miles higher than your typical skydiving altitude.
This is where SpaceShipTwo disintegrated Friday, high above the Southern California desert. Pilot Peter Siebold survived the crash. Co-pilot Michael Tyner Alsbury died.
Why one died and the other lived is unknown, said CNN aviation consultant Miles O’Brien.
“There’s a million things,” he said.
“What’s amazing is that Siebold is alive,” he said. “There must be an amazing story of either luck or sheer will that he’s living.”
Whatever happened, it’s a sure bet the pilots thought about and trained for failures similar to the one that caused the experimental craft to break up, he said.
“Everything about test flying is, ‘What’s going to go wrong now,’ “ O’Brien said.
Test pilots, by definition, work in extreme environments. They’re always chasing some superlative—higher, faster, never done—usually in unproven aircraft fresh off the design board.
And they often do it with a minimum of survival gear, which adds complication and weight to airframes under testing, O’Brien said.
Like the space shuttle and other spacecraft before that, SpaceShipTwo didn’t have ejection seats, O’Brien said. To escape a disintegrating, spinning craft, its pilots might have to claw their way along a rope mesh to escape.
And while they wore helmets with oxygen, they weren’t dressed in full-pressure suits—not that it would have probably meant much in this case, O’Brien said. After all, Siebold survived without one.
Still, the thin air makes quick reactions crucial. At 45,000 feet, pilots who lose their masks or oxygen canisters can expect to have nine to 15 seconds of effective performance time before the lack of oxygen takes hold, according to the Aircraft Owners and Pilots Association.
And while some on the ground are looking up, questioning the value of expending human life on commercial spaceflight, the test pilots themselves have a different perspective, as Virgin Galactic owner Richard Branson told CNN’s Poppy Harlow on Monday.
“Yes, the risk is worth it,” Branson said. “And as I say, Mike would have been the first to say that. I’m sure his parents and his wife and his sisters would not say that, but test pilots would say that because they know the risk they’re taking. That they know the importance of what they’re doing.”
O’Brien, who has been critical of the ease with which Branson and others have portrayed future space travel, agreed.
“These guys go into it completely with their eyes wide open,” he said. “There’s always going to be people who want to do that.”
The-CNN-Wire™ & © 2014 Cable News Network, Inc., a Time Warner Company. All rights reserved.