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The genius vs. the g's

When one of the world's most famous and frailest physicists goes on a zero-G adventure in April, it's not the weightlessness he'll have to worry about. Instead, the most trying time for British cosmologist Stephen Hawking will come after each half-minute of floating, when he'll feel as if he's sinking into the floor. And if that sounds daunting, the ordeal associated with flying to the edge of space would be much more stressful.

April's flight from NASA's Kennedy Space Center would represent one small step toward Hawking's ultimate goal of riding on Virgin Galactic's suborbital rocket plane to the edge of outer space, more than 62 miles (100 kilometers) up. That's where the best-selling author ("A Brief History of Time") would be able to glimpse the black sky of space, with the curving Earth spread out below.

Such a flight wouldn't be available to him until 2009 or 2010 at the earliest, but Hawking has already started on the preparations.

"We have had now a first detailed meeting with Stephen personally and with his team of caregivers and medical advisers," Stephen Attenborough, Virgin Galactic's head of astronaut relations, told me Thursday. "We're happy to continue with this process. Everybody recognizes that it's going to take a bit of planning - it won't be easy."

Hawking's ride on Zero Gravity's specially modified Boeing 727 jet will involve short shots of weightlessness, followed by a pullout that would produce a sinking feeling. Hawking could experience as much as 1.8 g's, or 1.8 times the normal pull of gravity. That kind of "hypergravity" tends to pull the blood down from your head, leading to lightheadedness and potentially motion sickness. To minimize the effect, Hawking will lie down on his back during the hypergravity phase of the flight - and medical personnel will make sure his cardiovascular system can cope with the fluid shifts.

Now consider what is expected to happen during a typical Virgin Galactic spaceflight. Attenborough said the rocket-powered ascent would involve a maximum of 3.5 g's or so, and the descent would be even more dramatic, with the sinking feeling reaching a peak of 6 g's. That's the equivalent of a hard-core roller-coaster drop.

"It's not the zero-G that's the issue," Attenborough said. "It's the high g's on the way down. That's what we need to focus on. For somebody in [Hawking's] condition, this is obviously a pretty demanding profile."

He said Hawking could conceivably go through "a program of progressive testing" to make sure he can handle the high-g transitions.

"A centrifuge is certainly a possibility," Attenborough said. "We're going to be giving our founder customers that experience later this year. ... That would certainly be an option, but to be honest we really haven't gotten to that stage."

Can you visualize Stephen Hawking strapped into a whirling centrifuge? Yes, it sounds daunting, but the Cambridge physicist has at least three factors working in his favor:

  • First of all, the 6 g's during the descent is what's called "Gx" acceleration - that is, a sense of being pushed back in your chair. The body tolerates that much better than the "Gz" acceleration that can make fighter pilots black out. This Web page explains G-forces in general and Gx vs. Gz in particular, while this article focuses in on suborbital spaceflights.
  • The second factor is that there are ways to reduce the G-load for Hawking's flight. "One of the nice things about the SpaceShipTwo technology is that we will have the option to fly slightly different flight profiles," Attenborough said.
  • Most importantly, Hawking seems to have enough can-do spirit to get him into the astronaut corps. At least that's the impression he made on Attenborough during their meeting a couple of weeks ago in Cambridge. "It was a fantastic meeting," Attenborough told me. "Stephen's obviously very enthusiastic to do this."