CAPE CANAVERAL, Fla. -- It might not seem like a brilliant idea, allowing a frail 65-year-old paralytic to float free from gravity aboard a rising and plunging rollercoaster stunt flight. But who's to argue with Stephen Hawking?

The celebrated British astrophysicist and black-hole theorist, author of "A Brief History of Time," paralyzed by Lou Gehrig's disease and communicating largely through eye twitches, has long wanted to visit space. Human survival depends on getting there, he says. An event yesterday in Cape Canaveral was described as his first improbable step.

Dressed in dark blue flight suits, Dr. Hawking and an entourage of caregivers boarded a Boeing 727 that roared out over the Atlantic Ocean and carved huge parabolic arcs in the sky, creating for passengers the floating "zero-gravity" effect of being in outer space.

While levitating, Dr. Hawking, who has been in a wheelchair for nearly four decades, was spun twice. Someone else floated an apple in the air alongside him in an allusion to Isaac Newton, whose esteemed chair Dr. Hawking now holds at Cambridge.

Once each of the 25-second spells of zero gravity ended -- as the plane headed to the bottom of each arc -- assistants ensured that the celebrated physicist's body was lowered to a mattress on the plane's floor as gravity kicked back in.

"It was amazing. ... I could have gone on and on. Space, here I come." Dr. Hawking said afterward, once again sitting slightly twisted in his wheelchair. His "voice" is actually the product of a synthesizer to which he dictates, using eye twitches.

Considered one of the giants among physicists pondering the beginnings of the universe, Dr. Hawking said he hopes to take a greater leap into the heavens in 2009 on a space plane being developed by Richard Branson's company Virgin Galactic.

Yesterday's flight was more than simply a step in one celebrity's eccentric whimsy. It was a platform for Dr. Hawking's strikingly bleak view of humanity's future on Earth.

"Life on Earth is at the ever-increasing risk of being wiped out by a disaster, such as sudden global warming, nuclear war, a genetically engineered virus or other dangers," the synthesized voice announced in a statement before boarding. "I think the human race has no future if it doesn't go into space. I therefore want to encourage public interest in space."

The flight benefited a charity for Lou Gehrig's disease, technically known as amyotrophic lateral sclerosis, Easter Seals and two other groups. But Dr. Hawking said sending a message about what people with disabilities can achieve was only a small part of his motivation. He wants to encourage copycats -- people who will say, 'If he can do it, I can, too.' I hope many people will follow in my path," he said.

The flight reached a level of 25,000 feet and then began a series of climbs at 45 degrees and then dips at 30 degrees. In profile, the flight path looks like a roller-coaster ride.

For about 25 seconds at the peak of each hill, passengers aboard are loosed from gravity, and they float. The effect is similar to a person on a roller coaster who feels a slight sense of levitation as it tops a rise. While crew members said they had hoped for one to three of the zero-gravity maneuvers, they completed eight.