Astrophysicist Stephen Hawking floats on a zero-gravity jet Thursday. The modified jet carrying Hawking, a handful of his physicians and nurses, and dozens of others first flew up to 24,000 feet over the Atlantic Ocean off Florida. Nurses lifted Hawking and carried him to the front of the jet, where they placed him on his back atop a special foam pillow. The plane made a total of eight parabolic dips, including two during which Hawking made weightless flips.

Cape Canavarel, 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 roller-coaster 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 here Thursday was described as his first improbable step.

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

While levitating, Hawking, who has been in a wheelchair for nearly four decades, was spun twice – pirouetting like a “gold medal gymnast,” a crew member said. Someone else floated an apple in the air alongside him in an allusion to Isaac Newton, whose esteemed chair 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,” Hawking said afterward, once again sitting slightly twisted in his wheelchair. His “voice” is actually the product of a synthesizer he dictates to by using eye twitches.

Considered one of the giants among physicists pondering the beginnings of the universe, 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. Flights like those taken here Thursday – once known at NASA as “vomit comets” – have been used to accustom astronauts to space travel.

But Thursday’s flight was more than simply a step in one celebrity’s eccentric whimsy. It was a platform for 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 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 danger for Hawking, his aides said, was not the sense of weightlessness but the extra gravity forces that came at the bottom of each descent. The extra force could have, for example, make it more difficult for him to breathe.

The fears proved unwarranted.