In an interview on Wednesday he said: "I therefore want to encourage public interest in space. A zero-gravity flight is the first step towards space travel."
On Thursday, at a news conference before the flight, he said: "Life on Earth is at an ever-increasing risk of being wiped out by a disaster such as sudden global warming, nuclear war, a genetically engineered virus or other dangers."
Hawking, 65, and an entourage of carers and other thrill-seekers took off from the space shuttle's runway at the Kennedy Space Center in a specially modified jet that dives through the sky to give passengers an experience of zero gravity.
|"Life on Earth is at an ever-increasing risk of being wiped out by a disaster such as sudden global warming, nuclear war, a genetically engineered virus or other dangers"|
Stephen Hawking, British physicist
They returned to the space centre in Florida about two hours later after doing eight of the dives, giving him four minutes of weightlessness.
Hawking acknowledged before the flight that experiencing weightlessness, even for a few seconds, would be sweet relief from the bondage of a daily life immobilized by a debilitating and irreversible neuromuscular disorder.
"I have been wheelchair-bound for almost four decades and the chance to float free in zero G will be wonderful," Hawking said.
The acclaimed cosmologist and best-selling author of "A Brief History of Time," who has posited theories to help explain black holes and other celestial phenomena, lost his ability for natural speech after a tracheotomy that followed a bout of pneumonia in 1985.
He speaks with the aid of a computer-controlled voice synthesizer.
In a reference to his affliction, amyotrophic lateral sclerosis, or Lou Gehrig's disease, Hawking noted that his flight also would serve as a demonstration that "everybody can participate in this type of experience."
Hawking hopes to make a suborbital spaceflight aboard a new passenger service being developed by Virgin Atlantic Airway's offshoot, Virgin Galactic, expected to begin in 2009.