Astrophysicist Stephen Hawking, paralyzed by Lou Gehrig's disease, floats during a zero-gravity flight; an apple, a tribute to Isaac Newton, drifts along with him.

CAPE CANAVERAL, Fla. — Free of his wheelchair and tethered only to heart rate and blood pressure monitors, astrophysicist Stephen Hawking on Thursday fulfilled a dream of floating weightless on a zero-gravity jet, a step he hopes leads to further space adventures.

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 jet then climbed to around 32,000 feet and made a parabolic dive back to 24,000 feet, allowing Hawking and the other passengers to experience weightlessness for about 25 seconds.

Hawking, a mathematics professor at the University of Cambridge who has done groundbreaking work on black holes and the origins of the universe, has the paralyzing disease ALS, also known as Lou Gehrig's disease...