In this photo provided by Zero Gravity Corp., Hawking floats on a zero-gravity jet on April 26, 2007. The modified jet carrying Hawking, some of his physicians and nurses, and dozens of others first flew up to 24,000 feet over the Atlantic Ocean off Florida and then dove down, allowing the passengers to experience zero gravity.

In this photo provided by Zero Gravity Corp., Hawking floats on a zero-gravity jet on April 26, 2007. The modified jet carrying Hawking, some of his physicians and nurses, and dozens of others first flew up to 24,000 feet over the Atlantic Ocean off Florida and then dove down, allowing the passengers to experience zero gravity.

In this photo provided by Zero Gravity Corp., Hawking floats on a zero-gravity jet on April 26, 2007. The modified jet carrying Hawking, some of his physicians and nurses, and dozens of others first flew up to 24,000 feet over the Atlantic Ocean off Florida and then dove down, allowing the passengers to experience zero gravity.

In this handout photo provided by Zero Gravity Corp., Hawking floats on a zero-gravity jet on April 26, 2007. 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 and then dove down, allowing the passengers to experience zero gravity.