After the bunny hop, Mr. Baumgartner hopes his body will slowly rotate so that he descends headfirst and breaks the 614-m.p.h. speed record held by Mr. Kittinger. After 34 seconds, engineers calculate, he will have fallen to 102,000 feet and accelerated beyond the speed of sound, which at that altitude is close to 690 m.p.h. (the exact figure depends on the temperature).

Image Credit... The New York Times

“I expect him to reach 720 miles per hour, about Mach 1.1,” said Art Thompson, the technical director of Red Bull Stratos and a former designer of the Stealth bomber. Mr. Baumgartner should remain supersonic for 20 seconds, until he reaches an altitude of 92,000 feet. Then the thickening atmosphere should slow him to subsonic speed, and eventually to a terminal velocity of 120 miles per hour.

Engineers and physicians are not sure what will happen if Mr. Baumgartner goes through the sound barrier. They realize he could be battered as parts of his body go supersonic or subsonic at different times, but the impact is expected to be manageable because of the thin air at that altitude — or so the engineers and Mr. Baumgartner hope.

“I know the consequences if something goes wrong,” Mr. Baumgartner said as he went through preparations over the weekend. “And it crosses my mind — what if I’m never going to see my family again? But I have learned how to control my fear so that it doesn’t get in the way.” He is single and has no children, but his parents and girlfriend were here for the jump.

Mr. Baumgartner’s biggest struggle occurred during training with his custom-designed suit. In early 2010, when he showed the pressurized suit and sealed helmet to this reporter at the training site in Southern California, he explained the problem: “If you’re sitting in there for hours all by yourself, you create your own little world, and at a certain point it becomes, man, I don’t like this. I want to get out.”

Later that year, when he was supposed to do an endurance test in the suit, he instead got on a plane and fled the country — the low point of his life, he now says. He feared the whole project was doomed, but he managed to get used to the suit with the help of a sports psychologist and other experts. They taught him mental exercises and set up a routine to keep him busy during the mission: a 40-item checklist that he goes through with Mr. Kittinger