An Austrian daredevil suited up on Sunday in hopes of making his delayed skydive from a balloon flying 23 miles above the planet and breaking the sound barrier.
Crews early Sunday began spreading out the massive but delicate helium balloon so it could be inflated to carry skydiver Felix Baumgartner, 43, into the stratosphere. The launch, scrapped several times during the previous week by high winds, depends on calm skies.
The 30 million-cubic-foot (850,000-cubic-metre) plastic balloon, which is about one-tenth the thickness of a Ziploc bag, or roughly as thin as a dry cleaner bag, can only be launched if winds are 2 mph or less between ground level and an altitude of about 800 feet.
The scheduled 7:30 a.m. (9:30 a.m. EDT/1330 GMT) launch time was pushed back to late morning in hopes of better wind conditions, the project sponsors said.
Baumgartner aims to break a 52-year-old high altitude parachute jump record held by project adviser Joe Kittinger. In 1960, Kittinger, now a retired U.S. Air Force colonel, jumped from a balloon flying at 102,800 feet and fell for 4 minutes and 36 seconds before opening his parachute. Baumgartner hopes to top that with a jump from 120,000 feet and freefall for 5 minutes and 35 seconds.
There is so little air in the upper reaches of the atmosphere that after about 30 seconds of freefall, Baumgartner should be moving faster than the speed of sound, which is roughly 690 mph at that altitude.
Among the risks Baumgartner faces is the chance that his supersonic body will trigger shock waves that could collide with the force of an explosion. But Baumgartner's medical team doesn't believe this situation is very likely because the air in the stratosphere would be too thin to carry the waves.
Baumgartner would stay supersonic for about a minute before hitting a thicker part of the atmosphere, slowing his fall.
No human has broken the sound barrier during freefall, at least not intentionally. On January 25, 1966, Bill Weaver, a U.S. test pilot aboard an SR-71 Blackbird aircraft, was ejected from his damaged plane at Mach 3.18 - more than three times faster than the speed of sound - and survived.
"It goes to show there are still challenges to overcome and you should never lose sight of trying to achieve them," Baumgartner said in an interview posted on the project's website.
Energy drink manufacturer Red Bull is sponsoring the skydive. The firm declined to disclose how much the project or its advertising campaign would cost.
Besides breaking several records, including highest-altitude freefall, longest freefall and highest manned balloon flight, Baumgartner and his team hope the jump will help engineers working on spacesuits for NASA and the budding commercial space tourism industry.
One company, Virgin Galactic, an offshoot of Richard Branson's London-based Virgin Group, expects to begin passenger suborbital spaceflights in late 2013 or 2014. SpaceShipTwo, the first of Virgin's planned fleet, is undergoing testing in Mojave, California, and is expected to make its first foray beyond the atmosphere before the end of the year.
Baumgartner's ascent into the stratosphere should take 2.5 to three hours. The descent should last just 15 to 20 minutes, more than half of it beneath the relative safety of his parachute's canopy.
When Baumgartner jumps from a capsule beneath the balloon, the position of his body will be crucial since there is no air in which he can reposition himself. If he falls in a way that puts him into a rapid spin, Baumgartner could pass out and damage his eyes, brain and cardiovascular system.
Baumgartner's safety gear includes a custom spacesuit to protect him from the low pressure and the extreme cold.
Temperatures are expected to be as low as about minus-70 degrees Fahrenheit (minus 57 degrees Celsius). The near-vacuum puts him at risk of ebullism, a potentially lethal condition in which fluids in the body turn to gas - literally blood boiling. Severe lung damage could occur within minutes.
Red Bull plans to broadcast the entire jump live on its website, but has built in a 20-second delay in the feed in case there is an accident.