On October 14, Austrian daredevil Felix Baumgartner jumped from a balloon-suspended capsule nearly 23 miles in the sky to break the sound barrier as he fell back to Earth. The 128,100 foot jump, sponsored and paid for by energy drink company Red Bull, broke three world records for the highest manned balloon, and the first man to break the speed of sound without mechanical aid. The jump was streamed live on YouTube and brought in a record-breaking 8 million viewers. Nine minutes three seconds after jumping, Baumgartner gracefully landed back on Earth, fell to his knees, and raised his fists in triumph.
The Austrian adventurer hit a an unofficial top speed of 834 mph—Mach 1.24—when he fell 119,846 feet during the 4 minute 20 second free fall. While Baumgartner jumped from 128,100 feet, the free fall represents the portion of the jump before he opened his chute, where the only forces acting on him were gravity and air resistance.
“It was harder than I expected,” said Baumgartner when returning to mission control in Roswell, New Mexico. “Trust me, when you stand up there on top of the world, you become so humble. It’s not about breaking records anymore. It’s not about getting scientific data. It’s all about coming home.”
Red Bull, the sponsor of the mission, created the NASA-like Red Bull Stratos program five years ago. The program, with its mission control in Roswell, includes 300 people, 70 of whom are engineers, scientists, and physicians. In addition to the dramatic feat of the jump, the Red Bull Stratos teams has been gathering data through their experimentations that will assist future pilots, astronauts, and maybe future high-altitude tourists. One of the biggest questions the team faced being, “what happens to the human body when it breaks the sound barrier?”
Baumgartner was assisted throughout the entire jump by Joe Kittinger, an 84-year-old retired Air Force Colonel who help the previous high-altitude jump record. Kittinger held constant radio contact with Baumgartner as he made as ascent, preparations, and descent.
The twice-delayed mission lifted off at 9:31 a.m. Mountain time and Baumgartner began his 2 hours 21 minute ascent. The first couple minutes of the journey were the most tense as Baumgartner rose up past the first 4,000 feet, a region called the “dead zone” because it is impossible to deploy a parachute at that altitude. The super-massive balloon carried the capsule upward at 1,200 feet per minute and expanded from 180,000 cubic feet of helium on the ground to 30 million cubic feet at the maximum height.
Kittinger worked through pre-jump procedures and checklists as Baumgartner rose past certain benchmarks: the Armstrong limit at 63,000 feet, where air pressure is low enough to make a person’s blood boil; Baumgartner’s previous test jumps at 71,000 feet and 97,000 feet; Kittinger’s record at 102,800 feet; and finally the hight manned balloon flight record at 113,740 feet. Beyond the capsule, the curvature of the Earth loomed and the thin blue line separating Earth and its atmosphere appeared whisper-thin.
Mission control transferred all life support to Baumgartner’s suit, depressurized the capsule, and Baumgartner opened the hatch. “There it is. There’s the world out there,” Kittinger said. “Start the cameras and our guardian angel will take care of you.”
“I wish you could see what I can see. Sometimes you have to be up really high to see how small you are. I’m going home now.” Baumgartner made a salute, and jumped.
Early on, not everything went as planned. Baumgartner was supposed to fall in the “delta position,” with his head down and arms back, but he started to tumble and spin shortly after leaving the capsule. Baumgartner was afraid that the spin would be too violent, he wouldn’t be able to get out of it, and he would enter a flat spin sending all the blood away from the center of his body due to centrifugal force. “At a certain R.P.M.,” he said afterward, “there’s only one way for blood to leave your body, and that’s through your eyeballs. That means you’re dead. That was what we feared most.” You can see Baumgartner lose control and regain it from the chest cam footage.
Baumgartner recovered quickly and spent the rest of the descent in a controlled fall where he was able to break the sound barrier. Deploy his parachute at around 6,000 feet and safely make it back to the ground.
Some of the most remarkable pieces to this story involve the physics behind the jump and the technology used to ensure Baumgartner made it safely back.
When most skydivers fall, they reach a maximum terminal velocity of around 120 miles per hour. Whenever an object is falling through the air, it has two forces acting on it: gravity pulling back to earth, and the drag force of air resistance pushing up against it. The upward drag force depends on the drag coefficient of the object based on its physical properties, the density of air, and the rate at which the object is moving. For most skydivers, when they reach around 120 miles per hour, the drag force and gravity are equal meaning they can’t fall any faster.
But when you reach incredible heights, strange things start to happen.
A primary thing to note is that the speed of sound is much slower at extremely high altitudes. Sound requires a medium, namely air, to travel through and the atmosphere is so thin at 120,000 feet that the molecules making up air are so far apart and cold there is a delay in their interactivity. Where Baumgartner jumped, the speed of sound is only about 450 miles an hour compared to 760 miles an hour under standard conditions at Earth’s surface.
Second, the density of air is incredibly small at 120,000 feet. It’s small enough that the water in a persons blood will easily boil. It’s also low enough that were Baumgartner not to be in a pressurized suit, he would swell up like a marshmallow in a microwave. The human body has a certain amount of pressure it pushes outward with, but on the surface of the Earth, this pressure is combatted by air pressure which keeps us from expanding. To give you a comparison, on the surface air density is about 1.2 kilograms per cubic meter. At 120,000 feet it’s 0.00073 kilograms per cubic meter. The powerful thing about air density is that the further you go away from the Earth the more drastic the difference in air density is. That means the difference in air density between 5,000 and 10,000 feet is much smaller than this difference between 115,000 and 120,000 feet.
Going back to the forces on Baumgartner as he falls. Gravity pulls down; air resistance pushes up. But air resistance depends on the density of air. Because it is so low, the total air resistance pushing against him is very small. This means gravity will keep accelerating him before he reaches a terminal velocity because gravity overpowers drag far and away. But as the altitude drops, air becomes more dense and therefore the air resistance grows until it over powers gravity. At this point he will begin to slow down—the air he is moving through acts like a brake. Eventually he will slow to a certain point where gravity and air resistance balance and he will neither speed up nor slow down.
The above graph, made by Rhett Allain for Wired, shows the relative speed of sound compared to Baumgartner’s speed. As you can see there is about a minute period where he will break the local sound barrier. But the graph also shows how he will first accelerate, then hit a point where air will act like a break and we will gradually slow down as he approaches terminal speed. The second graph compares Baumgartner’s fall with the relative terminal velocity as he falls.
The suit Baumgartner wore is more-or-less a super-durable spacesuit. The composite structure provides him with a 3 psi pressurized environment for the whole trip and protects him for the -100 degree Fahrenheit atmosphere, while the suit supplies a fixed amount of pure oxygen for the whole descent.
The suit contains a main and reserve chute which are designed to be deployed at 172 miles per hour meaning he has to have been slowed sufficiently by more dense air before he can pull the rip cord. A fail safe mechanism would have triggered the main chute if he was still moving at more than 115 feet per second at 2,000 feet of altitude or less.
Instruments on the suit monitored his health conditions, streamed live video from his chest camera back to the ground, and measured the pressure on various parts of his body throughout the journey.
Jump altitude: 128,100 feet (24.2mi, 39km, the middle of the stratosphere)
Free fall distance: 119,846 feet (22.7mi, 36.5km)
Free fall time: 4 minutes 20 seconds
Max velocity: 373 meters per second, or 833.9 mph (1342 kph)
Video of full jump.