I don’t know where the sunbeams end and starlight begins
It’s all a mystery.
– The Flaming Lips (noted spaceflight experts)

As I’m sure many of you did as well, this afternoon I watched breathlessly as Austrian skydiver Felix Baumgartner jumped from a balloon 39 kilometers (about 24 miles) above the ground. For the first minutes, he tumbled rapidly, his body spinning as he fell at an extremely high speed (600 km/hour! see update below) before he gained control of his rotation as the atmosphere became thicker around him. The entire descent lasted about 10 minutes, and mostly free-fall until the end, when he opened a parachute and landed on his feet. Baumgartner’s dive broke the previous record of 31 kilometers, established by Joe Kittinger in 1960. (Kittinger served as the radio contact for Baumgarten during his the dive.) Over all, this was an incredible accomplishment, and when you consider everything that could have gone wrong, the smoothness of the adventure was all the more remarkable. And it’s not simply a daredevil stunt: the technology used in a fall from such an altitude could save the lives of astronauts bailing out of spacecraft breaking up in the upper atmosphere—as in the Space Shuttle Columbia disaster.

Update: The official word is in that Baumgartner reached 1350 km/hour (839 mph), which is about 1.24 times the speed of sound at that altitude. (The speed of sound depends on atmospheric pressure and density, so it isn’t a constant even at sea level.) Breaking the sound barrier produces a shock wave in the air, though at such low density high above the ground it wouldn’t be as dramatic as the sonic booms near Earth’s surface.

One thing bothered me, though, about a lot of the coverage: many people said Baumgartner was jumping “from space” or “from the edge of space”. Don’t get me wrong—39 km is a long way up, about 4 times the altitude of commercial airliners, so I’m not denigrating this accomplishment at all. Atmospheric pressure is about 2% of its value at Earth’s surface at 39 km, and the temperatures are pretty cold, so Baumgartner had to wear a pressurized suit and carry an air supply. (If memory serves, the temperature was -7° C or 19° F when the dive began.) However, it’s not what is conventionally considered “space”: it’s within the region of Earth’s atmosphere known as the stratosphere (which also explains the project’s official name, “Stratos Jump”). So, if Baumgartner didn’t jump from space, where is the boundary of space?

The answer isn’t very clear cut, and depends on what is important for the particular reason you’re asking. Generally when I’ve taught astronomy, I referred to space as the lowest point in Earth’s exosphere: the region of the atmosphere where atoms and molecules are so loosely bound by Earth’s gravity, they can escape. Satellites can orbit within the exosphere, and despite the root “sphere” in the name, it fluctuates in size and shape. The air in the exosphere is so tenuous, it’s considered hard vacuum for the purposes of space flight. The base of the exosphere is about 500 km, though that also varies a lot, thanks to solar heating and other factors. The Space Shuttle orbited around 350 and 600 km, for comparison.

On the other hand, rocket engineers often consider altitudes around 100 km to be the boundary of space, since that’s too high for balloons to reach and oxygen is rarefied enough that combustion isn’t possible, so ordinary (non-rocket-powered) airplanes can’t go there either. (Thanks to Lockwood DeWitt for pointing these criteria out; Phil Plait has more.) This altitude is known as the Kármán line.

In a sense, it doesn’t really matter where the boundary of space is. Human beings are fragile creatures, limited to a relatively thin layer of Earth’s atmosphere before air density becomes too low to breathe. However, Felix Baumgartner and the engineering team who made the dive possible show that human ingenuity allows us to overcome our inherent limitations. As with the successful landing of Mars Curiosity, the Stratos Jump shows that the builders win.