On his 108-minute flight in 1961, the Soviet cosmonaut Yuri Gagarin, the first human in space, reached a peak altitude of 327 kilometres (203 miles), after blasting off the planet atop a mighty Vostok rocket. After launch shook his tiny capsule violently, Gagarin experienced the feeling of weightlessness, and saw the curvature of the Earth first-hand. By all accounts, he crossed the mysterious border between the Earth and space. Or did he? It has been more than half a century since Gagarin's historic journey, but there is still no universally accepted definition of where space begins.
The International Astronautical Federation marks the beginning of space, for aviation purposes, at the 100-kilometre mark, where the Earth's atmosphere is so negligible that conventional aircraft cannot travel fast enough to generate aerodynamic lift. There are other, more arbitrary numbers: In the 1960s, eight US pilots earned their astronaut wings by flying the arrow-shaped X-15 rocket plane above 80 kilometres. In the US, this standard still applies, meaning that any American who flies on one of Virgin Galactic's private spacecraft will officially become an astronaut.
Recent scientific discoveries have further muddled our terrestrial-celestial border confusion. In 2009, an instrument called the Suprathermal Ion Imager (SII) pinpointed 118 kilometres as the point at which charged particles from space begin to overwhelm the relatively mild particle flow of the Earth's upper atmosphere. That was the point, researchers argued, where space really begins. Headlines hailing the discovery of the 'edge of space' briefly splashed across the media, but the attendant stories were hesitant, bracketing any notions of finality with alternative edge-of-space definitions.
It could be argued that the question of where space begins intrigues us because it marks the borderline between a world where we are protected by Earth, and one where we must fend for ourselves. After all, the Earth's atmosphere is our first and last line of defence against lethal ultraviolet radiation and rogue meteors.
This great shield is built of five layers, and each can stake a claim to the 'beginning of space' title. Most familiar to us is the troposphere, the layer that extends from the ground up to between nine and 17 kilometres, and contains most of what we would consider 'atmosphere'. The troposphere is thick and stable, making it ideal for most commercial air travel. Next comes the stratosphere, up to 50 kilometres or so, then the mesosphere, which is where the US government says 'space' begins. The outermost layer, called the exosphere, is the vastest of the Earth's atmospheric layers, extending from about 690 to 10,000 kilometres above the Earth's surface. There's not much atmosphere here and, if you reach the exosphere, you can with some confidence claim to be 'in space'.
But it's the layer below the exosphere where the edge-of-space battle is most vigorously contested. Sandwiched between the mesosphere and exosphere is the thermosphere, which is the realm of the ISS, and formerly the space shuttle and the Mir space station. Huge temperature variables exist here, because there is little air pressure and very few atoms by which to transfer heat, which means temperatures well in excess of 1,500°C. In the thermosphere, the Earth appears decidedly curved, and beyond the terrestrial horizon all is very black indeed. Travellers here don't much feel the effects of gravity, which is why we have so many videos of space station denizens floating happily in their shirtsleeves.
All of these competing thresholds suggest science isn't the right tool to gauge where space begins. Its boundary lies wherever humans feel they have crossed a threshold of distance, or protection, or some other radical departure from terrestrial experience.
The Apollo 11 astronaut Michael Collins quipped that what the space programme needed was 'more English majors'. Baumgartner would probably agree, having confessed frustration in interview after interview that he could not say something more profound about his experience than: 'It was cool'. But maybe that's the point. Maybe we want to touch the edge of space because it's the one place left to us that promises a shivering sense of primeval, inarticulate rapture; a place where language, like air, ceases to matter.
Aeon is a new digital magazine of ideas and culture, publishing an original essay every weekday. It sets out to invigorate conversations about worldviews, commissioning fine writers in a range of genres, including memoir, science and social reportage.