Where Does the Sky End and Space Begin?

Where Does the Sky End and Space Begin?

We all know the distinction. The sky is that giant, enveloping space above our heads, sometimes blue, sometimes grey, sometimes a bit of both. Space is that unfathomably large, black, and empty…uh…space that surrounds our planet and reaches into the vast expanse. And we all know that, if you shoot into the sky high enough, you will eventually reach space. But where is the boundary? Where is the defining place that signals your departure from Earth and entry into outer space?

Well, we don’t really, conclusively know, as there’s no single place that declares your official exit out of Earth. It’s not like there’s a border crossing in the sky. The further up you go, the thinner the atmosphere becomes. Ask any mountain climber, and they’ll concur. But, at even greater heights, as the atmosphere becomes less dense, outer space slowly begins to intrude… and before you know it, you’re technically (if not officially) in the final frontier. So, entry into outer space is more of a gradual ascent than a switch being flipped. In fact, both the UN and US State Department don’t believe in making any one area an official designation of entry, as the process is so gradual and slow that there can be no clear indication of when it is.

But we know what you’re thinking. Surely there must be an altitude which is generally considered space. And you’d be right. It’s called the Kármán Line, and it’s 100 kilometers above sea level. This area was named after the physicist Theodore von Kármán, who determined that regular flight is impossible at this height. According to his studies, the atmosphere becomes so thin at this point that a craft would need to fly faster than the Earth’s orbit to generate enough lift to fly. At this height, the speed required is the same as what’s required for escape velocity. Therefore, the craft no longer follows Earth’s curvature and is ‘in outer space’. The Kármán Line has been officially designated as the point of Earth’s exit by the Fédération Aéronautique Internationale, or The World Air Sports Federation. According to them, once you reach the Kármán Line 100 kilometers above Earth, you are definitely in outer space and may be appropriately designated as an astronaut.

However, even this boundary is questionable and not universally agreed upon. For example, the United States Air Force categorises someone as an astronaut if they travel just 80 kilometers above sea level. This area signals the beginning of the thermosphere, where temperatures dramatically shift due to the absorption of X-rays, UV, and solar radiation. According to this particular logic, if the atmosphere is thin enough to be heated by the effects of outer space, then it is officially an area belonging to outer space. Which makes some sense, right?

Others agree that outer space starts at a location just above the Kármán Line, roughly 118 kilometers above sea level – so, 18 kilometers further from ground than the Kármán Line. This is where solar radiation and ions become abundant, taking over the Earth’s atmosphere. A 2009 study conducted by the University of Calgary determined that charged particles travel at an astounding 1,000 kilometers per hour at this altitude.

But even this location doesn’t signal the actual end of Earth’s atmosphere. Many people likely consider the International Space Station to be in outer space – it’s right there in its name, after all. But even it still feels Earth’s atmospheric effects. Despite maintaining an altitude of 330 to 435 kilometers, there are enough particles and molecules around it to cause drag. Given enough time, the particles would be strong enough to bring the ISS back down to Earth. To counter this, the space station accelerates with regular rocket boosts. So, we can see that even our more conventional understandings of outer space are still influenced by Earth’s atmosphere. You could perhaps argue that the space station isn’t officially in interplanetary space, as it still feels the effects of its nearest celestial body.

Above the thermosphere is the exosphere, and it’s here that our atmosphere finally merges with outer space. At this level, various gasses are present, including helium and carbon dioxide, but the particles are spread very far apart. However, they are still gravitationally bound to Earth. The beginning of the exosphere, officially known as the exobase or the thermopause, begins anywhere from 500 to 1,000 kilometers above the Earth’s surface, significantly higher than the International Space Station.

At this level, the barometric formula no longer applies, and temperatures remain constant (i.e. the freezing temperatures of space). The exosphere accounts for that distinct luminous ring that seemingly acts as a barrier between Earth and space. This ring is called the geocorona, and it can be seen in various satellite imagery. The geocorona extends roughly 10,000 kilometers above Earth’s surface, but even though it seems to end, it doesn’t. As you travel further into space (roughly half the distance to the moon), the sun’s radiation puts so much pressure on hydrogen particles that it finally exceeds the Earth’s gravitational pull. Therefore, we could claim that it’s here that you’re officially in interplanetary space, as Earth’s gravitational pull no longer affects atmospheric particles. In other words, there is no more atmosphere.

As the distance from Earth to the moon is 384,000 kilometers, and solar radiation affects hydrogen up to roughly half that distance, we can reliably estimate that the exosphere officially ends somewhere around 190,000 kilometers above sea level. While you might’ve called a cut-off point far earlier, at any one of the high altitude pitstops along the way, this feels like the final call. So, if you’ve made it this far, then the sky finally ends and interplanetary space begins when you’re halfway to the moon.

But, as we’ve repeatedly seen, the answer to this question depends on what your definition of outer space is. According to the US Air Force, you’re officially in outer space at just 80 kilometers above sea level, when the temperature begins to rise due to the absorption of solar radiation. Not much higher than this, at 100 kilometers, is the Kármán Line, where regular flight is no longer possible due to the lack of density in the atmosphere. A little further up, at 118 kilometers, is where ions begin to dominate Earth’s atmosphere and travel at incredible speeds. Yet even the International Space Station, orbiting roughly 400 kilometers above us, still feels the effects of Earth’s atmosphere – which, depending upon your definition, could mean it’s still ‘in the sky’. Between 500 and 1,000 kilometers above us is the beginning of the exosphere, which extends halfway to the moon. And, as solar radiation overcomes the power of Earth’s gravity, it’s here that Earth’s atmosphere officially ends – and the sky’s limits are finally reached.