Where Does Space Begin? | The Vacuum of Space Explained

Added: 2026-05-24

[00:00:00]If you got into a rocket right now and flew straight up, at what exact point would you officially be in space? Like, where do you draw the line? Is there a welcome to space sign or a cosmic border control officer? Because surprisingly, the answer isn't as clean as you think.

[00:00:18]Let's explore this.

[00:00:26]Hello there. Welcome back to Star Stuff, where we're continuing the second episode of the explaining the hard to understand stuff series. Some of you asked very nicely to explain where space actually begins and to go into a bit of detail about what the vacuum of space really means. So, let's start with the biggest misconception. Earth's atmosphere doesn't just stop. There's no solid boundary. There's no air ends here moment. Instead, it gradually fades out layer by layer. First, we have the troposphere where we live and complain about the weather. Then, we have the stratosphere with the ozone layer. Then, we have the meosphere and then the thermosphere and the exosphere. Each layer gets thinner and thinner and thinner until you're basically surrounded by almost nothing. So, already we've got a problem. If the atmosphere just fades away, where exactly do we draw the line? Most scientists and organizations use something called the common line, which is named after the Hungarian American engineer Theodor Vanc. This line sits at 100 km above the Earth's surface. And here's the logic. At that altitude, the air is so thin that you can't fly a plane anymore. To stay up, you need to go so fast that you're basically in orbit. So instead of flying like an aircraft, you're now moving like a spacecraft. Boom. Space. Because of course, humans couldn't agree on just one number. In the United States, for example, space is sometimes considered to start at 80 km up. That's what organizations like the US Air Force have used to award astronaut wings. So depending on who you ask, you could be an astronaut 20 km earlier. That's like finishing a marathon at kilometer 32 and still getting a medal. Not complaining, just saying. Here's where it gets even weirder. The International Space Station orbits at around 400 km up. Definitely space, right?

[00:02:36]Yes. But there's still some atmosphere up there. Not much, but enough to cause drag. That's why the ISS has to boost itself regularly to avoid slowly falling back to Earth. So technically, even at hundreds of kilometers up, you're still not completely free of Earth's atmosphere. At this point, you might be thinking, "Okay, cool." But seriously, where's space? Here's the honest answer.

[00:03:03]Space doesn't have a sharp beginning.

[00:03:07]It's more like a transition zone, a gradient, a slow fade from thick breathable air to near total vacuum. So the common line is just a useful boundary, not a physical wall. Okay, now let's talk about the idea that Earth needs a dome to keep the atmosphere in.

[00:03:26]Because on the surface, pun fully intended, it sounds reasonable. If space is a vacuum, shouldn't all the air just get sucked away? It's a fair question.

[00:03:37]It's just based on a slightly dramatic misunderstanding of what the word vacuum really means. Now, let's just imagine the dome idea is true. Just pause, visualize it. The giant invisible, perfectly sealed global wheelid. First of all, impressive engineering. NASA call them. But more importantly, what would that do to the air inside? Because gases in containers behave in very specific ways. You'd expect a clear boundary where the air stops, a noticeable pressure change near that boundary, or at least some kind of uniformish pressure distribution inside.

[00:04:20]Basically, something that screams, "Yep, I'm being contained in a GLASS CASE OF EMOTION."

[00:04:28]>> BUT what do we actually see? a smooth gradient from thick air at the surface to thinner air as you climb to extremely thin gas as you approach space. It's not stepping down like stairs. It's sliding down like a ramp. And the key thing is that's exactly what physics predicts for gas under gravity. Now, let's go back and address the word that causes all the chaos, vacuum. Because people hear that and imagine space as this aggressive vacuum cleaner on a caffeine drip. Look at all that air. Don't mind if I do.

[00:05:07]That's not how it works. The vacuum of space doesn't suck. It doesn't pull. It doesn't have intentions or goals or a LinkedIn profile. A vacuum just means very low pressure. That's it. So instead of thinking air next to nothing, think high pressure, low pressure, extremely low pressure. A smooth transition, no dramatic betrayal scene where the atmosphere suddenly abandons Earth. So why doesn't the air just drift away?

[00:05:41]Because gravity is doing its job.

[00:05:44]Gravity pulls gas molecules toward Earth. At the same time, those molecules are moving, colliding, spreading out. So what you get is a balance. Gravity pulling inward, motion spreading outward. And the result is a pressure gradient, not a sealed container, not a dome, just natural distribution. So now we've got two competing ideas. Model one, which is the dome earth, which requires a physical boundary, predicts noticeable changes near that boundary, doesn't naturally explain the smooth pressure gradient. And then you have model two, which is a spherical earth and gravity, which predicts a gradual decrease in pressure, explains why air gets thinner with altitude, and matches every measurement we've ever taken. The fun part is only one of these matches reality. So, what we know in reality is that air behaves exactly as physics says it should in a gravitational field. No lid required. What we see is a smooth pressure gradient, which is exactly what gravity predicts. So, the atmosphere isn't being kept in, it's being naturally arranged. Also, space doesn't suck. It's just minding its own business. Thank you so much for watching. If you enjoy this video, please like and subscribe. And if you'd like to support the channel further, you can join to become a member. Let me know if you have any questions in the comments. Cool beans.