Where does outer space begin?

where does our atmosphere end and outer space begin?
22 November 2022


Aerial view of Earth from space



Is there a definitive point above the earth where our atmosphere ends and outer space begins?


Rosemary - Yeah, that's a great question. The issue with astronomy and all things in general is there's very rarely a black and white answer. The atmosphere, it doesn't have an edge, so to speak. It kind of fades out. So scientists have categorized the atmosphere into a few different layers. The one that's most valuable to all of us, I guess the one that we're gonna be interacting with our whole lives, is called the troposphere. That's where 75% of all of the atmospheric particles lay. And it goes all the way out to the exosphere, which actually reaches about halfway to the moon. So it's this very large range for the atmosphere. But most people think that space probably starts a little bit before halfway to the moon, you know? So in the 1900s there was a scientist named Theodore von Kármán, and he basically calculated the highest point that an aircraft could function within the atmosphere, right? So planes rely on air to provide lift. And so he calculated about 84 kilometers, around there, was the highest point that a plane could fly, relying on lift. Now, he ultimately decided that a hundred kilometers is just a nicer number. There wasn't a lot of science behind that. But a hundred kilometers also happens to be about where a satellite can no longer stay in stable orbit because then you have to take into account the atmosphere interfering with that orbit. So it might destabilize. So the general consensus is around a hundred kilometers. It's not super important, space is space. The only reason that it was kind of concerning is politics. You know, who's controlling what aspects of the air and all that. So, great question. Thanks Emma.

Chris: So 100 kilometers is currently accepted but it's a bit of an arbitrary number because the atmosphere doesn't stop there, other aspects don't stop there, and the International Space Station that's about 400 kilometers isn't it up? So it's about four times further than that. But that's still in the atmosphere because I was talking to a space scientist and they were saying they have to periodically give it a boost because it is experiencing drag from the wisps and tendrils of the atmosphere that progress and progress even up there.

Rosemary: Yes, that's true. Yeah.

Chris: There you go. Now you know about the Kármán line. Thank you for that, Rosemary.


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