Where does space begin?
The question, “Where does space begin?” depends on your point of view. You could really ask, “Where does the atmosphere end?”
As you think of the atmosphere where we live and breathe here on Earth, it doesn’t just stop right above our heads. It doesn’t stop at Mount Everest. It doesn’t stop where the planes are flying. It goes on and on all the way up, and just gets less and less dense the higher you go. And it’s still there at a very high altitude.
When you go to where the Space Station is — only a couple hundred miles above the Earth — there’s still enough air there to slow the Space Station down. And if you didn’t re-boost it with rockets,
it would come back to Earth based on the air drag, just like when you’re driving your car.
So that region up there, even though it’s not like what we’re used to here — you couldn’t breathe there — there’s still enough atmosphere there to make it interesting. Not only is there air like we have on Earth, there’s ionized gas, there’s radiation. And all those things together make that environment really dynamic, and it has something we call space weather.
And in a sense, we live in the atmosphere of the Sun. So there’s this kind of dichotomy there where you go from the Earth, the Earth’s atmosphere, then you’re in the Sun’s atmosphere. And then at some point you’re outside that when you reach the heliopause and the heliosphere boundary.
So if the question is, “Where does space begin?” It depends on your point of view. If you want to know, “Where does the atmosphere end?” It’s about 400 miles over your head. But just remember that that space above that is not empty. It’s full of all kinds of interesting stuff. And that’s what we study at NASA.
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