# Where does space begin?

10 January 2017

## Question

Aaron asks: How far above ground does the "sky" begin, and does it end at some point in space?

Chris Smith put this to Imperial College's Stuart Higgins...

Stuart - So first bits first: where does the sky begin? Well, it kind of begins where the ground stops being - it’s where the atmosphere is. If you want to go into a legal side of it, you could argue that planes are not not allowed to fly below 150/300 metres. So a lot of discussions about drones and making sure you don’t fly them. But the sky begins at ground level.

Where it ends? Well, that kind of depends on what you think. So actually it’s fuzzy; there is no clear boundary where we say that's the edge of the Earth’s atmosphere. It extends up huge, huge distances. There’s something called the exosphere, which is this huge ball of low density gas that extends somewhere about 10 thousand kilometres up into the sky. Bearing in mind the width of the Earth is 12 thousand kilometres so to give you a size... that’s huge. But a lot of people like outer space to be defined as around 100 kilometres, or 62 miles.

Chris:: So just to clarify then - you’re saying that there are vestiges of the Earth’s atmosphere out there at least 10 thousand kilometres above the Earth?

Stuart - I know, incredible distance. The bits that you consider still have some kind of influence and link to the Earth. Beyond that, of course, there is some limit at which there will be no more particles and you can argue there is no influence of the Earth.

Chris - I mean we know this happens because satellites slow down in orbit over time, don’t they, and that’s presumably because there is drag against these vestiges of the Earth’s atmosphere out there?

Stuart - Exactly. And that means they have to be high enough to do that. My favourite thing is: if you want to become an astronaut you need to pass the Karman line, and that is the point where you go above 62 kilometres, about 100 miles, and then you can get your astronaut wings.