How far away is space?

NASA's Blue Marble image, taken by Apollo 17 astronauts,
and found at http://visibleearth.nasa.gov

That kind of depends on what you class as 'space'. Space is, really, everything in the universe. Even when you're on the Earth, you're still in space, because Earth is in space.

But that's an unhelpful and, even more importantly, uninteresting answer.

So we'll take it to mean something like 'how far would you have to travel to leave the Earth's atmosphere'. That makes sense, as Earth's atmosphere is a blanket of gasses that hug the Earth's surface and provides us with all manner of benefits, not least the ability to breathe: leaving it is certainly more of an event than popping out to the post office.

So where does the Earth's atmosphere end? This question isn't as easy to answer as it might at first seem- the Earth's atmosphere, like that of all planets, doesn't just stop once you get past a certain point. If it did, you'd only have to go about 8.5km directly upwards to leave it. In reality, if you were to rise directly upwards from where you are, and measure certain aspects of our atmosphere as you went, you'd find that it starts out fairly thick and gets gradually thinner the higher you get. Once you got up to about 5-6km you'd find that about half of all the stuff* that makes up the atmosphere was beneath you, but because it gets thinner and thinner as you rise, you've got a lot further than that again before you'll finally clear the atmosphere. After 10km you'd be above the altitude of most commercial aeroplanes. At 16km you'd be above around 90% of the Earth's atmosphere.

After this, the atmosphere starts to get really tenuous, and it would be impossible to say how far out you'd have to go to be absolutely sure of not seeing another molecule of Earth's atmosphere. However, at about 100km up, you're above roughly 99.99997% of our atmosphere, and it's this altitude that's usually quoted as 'the edge of space'.

So space is about 100km away. Quite a long way? If you could drive it, it'd take you about an hour and a half, and would be roughly the same distance as driving from London to Cambridge. Further food for thought: the UK is a bit under 1,500km from top to bottom, the Earth is over 12,500km in diameter, and the moon is more than 38,000km away. Our atmosphere is just a thin layer smeared over a planet which isn't even itself particularly large.

Space is big.




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* Mostly gases like oxygen, carbon dioxide and nitrogen as well as a bit of argon and tiny traces of other gases.