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Where does outer space begin?

Updated Wednesday, 21st July 2021

We don't really know where space begins, as Dave Rothery explains

WARNING: This blog post contains spoilers for the fourth programme in the Bang Goes The Theory series. Don't read it if you haven't seen the space challenge yet and don't want to know what happens.

While we were planning and filming an ambitious item for *Bang Goes the Theory* in which an 'action man' type-figure dubbed mini-Dallas is sent up to the "edge of space" by a balloon, there was a lot of discussion among the Bang gang about whether or not we could claim to be reaching 'space', and also whether Joseph Kittinger had really "parachuted from space" after his balloon ascent to 102,800 feet (31,333 metres) in 1960.

We got our mini-Dallas to pretty much the same height, but I’m afraid that the answer has to be ‘no’ in both cases, even though the sky looks gratifyingly black in our remarkable camera shots.

Mini Dallas from Bang Goes The Theory.

It’s pretty obvious if you think about it. Kittinger and mini-Dallas were both carried up by a balloon, and a balloon only goes up if it (plus its 'astronaut' payload) is on average less dense than the air that it displaces. That’s how buoyancy works.

There must still be air – albeit very tenuous – at the height reached by the balloon, otherwise, it could not float.

There is not a vacuum at the height reached by these extreme balloons, but the pressure is very low. In fact, it is about one-hundredth of the pressure at sea level. This means that 99% of the atmosphere’s mass is below, and only 1% of the mass of the atmosphere is above.

However, that does not mean that mini-Dallas was 99% of the way to the top of the atmosphere because the atmosphere becomes more and more tenuous with height. If you look at this diagram that shows how atmospheric temperature varies with height, you will see that 30,000 meters are only about halfway to the top of the stratosphere and that there are layers called the mesosphere and the thermosphere above that!

Temperature variation with height in the Earth’s atmosphere. The warming with height in the stratosphere and thermosphere is because the air molecules are warmed by absorption of ultraviolet and other radiation from the Sun

Temperature variation with height in the Earth’s atmosphere. The warming with height in the stratosphere and thermosphere is because the air molecules are warmed by absorption of ultraviolet and other radiation from the Sun.

There is actually no definite boundary that marks the top of the atmosphere, but eventually, it becomes so completely tenuous that for practical purposes it can be regarded as ‘space’. But where is this limit?

Well, I did some web searching, and I came up with this. Satellites can orbit 200 km above the Earth, free of any appreciable atmospheric drag. Clearly, at 200 km, you are in ‘space’ (the International Space Station orbits at 320-347 km). Lower orbits down to about 160 km are possible, but there is too much drag for these to be stable.

But in the USA, there is no single accepted definition of where space begins, perhaps because the US Government prefers to keep its options open.

However, the Fédération Aéronautique Internationale recognises 100 km as the lower limit of space, whereas an encyclopedia of international law suggests 80 km as a practical limit between ‘air space’, potentially reachable by an aircraft, and ‘outer space’.

However you look at it, sadly 30,000 metres or 30 km is less than half way there, but it was a bold effort nonetheless.