Science, Maths & Technology

Where does outer space begin?

Updated Friday 14th August 2009

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 Copyrighted image Icon Copyright: BBC
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 metres is 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 are 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 are 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 are 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.

The US government refuses to recognise a definition of where space begins, perhaps because it prefers to keep its option 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.

Find out more

John Zarnecki looks back over fifty years of space exploration

Consider The Open University course Planets: an Introduction

 

For further information, take a look at our frequently asked questions which may give you the support you need.

Have a question?

Other content you may like

What's space expanding into? Copyrighted image Icon Copyright: NASA article icon

Science, Maths & Technology 

What's space expanding into?

You can try imagining curved four-dimensional spacetime, but it might be a bit easier to start with some ants.

Article
Watching the weather Copyrighted image Icon Copyright: Used with permission © NASA free course icon Level 1 icon

Nature & Environment 

Watching the weather

This free course, Watching the weather, describes how meteorological observations are made looking upwards from the surface of the Earth, looking downwards from satellites in space and from aircraft and balloons within the atmosphere. This international network of observations is vital for scientists and forecasters and the results impact on everyones daily activities.

Free course
10 hrs
Sky notes: October Copyrighted image Icon Copyright: OU image library article icon

Science, Maths & Technology 

Sky notes: October

A guide to the what's happening in the night sky in October.

Article
Making stars Copyrighted image Icon Copyright: BBC video icon

Science, Maths & Technology 

Making stars

Dr Janet Sumner visits a project looking for a source of cheap, clean fuel, which gives us a glimpse of the birth of our sun

Video
10 mins
Sky notes: April Copyrighted image Icon Copyright: OU image library article icon

Science, Maths & Technology 

Sky notes: April

A guide to the what's happening in the night sky in April.

Article
Mars in 3D - NASA World Wind Copyrighted image Icon Copyright: NASA/JPL-Caltech article icon

Science, Maths & Technology 

Mars in 3D - NASA World Wind

Tony Hirst recommends some online applications for viewing the surface of Mars, Venus and Jupiter.

Article
Measuring the Solar System Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Measuring the Solar System

Dr Alan Cooper discusses the methods used to measure the Solar System.

Article
Genesis: flying to the sun Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Genesis: flying to the sun

The Open Minds programme looked into the Genesis programme prior to its launch in 2001.

Article
What does the AU mean? Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

What does the AU mean?

Dr Alan Cooper discusses the significance of the astronomical unit as a measurement

Article