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Where space begins

Rockets and landing · level 4, lesson 1 of 6

A question to start

If you could drive a car straight up at motorway speed, you would reach space in about an hour. But where exactly is the edge? Is there a line where the air stops?

The idea

There is no sharp edge. The air simply gets thinner and thinner as you go up. About half of all the air is below 5.5 km; about 99% is below 30 km. Above that there is still a little, thinner and thinner, out to hundreds of kilometres.

Height above the ground How thick the air is (ground = 1) What is there
0 km 1 you
8.8 km about 0.4 the top of Mount Everest
11 km about 0.3 jet planes
30 km about 0.015 weather balloons, at their highest
100 km about 0.0000005 the Kármán line
400 km almost nothing the space station

People agreed on a line to call the start of space: the Kármán line, at 100 km up. It is named after the engineer Theodore von Kármán, who worked out that about this high, a plane would need to fly so fast to be held up by the air that it might as well be in orbit. The US Air Force and NASA give astronaut wings for going above 80 km instead.

Even at 400 km there is a trace of air. It drags on the space station and slowly lowers its orbit, so it has to fire its engines every few months to climb back up.

Mars's air starts much thinner, less than 1% of the pressure on Earth at the ground. But it thins more slowly with height, because Mars's gravity is weaker. Landers count their entry from about 125 km up, where the air first starts to matter.

Mars Earth
Air pressure at the ground about 6 hPa (0.6% of Earth's) 1,013 hPa
Height over which the pressure falls to about a third about 11 km about 8.5 km
Where entry begins for landers and capsules about 125 km about 120 km

Worked examples

1. Driving to space. At 100 km/h straight up, you would reach 100 km in 1 hour.

2. The air at 30 km. Air at 30 km on Earth is about as thick as the air at the ground on Mars, a useful fact for testing Mars parachutes high above the Earth.

Try it

  1. Is there a sharp edge to the air?
  2. Where is the Kármán line?
  3. Why does the space station need to fire its engines every few months?
  4. Why does Mars's air thin out more slowly with height than Earth's?
Answers
  1. No, it gets thinner and thinner.
  2. 100 km up.
  3. A little air still drags on it and lowers its orbit.
  4. Mars's weaker gravity does not squash its air as close to the ground.

Common mistake

"Space is cold and has zero gravity because it is high up." Height is not what makes gravity weaker in orbit: at 400 km gravity is still about 90% as strong. Astronauts float because they are in orbit, falling round the Earth.

Where it leads