Where Does Space Begin?
― Just 100 km Straight Up. On a Globe, the Air Is Only 2.5 mm Thick
Space feels very far away. Yet the height usually taken as its edge is only 100 km straight up. That is about an hour's drive on a motorway. The air around us is a much thinner film than we tend to think.
Look up at the night sky and the stars seem incredibly distant. Space feels like somewhere far beyond our reach.
But suppose a car could drive straight up. Travel at 100 km/h and after one hour you would already be in "space". That is about the distance from Tokyo to Hakone.
If it is that close, why does the sky look endlessly deep? And why is the boundary set at 100 km?
Two reasons the boundary is at 100 km
Air decreases by a fixed proportion as you climb. Near the ground, it halves every 5.5 km or so. By 100 km, less than one-millionth of the ground-level amount remains.
A plane stays up by pushing air down with its wings. When the air is too thin, the speed needed to stay up keeps climbing. Around 100 km, that speed catches up with the speed needed to keep circling the Earth.
So 100 km is not a visible line drawn across the sky. It is a rule of thumb: "above here, things move the way they do in space, not the way they do in air." Let's take it step by step.
Air halves, and halves again
Air has weight. The weight of the air above presses down on the air below. That is why air is packed tight near the ground and gets sparser as you go up.
The thinning follows a set pattern. Every time you climb the same height, the air drops by the same proportion. Near the ground, climbing about 5.5 km halves it. Climb another 5.5 km and it halves again.
Try the slider in Figure 1 to change the height. At the summit of Mt. Fuji (about 3.8 km), the air is about 60% as dense as at the ground. At the height of airliners (about 10 km), it is down to about a quarter. Above 50 km, only 0.1% of all the air remains.
Above 100 km, you stay up by circling, not by wings
A plane pushes air down with its wings, and the reaction lifts it. The lift is proportional to both the density of the air and the square of the speed. If the air gets four times thinner, you must go twice as fast to get the same lift.
What if the air were a million times thinner? You would need 1,000 times the speed. An ordinary plane cannot come close.
There is another way to stay up. Travel sideways at about 7.9 km per second (roughly 28,000 km/h), and the ground curves away beneath you as fast as you fall. You keep falling, yet never hit the Earth. Satellites and the space station stay up this way.
At around 80 to 100 km, the speed needed to stay up on wings is thought to overtake this "circling speed". Above that, wings are no longer any use. The 100 km line is called the "Kármán line", after the scientist who first put forward this idea.
How thick is the air on a globe?
100 km sounds quite high. But the Earth is about 12,700 km across. Next to that, 100 km is only 1/127 of the diameter.
Look at Figure 2. On a globe 30 cm across, the edge of space is just 2.4 mm above the surface. The layer with clouds and rain is 0.26 mm, about the thickness of three sheets of copy paper. Even the space station orbits only about 1 cm above the surface.
Many astronauts who have seen Earth from space say the air looked like a surprisingly thin blue line. The air we breathe is a paper-thin film lightly wrapped around the planet.
The air does not drop to zero at the boundary. The space station, orbiting at about 400 km, still runs into a tiny amount of air and slowly loses height. So it fires its engines from time to time to climb back up. In the United States, 80 km (50 miles) is sometimes used as the mark for being an astronaut. The boundary was not drawn by nature. It is a convention chosen by people.
Summary
The edge of space is just 100 km straight up. Air halves every 5.5 km or so, and by 100 km it is less than one-millionth of the ground-level amount. Beyond that, wings cannot hold you up, and the only way to stay aloft is to keep circling the Earth.
Space is not far away. It is "just above us".
On a globe, we live at the bottom of an air film 2.5 mm thick.
Why it gets hard to breathe on high mountains is explained in the altitude sickness article, and what shooting stars glowing at around 100 km really are is explained in the shooting star article. For the weight of air itself, see also How Many Kilograms Is the Air in a Room?
- If your smartphone has a barometer, open an app that shows air pressure. Read the number on the ground floor, then go up by lift or stairs and read it again. For every 10 m you climb, it should drop by about 1.2 hectopascals.
- Take a globe or a ball and measure its diameter. Divide the diameter by 127. That length is the "edge of space" for that ball. Hold a ruler against the surface to see how thin it is.
- Stack three sheets of copy paper and pinch the thickness between your fingers. On a 30 cm globe, the layer where clouds and rain form is about this thick.
Air pressure also changes with the weather. When you compare, measure the upper and lower spots one right after the other, within a short time.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school Earth Science Basics and Physics Basics
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity courses not taught in high school (atmospheric physics, space engineering)
- ResearchTopics researchers are still studying, not yet taught as settled fact even at university
Middle schoolTerms: this phenomenon has names
- Atmospheric pressure (air pressure): the force with which the weight of the air above pushes on a surface. At the ground it equals the weight of about 1 kilogram per square centimetre.
- Atmosphere: the layer of air wrapped around the Earth. From the bottom it is divided into the troposphere, stratosphere, mesosphere and thermosphere.
- Kármán line: the boundary between the atmosphere and outer space, drawn at a height of 100 km. It is a guideline set by the International Aeronautical Federation.
Middle schoolHigh schoolChecking with a formula: how much air remains at 100 km, and how many millimetres on a globe
Air pressure is "the weight of the air above that height", so the pressure ratio is exactly the share of air remaining above. We calculate this ratio, and the thickness when shrunk to a globe.
| In symbols | p ＝ p₀ × e^( −h ÷ H ) Thickness on the globe d ＝ h × D ÷ L |
| In words | Pressure at height h ＝ ground pressure × (the factor of about 0.37 per height H climbed, repeated). Thickness on the globe ＝ true height × globe diameter ÷ Earth's diameter |
| Where the formula comes from | For a thin slice of air, gravity balances the pressure difference between its top and bottom (hydrostatic balance). Combine this with the fact that gas density is proportional to pressure (the ideal gas law) and you get a pattern that falls by the same proportion for each fixed height. The second half is just a scale (proportion) formula. |
| Symbol | Meaning and unit |
| p | Pressure at height h (thought of as a multiple of ground pressure) |
| p₀ | Ground pressure (about 1013 hectopascals) |
| H | Scale height. The height at which pressure falls to about 0.37 times (unit: km) |
| D・L | Diameter of the globe (mm) and diameter of the Earth (km) |
| Scale height H (rough average for 0–100 km) | About 7 km (about 8 km near the ground, about 6 km in the cold upper air) |
| Height of the edge of space h | 100 km |
| Earth's diameter L | About 12,700 km |
| Globe diameter D | 300 mm (30 cm) |
| Thickness of one sheet of copy paper | About 0.09 mm |
| The exponent (h ÷ H) | 100 ÷ 7 ≒ 14.3 |
| The fraction left after shrinking by that much, with base e | About 0.0000006 (about 1 in 1.6 million) |
| Thickness on the globe: first h × D | 100 × 300 ＝ 30000 |
| Divide by the Earth's diameter | 30000 ÷ 12700 ≒ 2.36 |
| Cloud/rain layer (about 11 km): 11 × D | 11 × 300 ＝ 3300 |
| Divide by the Earth's diameter | 3300 ÷ 12700 ≒ 0.26 |
| How many sheets of paper | 0.26 ÷ 0.09 ≒ 2.9 |
The air remaining above 100 km is about 1 in 1.6 million of the total (with the standard values based on real observations, it is even less, about 1 in 3 million). Shrunk to a globe, the edge of space is about 2.4 mm above the surface, and the cloud and rain layer is about three sheets of paper thick.
High schoolHigh school+The structure of the air layers, and why the decrease is "exponential"
High schoolIn Earth Science Basics, the atmosphere is split into four layers by how temperature changes. From the bottom up to about 11 km is the troposphere, where nearly all weather such as clouds and rain happens. Above it, the stratosphere holds the ozone layer. The mesosphere starts at about 50 km, and the thermosphere at about 80–90 km. Shooting stars and auroras glow from the upper mesosphere into the thermosphere.
High school+A decrease by "the same proportion for each fixed height" arises from the rule that the more there is, the more is lost. Where there is more air above, a climb loses more weight. This gives a formula with the same form as the half-life of radioactive material.
UniversityThe barometric formula, the International Standard Atmosphere and the Kármán line
Combining the hydrostatic balance equation with the ideal gas law gives the "barometric formula" for how pressure varies with height. Here the scale height is set by the gas constant, temperature, mean molecular mass and gravitational acceleration. Because temperature changes with height, in practice a model called the "International Standard Atmosphere" is used, built up layer by layer. The 100 km boundary rests on Kármán's estimate of the height at which the speed needed for aerodynamic lift exceeds the first cosmic velocity (circular orbit speed).
📖 Derivations and further reading: Scale height (Wikipedia, in Japanese) ／ International Standard Atmosphere (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- Where does the atmosphere truly end? Around the Earth lies a faintly glowing layer of hydrogen. A study that re-examined old data from a solar observation satellite reported that this layer reaches beyond the Moon's orbit.
- Predicting the density of the upper air. The density of air at several hundred kilometres depends strongly on solar activity and magnetic storms. In one case, many newly launched small satellites fell because of air drag right after a magnetic storm, so better forecasts are needed.
- A blind spot in observation. The region from about 50 to 150 km is out of reach of balloons and too low for satellites to orbit. Ways to measure it directly are limited, and it is considered a layer where many phenomena are still poorly understood.
In other words, even this article is "an explanation within what is known today". The boundary of space itself may be revised as science advances or depending on the purpose it is used for.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, Year 2: "Weather and its changes" (atmospheric pressure) | The explanation that the weight of air presses on the air below |
| High school | Earth Science Basics: "Structure of the atmosphere"; Physics Basics: "Pressure" | Troposphere, stratosphere, mesosphere, thermosphere; the bands in Figure 1 |
| High school+ | Maths: "Exponential functions" | Decrease by the same proportion for each fixed height |
| University | Atmospheric physics, space engineering | Barometric formula, International Standard Atmosphere, comparison with the first cosmic velocity |
| Research | Upper-atmosphere physics, space weather | The edge of the atmosphere, predicting upper-air density, observation blind spots |
| ― | Links to daily life | Why high mountains and flights cause breathlessness or ear discomfort; the barometer in a smartphone |
- Wikipedia, "Kármán line" (カーマン・ライン, in Japanese)
- Wikipedia, "International Standard Atmosphere" (国際標準大気, in Japanese)
- NOAA, NASA and the US Air Force, "U.S. Standard Atmosphere, 1976"
- Fédération Aéronautique Internationale (FAI), statement on the boundary of space (2018)
- JAXA Human Spaceflight Technology Directorate (有人宇宙技術部門), "International Space Station"
*This article is a general-audience science explainer. The figures given are rough guides to help you understand how things work. The amount of air at great height changes with temperature and solar activity, and the height of the boundary can differ by organisation and purpose.