Where does space begin? Let’s look up into our planet’s atmosphere, that shell of nitrogen (about 78%), oxygen (about 20%), various other gases (2%) that makes life on Earth possible, to find out. The atmosphere gets thinner as you go further up, in fact 90% of the Earth’s atmosphere by weight is in the bottom 10 miles (16 km).
The atmosphere is stratified, that is divided into layers based the bulk properties and behaviours of the air at that altitude. Most familiar of these to us, because we live in it, is the troposphere. Most massive (about 80% of the atmosphere), warmest and most turbulent of the atmosphere’s layers, the troposphere starts at ground level and extends about a dozen kilometres on average above our head. The troposphere is warmed by heat from the Earth’s surface, and as any fule kno warm air rises. This is exactly what happens in the troposphere, great columns of warmer air rise from the surface to higher altitudes cool off and sink down again. Effectively the troposphere is churning like the fluids in a lava lamp, hence its names, derived from the Greek word trope, meaning “turn” or “overturn”. Its turbulent nature means that the upper extent of the troposphere (called the tropopause) varies, over the chilly poles it reaches 9 km (30 000 ft) and a height of 17 km (56 000 ft) at the warmer equator. It is cold up there at the tropopause perhaps −60 °C (−76 °F).
Just to put things into perspective, Mt. Everest’s summit is 8848 m (29 029 ft) above sea level and commercial airliners commonly cruise about 10 km (33 000 ft) overhead.
Rising through the tropopause we come to the second best -known layer of the atmosphere, the stratosphere. The stratosphere encompasses a much greater volume than the troposphere as it extends much further above the Earth, to an altitude of almost 60 km. In the central portion of the stratosphere we find a sub-layer which is without a doubt the best-known layer of the atmosphere: the ozone layer.
Ozone is a molecule made of three oxygen atoms (the oxygen molecules in stuff we breathe is made of two oxygen atoms; an extra atom makes a huge difference as ozone is horrendously toxic). Ozone in the stratosphere is made by sunlight. Ultraviolet light (UV) from the Sun rips apart oxygen molecules (made of two oxygen atoms) into individual oxygen atoms. Eager to have company again, these lonely atoms combine with unbroken oxygen molecules to create molecules of three oxygen atoms. These three oxygen atom molecules are ozone molecules and they are continually forming in the stratosphere (heat is released too during all this, so surprisingly the stratosphere is considerably warmer than the upper troposphere). If you think about this for a minute, this suggests that if this goes on all the oxygen molecules in our atmosphere are eventually going to be turned into ozone… The reason this does not happen is because as ultraviolet light shines on ozone it breaks it up again into a molecule of oxygen and an isolated atom of oxygen. This on-going process creates a layer about 10 to 50 km (33 000 to 160 000 ft) above Earth’s surface where there is a steady concentration of ozone (for a while we humans did our best to put a stop to this, but we have since mended our ways and the ozone layer seems to be recovering). This is all so important because all that UV absorbed in tearing up ozone atoms in the stratosphere does not get to reach the Earth’s surface. If it did, the green hills of Earth would be as barren as the rocky dunes of Mars.
At the top of the stratosphere, the stratopause, we are in an alien environment: the atmospheric pressure here is only 1/1000 of that at sea level. Even the wispy near-nothingness that is the Martian atmosphere is thicker than this.
Above the stratopause we have the mesosphere which extends upwards to 80–85 km (50–53 mile) above the surface. This is the most mysterious and poorly understood region on Earth, too high for aircraft and too low for satellites, it is hard to investigate. We do know the air (what there is of it) in the mesosphere gets steadily colder as we rise through it and there are strong east to west winds. This is also the home of red sprites and blue jets, vast and spectacular electrical discharges considered to be meteorologists’ tall tales until a sprite was photographed in 1989.
Despite its inaccessibility, you can see into the mesosphere any clear night when you watch the bright streak of a meteor. Most meteoroids burning up upon entering the atmosphere meet their ends in the mesosphere. It is also here that clouds of fine ice crystals form the noctilucent clouds observed on some summer nights.
Rising through the mesosphere, the temperature grows steadily warmer, so much so that the layer above the mesopause is called the thermosphere. It is well named for it is hot up there, in fact the temperature could be as high as 1500 °C (2730 °F). This seems fantastic, how can this be? How come rockets passing through it are not vaporised? Firstly, the rare atoms and molecules of air here are warmed by solar radiation. An energised atom is a fast moving atom; a fast moving atom is a hot atom. The second question is answered by the fact that there is virtually nothing there. The thermosphere is a fine approximation to a vacuum. The gases here are too dilute to transfer any heat to a passing traveller.
Remote and all but empty, the thermosphere may seem of little interest but a century ago it became of great practical use. Pioneers of radio communications were amazed to discover that their signals (which ought to travel in straight lines) could be received far away, right around the curvature of the Earth. It was as though hundreds of kilometres overhead there was a huge invisible mirror reflecting radio waves around the planet. Indeed there was. This is the ionosphere, a shell of free electrons and ionised gas molecules. This is another product of sunlight, up here the Sun’s rays are powerful enough to tear electrons away from atoms. As the ionosphere is created by the Sun shining on the atmosphere, the extent and density of the ionosphere over a particular location varies with the seasons. For a brief few decades the ionosphere played a vital role in intercontinental radio communications, but today is little used thanks to the ubiquity of communication satellites.
The thermosphere encountered its first visitor from the Earth’s surface some time in 1944 in the sleek shape of an A4 (aka V-2) rocket launched from Nazi Germany. Within a decade this first invader had been followed by literally thousands more rockets, and soon rockets (some with people on board) would be going higher still. In the mid-1950s it was decided that we needed a boundary between Earth and space and this would be in the thermosphere. The Fédération Aéronautique Internationale declared that this would be at an altitude of 100 kilometres (62 mles) above sea level. This is often called the Kármán Line (named for a Hungarian scientist who calculated that at this altitude a vehicle could not rely on aerodynamic lift to stay aloft).
The thermosphere ends as you might expect at the thermopause. Where exactly this is varies, being higher in direct sunlight as the Sun’s energy allowing the scarce but fast-moving molecules to rise high above the planet. Depending on the time of day and season the thermosphere can be 500–1000 km over any given location. Most satellites and spacecraft orbit the Earth at these altitudes, and the faint traces of atmosphere do exert a slight drag to the satellite’s motion hence the phenomena of a satellite’s orbit decaying until it falls to Earth (unless it is periodically reboosted to compensate as the ISS is).
Uppermost of the Earth’s atmospheric layers is the exosphere where the atmosphere gradually turns into outer space. Here there are only the rarest molecules of hydrogen, helium, carbon dioxide and oxygen atoms. They are as much under the influence of the cosmic environment as they are Earth’s and in fact some will escape into space, never to return. Some estimate the exosphere ends about halfway to the Moon.
Normally we are more concerned with what is beyond the Earth’s atmosphere, I hope you have found it as fascinating as I have to look into, rather than through, our atmosphere!