Before rounding off this article, we might consider two other definitions.

Where the Atmosphere Ends

                   One is natural enough and seems to be the most satisfactory from the straightforward physical point of view.

                    It concerns where the Earth ends; and since the peripherals of the Earth, its outermost layer is formed by its gaseous part, therefore it is reasonable to assume that the Earth ends and Space begins where the atmosphere ends.

However, the thing is, the atmosphere has no sharp, prominent boundary. Its density decreases from the Sea Level upwards, and it gradually and smoothly merges with the vacuum of Space.

Nor is Outer Space completely empty. In the space amongst the planets and the Sun, there are – on an average –  10 million particles in every cubic metre. (This is a relatively small number.. the number of particles (in this case molecules) in the space just above the Earth’s opaque surface, is 10 trillion trillion (10²⁵) for every cubic metre).

So the distance from the Earth at which the number of particles per unit volume of space decreases to a value equal to that, say, mid-way between the Sun and the Earth, or the Earth’s orbit and Mars’ – that is, in the general interplanetary space of the solar system – can be said to be the distance from the Earth at which Space begins.

                      Also, the composition of the Earth’s atmosphere is different from the system of particles that fills the solar system’s deep space. The former consists almost entirely of nitrogen and oxygen molecules, and the latter, of fast-moving protons, a few helium nuclei, electrons etc. 

                       It’s not the same kind of particles.

In the Deep Space of the solar system, almost all of the particles are in flux i.e. they are moving at high speeds (generally away from the Sun, unless things like the magnetic field of a planet swings it into another path. -Because they emanate from the Sun).

Less than one-tenth of the particles in such motion, are the particles which do not come from the sun, and move differently in relation to the Solar System. -These are hydrogen atoms and as per my current understanding, most, if not all of them come from interstellar space. (Although about this last point, the author is not sure. If needed, this part will be rectified later).

[ The reader may wonder whether in this inter-planetary space, there are any hydrogen or helium atoms that are the direct products – or leftovers – of the Big Bang itself. -Or dust-like, very minute particles that have been thrown out from stars in supernovae. This author does not have proper knowledge of this, but it can be guessed that atoms of hydrogen or helium which were always in the space of the Solar System and which originated directly from the Big Bang, must have been long back pulled in by the Sun, and incorporated in itself, even when it was a gas cloud and had not yet transformed into a star. Similar action must have been exerted by the gas giants like Jupiter and Saturn. Once it became a star, the radiation pressure of the sun, must have blown away any stray particles over the aeons.

Again, this is only a guess of this author ].

*

                   The atmosphere becomes rare above an altitude of about 17 kilometres, it steadily diminishes in quantity higher up from there.

                   98% of the mass of the atmosphere is present in the 30 kilometres from the ground.

                    And above five hundred to a thousand kilometres (depending upon the phase of the sunspot cycle that is ongoing)- on an average about 690 kilometres, it becomes extremely thin. Above the last level, the part of the atmosphere is called the ‘exosphere’.

This layer of the atmosphere is so tenuous that unlike in a typical sample of gas, the atoms in this region do not undergo collisions with one another. They are separated by much greater  distances, and an atom may travel for hundreds of kilometres before bouncing against another.

Though they still move under the effect of gravity, that is the charged particles of the solar wind or the energy imparted by the Sun’s radiation cannot yet drive them away from the ‘gravitational sphere’ of the Earth. They move in ‘ballistic trajectories’ i.e. parabolic paths towards the centre of the Earth.

                     The exosphere – though it contains atoms of gases – has no air as we know it. It is comprised of mostly hydrogen, some helium, and some atomic or molecular oxygen and carbon-di-oxide at its very base. It is still a part of the planet of course.

                    The American space agency NASA and most other experts it seems, consider the exosphere to extend up to a distance of 10,000 kilometres. This is where the atmosphere physically ends.

                    Therefore beyond this distance, is Space.

                    However, some atoms of gases do exist centred upon the Earth, even beyond this distance, -at higher densities than the average density of atoms in the nearby interplanetary space.

                    One sign of this is the geocorona.

*

                     Only at a distance of 1,80,000 kilometres and beyond, are gas atoms no longer bound to the Earth by gravity. (But this is not in the sense that the gravitational force between the Earth and the gas molecules here, is too weak in itself- because I guess normally these atoms still rotate along with the Earth – but because certain external forces are, at times, stronger). These gas atoms can be and are torn away and flung out by the radiation pressure of the Sun. Perhaps more during the periods when the Sun’s radiations increase in intensity, every 11 years or so. (These peaks are called ‘solar maxima’). And especially during periods of solar activity, for example high speed solar winds, solar flares etc.

So, the Earth in the normal course, keeps on losing some molecules of its outermost fringes of atmosphere. The European Space Agency said in 2016, that every day, around 90 tonnes of material escapes from our planet’s upper atmosphere and streams out into space.

The Earth’s gravitational attraction here is too weak to hold them together anymore. (However we may bring to mind that at much greater distances than this, it is still strong enough to make the Moon go around the Earth! One may think about the reason behind that. However, one may orbit the Earth under its gravitational influence and still be very much in ‘Space’). 

                     So, to sum up, everything considered, it should be fair to say that the Earth’s atmosphere ends, and therefore physically, the Earth ends at about 10,000 kilometres from its solid surface. And just outside of it, is space.

*

                      Conceptually, the environs beyond the 100 kilometres designated by NASA and the FAI do have some qualities which make it seem more like Earth than Deep Space. For example the drag experienced by satellites even at altitudes of thousands of kilometres.

Things like difficulty in breathing, the ‘sky’ being black, an extremely low atmospheric pressure, the absence of sound are all signs of a much reduced atmosphere.

[ There is no sound above a distance of 160 kilometres from the Earth’s surface, because even though there is an appreciable amount of atmospheric gases present here, the molecules are not close enough to one another, to be able to bump against one another along a certain direction, and thereby transmit sound ].

~

Aeronautics and Astronautics

                     There is another definition of Space based on another manifestation of the density of the atmosphere at a certain height above the Earth’s surface.

                     The minimum (vertical) distance from the surface of the Earth, at which an aeroplane cannot fly, is where Space begins.

This is according to the Fédération Aéronautique Internationale (FAI), a private body headquartered in Lausanne, Switzerland, which is the world governing body in air sports. It encourages and oversees aviation events. It also certifies and maintains records of various kinds.

Naturally, they would have an interest in the maximum height at which an aeroplane can be flown.

*

                    To remain above the ground despite the Earth-ward pull of gravity and also to move actively (i.e. by its own energy) in any direction, an aeroplane needs the presence of a sufficient amount of air beneath its body.    

It is a vehicle that depends on the medium (the surrounding gas) to float above the ground, and also to propel itself (by sucking in some of the medium at the front and thrusting it out at the back).

This is why an aeroplane is also called an ‘air-breathing’ vehicle.

(Actually, to remain afloat, a plane must both have air beneath it and also be in a state of motion).

                    The less air there is beneath the body of an aeroplane, the faster it would need to fly in order to remain at the same level.

As we know, the density of the atmosphere is lesser at higher altitudes.

So a plane would need to fly faster at higher altitudes.

And eventually, there comes a certain altitude at which, in order to maintain its height, the plane would need to fly so fast that it would enter into orbit around the Earth.

[ That is, the minimum speed required to maintain level flight at this altitude is equal to the orbital speed for this altitude ].

To fly in a level plane at altitudes higher than this threshold, its speed would need to be greater than ‘the orbital speed for that altitude’, and the plane would simply fly off the Earth’s environs, its sphere of gravitational influence, and enter outer space (if its engines kept running all the while and there were an unlimited supply of fuel).

[ A helicopter on the other hand, only needs to have air beneath its rotors to float above the ground. One could say that the continuous motion in its case (which generates the lift), is that of a part of the vehicle around a closed path and not the entire vehicle moving in space ].

                   So a propeller-driven or jet engine plane is not able to fly at such heights. 

                    If it tried to, it would simply drop to a lower height, at which the air would be dense enough to support the weight of that particular plane flying at that particular speed.

(It has to be taken up there by another craft of course, because again, it would not be able to climb to such an altitude from a lower level, in the first place).

                     Only vehicles which travel by rocket propulsion, would be able to move at those distances from the Earth’s surface.

(Including rocket-planes which are shaped like and look like aeroplanes, but are propelled by rocket engines rather than jet engines. Examples are the Heinkel He 176, the Bell X-1, and the famous North American X-15). 

This kind of a means of propulsion does not depend upon the surrounding medium for staying above the ground against gravity or for moving in the desired direction.

*

                    This height from the ground is about a 100 kilometres.

(Actually, the height of the ground may vary from place to place, so in the definition- the figure is in relation to the Mean Sea Level).

                     It was first attempted to be calculated by Theodore von Karman, a mathematician, aeronautical engineer and physicist, who was born and studied in Hungary, but later settled in the United States.

He headed the aeronautical research laboratory at the California Institute of Technology and later, in 1944, helped found the Jet Propulsion Laboratory there.

                      In 1957, he tried to determine a theoretical limit of altitude for airplane flight.

                      In 1959, Andrew G. Haley, an American lawyer who devoted much of his legal career to communications law, and is said to have been the world’s first practitioner of ‘space law’, coined the term ‘Karman Line’ for the above limit.

                      What does the reader think? Is the Karman Line a single, constant value or can it change due to some reasons?

                      For example, can the location of the Karman Line be raised higher by humans?

                       Where the Karman Line lies, depends upon the density of the atmosphere at different heights above the ground, but it also depends upon the aircraft itself.

Can any design or engineering improvements in the aeroplane help it to fly at a higher maximum altitude?

It seems that increasing the horizontal area of its surface, reducing its total weight, and making its engine more powerful- that is it would produce more thrust, would all help in increasing its maximum altitude of flight. (Also called the plane’s Absolute Ceiling).

Of course, different kinds of airplanes may have different Absolute Ceilings.

                     So, von Karman calculated the Karman Line based on the most advanced aircraft in this context, of his time – the Bell-X2.  And he arrived at a value of 83.82 km. But at present, due to advancements in airplane technology, the Karman Line is taken to be about 100 km.

                      It must be stated that the figure of 100 kilometres is not a definite boundary; it is rather a working definition which distinguishes between aeronautics and astronautics.

                       The FAI – mentioned above – has set this figure; and it is also subscribed to by the United Nations. It helps in the making of international laws and treaties, in the demarcation of the airspace of a country (below the Line, are national airspaces, and above it is Space, which belongs to no country).

                  -Although no International Law or Treaty yet has specified the boundary of Space, by a number.

                  It is conceivable that the Karman Line can be extended further in the future.

                   Although it is a very interesting question, whether there is an absolute limit to the Karman Line for a given planet. -A limit based on the laws of nature. No matter how far aeronautics (further streamlining the body thereby reducing drag), materials science (making the body of the aircraft lighter while also making it less subject to heating up and more heat-resistant) etc. progressed, a height above which an air-breathing aircraft cannot be able to fly.

                   By the way, how high approximately do you think the Karman Line over Mars or Venus is located?

(The reader may look in the ‘Notes’ page for this article, for a (truly) brief discussion on this).

*

                  The United States military is said to have adopted a definition of Space that is a little different.

                   It is based on the lowest perigee attainable by an orbiting space vehicle (including a satellite). -This is for a body orbiting naturally, i.e. without a periodic boost by a thruster/small rocket to hold up the object against the effect of atmospheric drag, which acts to gradually lower and ground a satellite.

                   For a circular orbit, this is 150 kilometres from the surface of the Earth.

The closest approach is much greater for a satellite orbiting in an elliptical orbit; in which case, the perigee would be about 90 kilometres.

                   However, this author did not look for an authoritative source or verification on this. It seems to be an ‘on paper’ calculation.

                   In reality, the lowest altitude at which a satellite has orbited the Earth – but with propulsion to maintain its height – is 167.4 kilometres, achieved by the Tsubame (also called Super Low Altitude Test Satellite (SLATS)), launched by the Japanese space agency JAXA. It operated from 2017 to 2019, but held this lowest orbit only for 7 days.  

[ Satellites flying very close to its parent body will not only soon be grounded due to the substantial drag of the atmosphere (larger planets or moons generally collect more gases around themselves), but also may suffer perturbations in their orbits due to ‘gravitational anomalies’ i.e. a non-uniform gravitational field over the body’s surface ].

Had there been no resistance due to the atmosphere of the heavenly body and a perfectly uniform gravitational field over its surface, then theoretically, I guess a satellite could orbit indefinitely even at a height of a few metres from the ground.

*

                 The National Aeronautics and Space Administration (NASA), it seems, does not expressly define the boundary of space. But it can be said to consider the boundary at 80 kilometres from the Earth’s surface. -Not that it appears to carry much practical significance for this organization, which is engaged basically in Engineering.

                   Till 2021, the United States, in the form of the Federal Aviation Administration, recognized as an astronaut, any person who had travelled to an altitude of 80 kilometres. But since then, it has ceased this practice, reportedly because there had begun to be too large a number of such persons, due to commercial space voyages or ‘space tourism’.

*

                   Although the location of the boundary of Space, as deemed by NASA is the 80 km mark, NASA Mission Control (for space flight missions) considers space to begin from 120 km above the Earth. Because it is from this altitude that atmospheric drag upon spacecrafts during re-entry, becomes noticeable. This seems to be a functional definition, specific to spacefaring.   

~

The Limits of the Atmosphere

                   If the physical limit of the atmosphere is what we are interested in, then it may be mentioned that it now appears that the very outermost fringes of the Earth’s gaseous envelope extends up to 6,30,000 kilometres. This approaches double the distance between the Earth and the Moon.

                   In early 2019, a scientist of Russia’s Space Research Institute, Mr. Igor Baliukin along with some others who worked in the project, published a paper in a scientific journal, in which they presented an analysis of certain data received from the space probe SOHO.

The latter is a joint project of the European Space Agency and NASA, which collects its data while orbiting an empty point in space (the ‘Lagrange 1’ (L1) Point) about 1.5 million kilometres from the Earth, toward the Sun.

                   The space probe SOHO has a number of instruments, one of which is the SWAN. This detected ‘signatures’ of hydrogen atoms.. a faint geocorona, even at the above mentioned distance from the Earth i.e. 6,30,000 km.