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          This is the first article in a series on topics related to space, including rocketry, imagined settlements on other planets, and journeying to distant worlds.

          On the first day, we might like to think about what space is. Where does it lie?

[ The word ‘space’ in Physics, denotes a basic concept. It is the physical entity which is necessary for an object to have volume or for distances and locations to exist. It is the ‘room’ required for an object to exist. But this is not our concern here ].

           Of course, we all know the vast expanse of room beyond our Earth, in all directions, in which are hanging the stars, planets going around them, clouds of gas and dust, and huge re-occurring clusters of these things.. is what we think of as ‘space’.

And our earth is our home, a familiar, beloved place. At times beautiful and at times a little mundane. Space is on the outside.

But how far exactly? From an Earth-dweller’s perspective, how far above the ground do we have to travel, in order to enter Space?

         Well everyone can form his or her own concepts in this matter. The top of one’s local water tower is not it, and a comet beyond the planet Neptune, is definitely part of it.

         So what is the defining boundary?

~

A LIFE-SUPPORTING ENVIRONMENT

        For example, a person might think that the height above the ground, at which there begins to be no air, or too little of it, so that a person wouldn’t be able to breathe and therefore live in, is where space begins.

         But do we need to go up all the way to what is commonly thought of as ‘space’, to experience that?

          Human beings cannot reside above an altitude of about 5300 metres.

-that is, a human being cannot stay permanently or for an extended period of time (say a few days or weeks) normally and naturally, in such places. Even if they be on the surface of the Earth.

[ Here ‘normally’ means – without medical problems or serious physical distress, and while maintaining the routine level of physical activity.

‘Naturally’ means – without any artificial aid to survival such as supplemental oxygen or medicines.

Altitude (of a given place) of course means the height of that place above the ‘mean sea level’ ].

        The three highest human settlements in the world happen to be located at elevations of 5,000 to 5100 metres. (They include a town in the Andes mountains of Peru and a couple of villages in Tibbet).

[ Here, only permanent, round-the-year settlements serving as home to common people are being considered; so a research station manned by scientists or an expeditionary camp in the Himalayas would not count ].   

It may be added that on the website mounteverest.info, Mr. Graham Hoyland, who is himself a mountaineer, writes ‘humans have survived for two years at 5950 metres, which is the highest recorded permanently tolerable altitude.’

         With adequate rest intervals, a healthy and very fit person could climb to a height of even 6100 metres (which is a little more than the height of Mt. Kilimanjaro) without an oxygen canister, and breathe normally there at rest.  

          But he would not be able to stay there permanently, or even for more than a day or two (going to sleep becomes very difficult or impossible; and the person may wake up in distress shortly within dozing off. This is due to the scarcity of oxygen; though initially, at rest, there may be no difficulty in breathing).

Food would not be digested properly. And he would probably be seriously out of breath if he attempted any substantial physical exertion.

*   

This is slightly divergent from the topic, but we will continue for just a while.

-Depending on his individual tolerance of high altitudes and at what rate he has made the ascent – the person may also develop Acute Altitude Sickness, the mildest and most common form of which is Acute Mountain Sickness. AMS is characterized by a set of symptoms including headache and possibly : lethargy, nausea, dizziness, shortness of breath, disturbed sleep, and in severe cases impaired mental functions/confusion. AMS occurs 6 to 48 hours after the altitude exposure begins. And depending upon certain factors, it may resolve on its own or worsen.

In severe cases of Acute Altitude Sickness, the person may develop pulmonary edema or cerebral edema (aka HAPE and HACE).. i.e. the accumulation of excess fluid in these organs. Acute Altitude Sickness can be fatal, but is so in less than 1% of all cases.

*   

                   However, this being an affair of Biology, there are exceptions, departures from the pattern too.

                    Men in their prime, including trekkers and natives of mountainous regions have been known to die of Acute Mountain Sickness after ascending to elevations of 4,000 metres. (Although this author didn’t find out whether they happened to have a relevant – perhaps undiscovered – pre-existing medical condition).

                    So there are considerable differences even among healthy individuals of the same age group in respect of their ability to withstand low oxygen concentrations.

     Interestingly, the physical troubles do not start with the difficulty in breathing itself, but with the effects of less quantity of oxygen in the air inhaled. Mostly headache.

     The cause of all of this is basically the lesser mass of air – including the lesser mass of oxygen – that a person is inhaling in each breath, as he/she ascends from the sea level. Beyond a certain point, it becomes impossible for the human body to tolerate it or get habituated to it.

      The atmospheric pressure – which is an indicator of the density of air – at the altitude of about 5600 metres is half of what it is at Mean Sea Level. This is a little higher than the Base Camp for Mount Everest. At the elevation of Mount Everest, the highest point on the Earth’s surface, it is only one-third of that at sea level.

       What could be the reason behind the progressively lower density of the atmosphere at greater distances from the Earth’s centre ?

*

        One might wonder, at what altitude would a human be unable to breathe normally, with immediate effect and even at rest, despite any amount of attempted acclimatization, and therefore be unable to survive for more than a short while?

        That height, it seems, is around 9.37 kilometres.

        The air at this level is so rare (i.e. of such low density), and therefore the atmospheric pressure is so small, that the air present beforehand in one’s lungs would be at a much higher pressure, and therefore flow out of the lungs into the space outside.

          It would simply not be physically possible for a human being to expand his lungs and chest cavity by such an extent that the pressure inside them becomes a little less than that of the air outside, and thereby that air may flow into the lungs.

[This is the current understanding of this author, which might not be completely accurate. He hasn’t done substantial thinking or study on this].   

          To survive at this height and beyond, any human being, no matter how good his physical condition, would need to breathe artificial oxygen. This needs to be ‘100% oxygen’.

(Can the reader put this in perspective, for example placing it besides the height at which a passenger aeroplane flies)?

*

           And from about 15.6 kilometres (~50,000 feet), the atmosphere is so thin that ‘the lungs cannot be inflated even using oxygen, necessitating a spacesuit’ or pressure-suit.

(A reader might recall that this is what Mr. Felix Baumgartner was dressed in, when he had jumped off a Helium balloon from an altitude of 39 kilometres in 2012).

           We might look at one other such threshold before moving on – at the height of 18 to 19 kilometres from the ground, the atmospheric pressure is so low (0.06 atmospheres) that the boiling point of water is the same as the normal temperature of the human body.

What this means is, if a human were present in such a region, exposed, then the water present in large amounts in his body tissues would begin to boil; the blood, mucus and other body fluids would start boiling. The person would rapidly lose consciousness and die if not restored to ‘normal’ ambient pressure within 90 seconds.

       -So even inhaling oxygen at high pressure would not save his life.

This level is called the Armstrong Limit. (Not after Neil Armstrong, but after Harry George Armstrong, an officer in the American air force, a doctor, and a pioneer in aviation medicine and aerospace medicine. He saw service till 1957 and authored many papers).

Of course, as we have seen, astronauts do space-walks at much greater heights and perform repairs to the International Space Station, because again, they are wearing space-suits which – among their other interesting and necessary provisions – are also internally pressurized to guard against this very problem.

*

       Continuing, above the elevation of 8,000 metres (the peaks of fourteen mountains in the world are this high, the highest being Mount Everest at 8849 m), not even expert mountaineers could survive for more than a few days. – Unless they are consuming supplemental oxygen. Without breathing in from their oxygen cylinders, after sometime, they would be steadily deteriorating.

There may be the permanent loss of some brain cells, too.

      Acclimatization beyond 8,000 metres is simply not possible.

[ It is for some reason that the areas of mountains above 8,000 metres have been called  – somewhat unceremoniously – the ‘death zone’ ].

By the way, the cold at these altitudes, the reduced levels of humidity and to some extent the elevated levels of ultraviolet rays in the sunlight – all make respiratory function and general human well-being, difficult.

     But the views are divine.

      If only the person makes use of some technology and knowledge to prevent any harm to himself.

       It certainly doesn’t look abnormal, looks quite natural, from these heights.

*

    If the reader is interested.. if they are properly acclimatized and were using supplemental oxygen during the climb, then an average Everest mountaineer can survive without oxygen on the top for about 12 hours. -Perhaps a little more.

The aforementioned Mr. Graham Hoyland, once took off his oxygen mask upon reaching the summit and remained so for half an hour, and he ‘was okay’.

The record in this respect is 21 hours by Babu Chiri Sherpa (of Nepal) who in May 1999 stayed at the summit for that duration.

One might think, if a person from around the sea level is air-dropped directly on to the summit of Mount Everest, what the effect upon him would be.

As a matter of fact, he would very soon ‘lose consciousness and die within minutes’.

*

      So, to sum up, should we define the boundary of Space by the criterion of whether at a certain distance from the surface of the Earth, there is sufficient air to sustain human life normally?

(And permanently).

       If we do, then Space would begin from a height of about 5500 metres to 6000 metres from the sea level. And most of us would not feel like calling this ‘Space’ !

     For one, it does not look like Space – what layman’s conception we have of it. It is blue skies, white clouds and terra firma all around.

Besides, there are many places on Earth, the slopes and peaks of the highest mountains, which exceed this height and where one can still stand very much on the ground.

      [Can the region of space just above the peak of the highest mountain of the Earth be called Space?

     I suppose it is a subjective view. A way of thinking.

     There does not seem to be any natural justification or necessity to call this Space].

     The same can be said of the thresholds of 9.37 kilometres and 15.6 kilometres (approximately) of altitude, outlined above.

     It also seems to this author that in a way, these lines are too human body-centric.

     Could the criterion be more ‘natural’ i.e. Physics- or Earth Sciences-based?

~

SKY OR SPACE ?

    Does one feel that space begins where the sky begins to look black? —-Rather than the blue or other hues it has when seen from near the ground.

     This would be a visual or ‘impression’-related conception of Space.

I find it naturally appealing.

  There are certain heights above the Earth where around and above oneself, the surroundings appear black – except if one is looking directly at the sun – and stars are visible even during the day (‘day’ at such locations simply means ‘the duration of time in which one happens to be over the half of the planet which is facing the sun’).

     One could simply say that this is the height above the ground beyond which there is no sky, but the open regions of Space.

     From this altitude onwards, the background or the empty parts of one’s field of view is black.

Though we must be aware that in technological activities like space-faring, and the various technical calculations and considerations that go into it, as well as in basic sciences like astrophysics, definitions like the one above may not find much application.

*

                   Before coming to what this height is, we may quickly examine why this change in colour happens.

                   We see the sky as blue.

(And containing some other colours, at some other times).

                   But why should it be so?

                   Has it occurred to us to that question may arise, why we should see any colour or light at all, when we are facing away from the sun and looking above the ground?

*

                   In a part of the scene in front of us, we see a given colour, because light of a particular range of wavelengths – which is specific for that colour – is coming from that part of the scene and striking our eyes.

Each specific range of wavelengths (in the visual segment of the spectrum of electromagnetic radiations) produces the effect of ‘seeing’ a particular colour, in human beings (and some other creatures too).

[ Electromagnetic waves outside of the ‘visual range’ that is specific for human beings, do not produce the sensation of vision or colours in human beings ].

So, a particular range of wavelengths in the visual spectrum corresponds to a particular colour, and there are seven such wavelength ranges and their own respective colours.

*

                  So, when we see the sky as blue, it is a simple thought that light of blue colour must be coming from that part of the sky and entering our eyes.

[ When sometimes we see the colour of some parts of the sky as reddish or orange, it is because due to some reason (that are known), lights of those wavelengths – corresponding to those colours – are entering our eyes from those parts ].

                   Sunlight, as school books tell us, is white in colour – it is a mix of all the seven colours of the visible range. And indeed when we look at the sun itself, we see it as white (NEVER LOOK AT THE SUN DIRECTLY i.e. STRAIGHT-ON, AT ANY TIME OF THE DAY. IT MAY CAUSE PERMANENT DAMAGE TO YOUR EYES). Although brilliant and dazzlingly so.

When we see the colour of an object as white, it is because light of all the seven colours are coming into our eyes from the object at the same time, and the resultant experience of that in our visual perception – how our eyes sense this mix of different wavelengths and how our brain interprets it – is what we call ‘white’.

                   And similarly, when we see the sky as black, for example on a moonless night, it is because no light is coming from the direction of the sky to our eyes in such times.

Because ‘black’ is the colour that is perceived as the effect of no waves of the visual range reaching our eyes. So actually, it is the absence of any colours. -From the physics point of view.

                  Space too appears black, unless one happens to be gazing at a body that is luminous, like a star, the Moon or a bright nebula.

So obviously, there is no light of any visible wavelength that comes from the ‘empty’ parts of our field of view, in space.

                   But has it occurred to the reader, why we should expect the sky to look blue from the ground. All of sunlight is made up of rays of light (or photons, as you wish) travelling parallel to one another. And sunlight, before/unless it has been reflected or redirected in any other way by any material on or near Earth, comes only from one direction.

So why is it that when we look up at the sky, in any given direction away from the sun, we see blue? Even if we have our back to the sun?

We should see no colour or light at all, because the perception of vision is produced only when light is entering our eyes. Light that passes us by, -on the sides, or is moving away from us, would not produce any sensation of vision in us.

So, we should see such parts of our field of view as black.

                   If we see a colour even when we are facing away from the sun, it must be because light of this colour is coming from that direction to us, and indeed from all directions above the ground .. on the sides and at all angles above the ground.

                    What could be the source of this light?

                   Is sunlight being reflected by the air molecules?

Then why would light coming from a single direction be reflected in all directions, seen from any and every point on the ground; and even from airplanes- below oneself? Is that because the air molecules are moving randomly in all directions, and also their atoms are spherical, and there are a very large number of them, so collectively they are reflecting incoming light in all directions. Some of the sunlight passes straight through the upper layers of the atmosphere, because of the large gaps between the gas molecules, to be reflected by the lower layers or reach the ground.

Turns out, that is not what happens. Because the sizes of the different types of molecules of air are much smaller compared to the wavelengths of light, and also because their numbers per unit volume of air is too little, air molecules do not reflect light to a significant extent. -Not enough for our eyes to see.

                   But is there nothing else that can act as the source of the blue light or redirect the blue part of the sunlight to people’s eyes. And what happens to the rest of the colours of light present in the incoming sunlight?

                   The truth is, it is air that is the source of the blue light reaching our eyes from every direction above the ground.

And this causes the field of view above the ground to look like.. well, like the ‘sky’. – A layer or sheet above us, coloured most of the times in blue, spreading up from the horizons in a very large and lightly curving manner.

It does not look limited, blocking our view, or limiting.

It is because of the uniform and all-pervasive manner in which air, at all heights and regions above the ground (till a certain limit), sends out mostly blue light in all directions.

                    But if it is not by reflection, then how?

                    Well, the optical phenomenon responsible for causing this appearance is Rayleigh Scattering. This is a particular type of Scattering of light which is done mostly by air molecules and a small portion by dust particles suspended in the air. 

                    In it, there is an interaction of the molecules of air (and in general – to a lesser extent – of any transparent material) with electromagnetic radiation (including light) that is incident upon it.

Rayleigh Scattering occurs only when the size of the particles of the medium in question, is smaller than a particular limit. – smaller than about 1/15^th of the wavelength of the radiation.

*

                 The molecules are made up of atoms.

                  Normally in an atom, the positively charged nucleus is at the centre and the electrons – which are negatively charged – are present around it, in a symmetrical way, in their respective, fixed geometric regions of space called orbitals.

Since under normal conditions, in the atom as a whole, there is no net separation of positive and negative charges between any two opposite points on its boundaries, therefore the atom is said to have no dipole moment.

                    An electromagnetic wave is basically periodic changes (within a range of values) in the intensity of an electric and a magnetic field (oriented perpendicular to each other) which together make up the electromagnetic field. These changes are also called ‘oscillations’ of the field.

                    When an electromagnetic wave is incident upon these atoms, then the oscillating electric field of the wave – i.e. the rhythmically changing intensity of the electric field which pervades the space in which these atoms are present, –causes a force to act upon the electrons and the nucleus of the atom, because of course they are electrically charged. And because they are oppositely charged, the respective forces they experience due to the external electric field, are in opposite directions, and these forces cause them to be displaced a little in opposite directions. Thus, the atom becomes electrically polarized – an electric dipole is created.

[ NOTE : This author does not know if the nucleus too is actually displaced. It is order of magnitudes heavier than the electrons, while being only 7 or 8 times more charged than an electron, so its displacement i.e. oscillating motion might be negligible compared to the electrons’.

But even if only the electrons oscillate, i.e. are displaced a little away from the nucleus, an electric dipole would still be created, because of the non-balanced distribution of electric charge within the atom ].     

                   Now, if you remember, an electromagnetic wave essentially is made up of an electric and a magnetic field, such that at each given point within the field, the strength or intensity of the electric or magnetic field is varying regularly over time, repeatedly.

                   So the charged particles of the atom – the electrons and the nucleus – must also be experiencing forces upon themselves which are varying in strength periodically (a very small period) ; and therefore, the amount by which they are being excited or displaced is also varying in time. And therefore, the electric dipole moment created by their displacements must also be varying in magnitude periodically.

                   And here we come to the crux of this natural phenomenon.. according to the classical electromagnetic theory (Maxwell’s Equations), an accelerating charge generates electromagnetic waves. The oscillating electron is such a thing.

This oscillating electric dipole in turn creates its own electromagnetic waves. These waves are emitted by the atom in all directions, (–equally, in Rayleigh Scattering; in some other forms of Scattering, the intensity of emissions is not uniform in all directions).

What wavelength or mix of wavelengths the emitted radiation will have, is dependent upon what wavelength/s are present in the incident radiation, and the size and nature of the scattering particles.

                   So, in this way, air molecules receive sunlight and then re-emit, i.e. ‘scatter’ it in all directions.

Do you think a given air molecule, over a period of time, is scattering only the sunlight coming directly from the sun?

*

[ If the reader is interested, there is a succinct and good explanation of the mechanism of Rayleigh Scattering here :

https://physics.stackexchange.com/questions/235535/how-does-wavelength-affect-scattering-of-light-rays ].

*

                  Sunlight contains waves that are ordered in seven consecutive bands of wavelengths, corresponding to seven colours – the VIBGYOR that we know well.

                   But not all wavelengths of the received sunlight are re-emitted equally.

                    All wavelengths of light undergo scattering, indeed radiations of wavelengths above or below the visual range for human beings i.e. wavelengths other than those of ‘light’, also do, but the smaller wavelengths of radiation undergo scattering much more strongly.

[ ‘Strongly’ here means that the intensity or share of energy, in the overall scattered radiation, is much more for smaller wavelengths, than what would have been by arithmetic proportions within the incoming spectrum ].

                   Now if we look at the pattern of the wavelengths in sunlight, (we may

recall the short-form VIBGYOR from school), we see that blue light has a short range of wavelengths.

                   Well, violet and indigo have even shorter wavelengths, but the colour of the sky is almost never violet. This is for two reasons.

The proportion of violet light present in sunlight is much smaller than that of blue. That is the very nature of the overall radiation emitted from the sun. In fact, the amount of green light is the most in sunlight.

 Secondly, the human eye is more sensitive to light of blue colour than violet or indigo.

                    About one-quarter of the incoming sunlight is scattered in this way.

(The rest passes through to the ground; a little is absorbed by the air, which causes it to become warm).

[ Actually, as may be guessed, the blue light that reaches us on the ground is not coming from having been scattered at one place only. It has been scattered and re-scattered numerous times by the time it has reached our eyes ].

The question may also arise, if air causes sunlight to be scattered into predominantly blue colour and reach our eyes, such that a mass or sample of air in front of a person, sends blue light toward him (and in all other directions) when the sun is not in line with the two, then how can we see a highrise building say two kilometres from us? Should the intervening mass of air not make the whole scene appear blue?

And how about a far away hill?.

*

                  Unlike our Moon, the rocky planets around the Sun do have atmospheres (except Mercury, whose is extremely thin).

                   So could we surmise that the skies on these planets are blue, too? Because it is this colour of light in the solar spectrum (i.e. the visible part of it) that has the smallest wavelength, and therefore is scattered most strongly?

But in the atmosphere of Mars, the proportion of dust particles is much more than in that of Earth.

And the density of gases is less than one percent of that of Earth’s.

Further, the composition of Mars’ atmosphere, -the predominant gas which constitutes it, is different from that of Earth’s. (95% of it is carbon dioxide). Therefore due to the different constituents of the Mars’ atmosphere, sunlight is scattered in a different way in the sky of Mars.

The colour of the sky on Mars is not blue, but reddish for most of the day. When the sun is near the horizon, that is around the time of sunrise or sunset, the sky takes on a pale bluish hue.

The sun – being almost double the distance from Mars as from Earth – looks quite a bit smaller than from Earth. It is also dimmer. (-despite the tenuous atmosphere).                       

*

              Now, we know that Earth’s gravity – the force of its attraction towards its centre – is strongest at its surface (i.e. the surface of its solid/liquid part), and decreases in strength with the increase in distance from the surface. 

            So naturally, most of the air around the Earth is present in the lower altitudes, and not evenly distributed through the extent of the atmosphere. In other words, the density of the atmosphere decreases steadily with the increase in height from the ground.

In fact, the decrease is almost exponential i.e. very steep, in the upper reaches.

              Away from the Earth, outwards, lies the near vacuum of space. The atmosphere gradually becomes fainter and fainter until it becomes non-existent; the extremely thin mass of atoms spread wide apart, merges into space.

*

              It would seem sensible to assume that since the blue colour of our general field of view, as we look up from the surface of the Earth, is caused by the Scattering of sunlight by air molecules, therefore the heights above the Earth where there is no more air, the surroundings would look black.

              But do the open, unobstructed parts of one’s field of view appear black only from where the atmosphere ends? Or does the scattering effect become sufficiently weak at a certain height above the Earth – because the air in those regions is sufficiently scanty – such that it creates the appearance of blackness – at least to the sensitivity of the human eye – even before the atmosphere ends completely? (And as common sense would suggest, on the way to becoming completely black, the blue sky would gradually assume a darker and darker shade of blue).

              If we return to the Mount Everest for a moment, it is mentioned that the air pressure here is only one-third of what it is at sea level.

              So the amount of air per unit volume of space here must be one-third of that in the plains. And so the Rayleigh Scattering effect must be much less.

               And this arrangement of nature does have the expression that one would expect it to have. From the summit of Mt. Everest, the sky above does look a considerably darker shade of blue, than the parts of the sky below.

In fact, one can observe the beautiful pattern of the sky becoming darker and darker, as one raises one’s gaze from the horizon below, toward the zenith.

                The same effect can be observed from an airplane if one bends his neck a little to look upward. The sky above is of darker shades of blue than below. A passenger plane of today flies at an altitude of about 10 to 13 kilometres (its ‘cruising’ height). 

(contd.) –

                 Long before the Earth’s atmosphere ceases to exist, the ‘sky’ or the open parts of one’s field of view becomes black.                   

                 The height from which the air begins to be so rare that the scattering effect is negligible, are the regions where during the day, if one is not directly looking at the sun, the sky around and above him is dark. If we wish, we may call this region ‘space’.

*

                   And what is this height above the ground?

                   It is about 80 kilometres.

                    And stars are seen scattered in it, even ‘during the day’.

*

                    But if one imagines space in this way, he needs to be aware of some other things.

                    For example, there is no atmosphere over the Moon. Even at ground level, gas molecules are practically absent. So, if on the surface of the Moon, a person were to look up, what would his view be like?

                    If blackness of the sky above a certain altitude from the surface of a celestial body is taken as the criterion for defining Space, then would Space begin at the same distance near all planets and moons? While many solid celestial bodies have an atmosphere around them, many others – like our Moon and Mercury – have very little or none to speak of. So there would be not be the effect of a coloured sky looking up even from the ground, on these bodies. The sky from the ground on such heavenly bodies appears black.

*

              The sky is completely black looking up from the surface of the Moon or of Ceres, a dwarf planet (earlier considered an asteroid) between Mars and Jupiter. (Needless to say, -even during the day). So does that mean that Space starts just above the surface of the Moon or Ceres? Strangely enough, I feel that that notion makes sense.

Whether in terms of the appearance of the outward environs, the strength of the solar wind and cosmic radiation at the ground itself, or the extreme differences of temperature between day and night, -and between sun and shade, the conditions on the ground are indistinguishable from those in Outer Space.

The conditions from the point of view of the safety of human beings standing on the ground of these celestial bodies, are very similar, if not practically identical to those in Deep Space.

                The impression which is created in the mind of someone, is that a person standing on these celestial bodies is ‘exposed to Space’. And true to that idea, people who have visited these worlds or might some day, did or would have to wear spacesuits, just like an astronaut on a spacewalk outside her spaceship.

                All the moderating and softening influences of a healthy, substantial atmosphere are absent. The sun rays are stark and intense. Indeed, it begins to be harmful to human beings within a short period of time.

~

VIEW OF THE EARTH

                  Some people could think of ‘space’ as the regions above the Earth, from where if a person were to look ‘down’, (i.e. toward the Earth), she could get a sense of the roundness of the Earth.

-That is, the heights from where a sufficiently large portion of the Earth is visible such that it is seen to have a curved outline.

                   Or that space begins in the region from where one is able to see the whole of the Earth. -Its complete round figure.

                  -Or where, still farther away – in the scene in front of oneself, the Earth is only a modest, minor object, and most of what one sees is the empty expanse of space with stars dotted in.

                   This is a subjective thought of space.

                   Children unconsciously, and many other people in their common moods, may think of space in this way.

(Though children may often have a more advanced and nuanced understanding too).

                   These definitions are predicated upon simply the distance of a location from Earth.

                   Is there, can there be a sharp boundary or cut-off point in such a concept of Space?

                  Technically, it seems there can be.

*

                  The height beginning from which the curvature of the Earth becomes visible is about 10.6 kilometres above the sea level. (Provided that one’s angle of view is at least 60 degrees, and of course the horizon needs to be almost free of cloud).

However, just like the very first moments of the appearance of daylight at the end of night, are almost imperceptible and go unnoticed unless we were particularly trying to sense it, – indeed if we happen to be waking at the time, we are not even sure that day has just begun; we usually realise only after the first few minutes – similarly at the above mentioned altitude (which is a little less than 35,000 feet), the Earth’s curvature as evinced by the outline of the horizon, is very slight. And barely perceptible.

Only people with the most acute eyesight and making the best conscious effort would be able to detect it.

                      Below is the view from the window of a passenger airplane.