Near space photography, high-altitude balloons and the Earth's atmosphere

I have recently decided to get involved in a hobby called Near Space Photography, which is very closely related to High-altitude ballooning. This hobby consists in being able to take photographs of the Earth as seen from the Near Space, usually (if not always) using an high-altitude balloon. When the balloon reaches its maximum altitude, it is possible to take pictures of the black canopy of space, and viewing clearly a big swath of the earth with a curved horizon out to several hundred Km's. Before explaining where it is located the Near Space, let me briefly present the Earth's Atmosphere which encompasses a set of main layers as follows (the description bellow regarding the atmosphere and its layers was adapted from the 'Atmosphere of Earth' entry in Wikipedia, together with other sources):

Atmosphere layers
(@Public Domain,
not to scale)

Atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. An altitude of 120 km is where atmospheric effects become noticeable during atmospheric re-entry of spacecrafts. The Kármán line, at 100 km, also is often regarded as the boundary between atmosphere and outer space. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×10¹⁸ kg, three quarters of which is within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. 

Air is the name given to atmosphere used in breathing and photosynthesis.

Exosphere

The exosphere is named from the ancient Greek "ἔξω éxō" meaning outside, external, beyond. The outermost layer of Earth's atmosphere extends from the exobase upward. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind.

Thermosphere

Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. This is why this layer  is named from the Greek "θερμός" (thermos) meaning heat. Unlike in the stratosphere, where the temperature rise is caused by absorption of radiation by ozone, in the thermosphere the temperature rise is a result of the extremely low density of molecules. The temperature of this layer can rise to 1,500 °C (2,700 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The International Space Station has a stable orbit within the middle of the thermosphere, between 320 and 380 kilometres. Auroras also occur in the  thermosphere . The top of the thermosphere, called the exobase, varies in height with solar activity and ranges from about 350–800 km.

Mesosphere

The mesosphere is named from the Greek word mesos (middle). This is related with the fact that together the stratosphere, mesosphere and lowest part of the thermosphere are collectively referred to as the "middle atmosphere", which spans heights from approximately 10 to 100 km. The mesosphere extends from the stratopause to 80–85 km. It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring place on Earth with temperatures of 190 K (−83 °C).

Stratosphere

The stratosphere extends from the tropopause to about 51 km. Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer, which restricts turbulence and mixing. The ozone layer is mainly located in the lower portion of the stratosphere from approximately 20 to 30 kilometres (12 to 19 mi) above Earth, though the thickness varies seasonally and geographically. The ozone layer absorbs 97–99% of the Sun's medium-frequency ultraviolet light (from about 200 nm to 315 nm wavelength), which potentially damages exposed life forms on Earth. While the temperature may be −60 °C at the tropopause, the top of the stratosphere is much warmer, and may be near freezing, in fact, the top of the stratosphere has a temperature of about −3°C (270 K), just slightly below the freezing point of water. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km. The pressure here is 1/1000 of the pressure at sea level.

Troposphere

The troposphere begins at the surface and extends to between 7 km at the poles and 20 km at the tropics. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word "τροπή", trope, meaning turn or overturn. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km deep depending on the landform and time of day. The tropopause is the boundary between the troposphere and stratosphere and is a temperature inversion. In the tropopause it can be found the main jet streams which are fast flowing, narrow air currents with wind velocities that can go from 92 km/h to 398 km/h, typically from West to the East. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round. This phenomena is not so typical over the Portugal latitude, so it should not affect a balloon launched from there. While air content and atmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres. Most of the phenomena we associate with day-to-day weather occur in the troposphere.

In order to better illustrate the differences between the atmosphere layers and also because this will be useful for a next Post where I will present the physics and maths applied to a weather balloon, the figure that follows show how the air density, pressure and temperature change as the altitude increases and the several atmosphere layers are traversed.

Comparison of the 1962 US Standard Atmosphere graph of
geometric altitude against air density, pressure, the speed of
sound and temperature with approximate altitudes of
various objects (@GNU Free Documentation License)

Regarding this figure I would like to highlight the following. From the Earth surface, the air density and pressure drops consistently up-to between 30 to 40km of altitude when both reach 0. Regarding the temperature things are more complex. The temperature in the Troposphere drops a lot from 290 kelvin to a bit less than 220 kelvin. Then in the stratosphere the temperature initially, and during almost 10 km, maintains constant, but then it raises from 220 kelvin to 270 kelvin. Then in the Mesosphere it drops again. An analysis of the temperature profile in the Atmosphere reveal that the temperature is in fact a good metric to distinguish among atmospheric layers, because:

• We know that the general pattern of the temperature profile with altitude is constant;
• The temperature profile pattern changes coincide with changes in the atmospheric layer- the temperature stops dropping when we reach the end of the Troposphere (called the Tropopause) and starts increasing in the Stratosphere up-to its limit (the Statopause) when it is maintained constant for a while until we enter in the Mesosphere where a drop of the temperature occurs.

In any case the discussion regarding the air density, pressure and temperature patterns with altitude is only included in this Post because this characterisation is relevant for the next post where I explain intuitively how a weather (gas) balloon works and is able to go up into the atmosphere.

Getting back to the Near Space, different (not many) definitions exist about it, but to be honest I do not know to which point the Near Space is a formal, standard concept, or if it is just a more informal concept created to designate a given region in the atmosphere. The typical references state that the Near Space refer to an area of the Earth's atmosphere between 20 to 100 km above the sea level, encompassing the stratosphere, mesosphere, and thermosphere. The Near Space is an area of Earth's atmosphere where there is very little air, but where the remaining amount generates far too much drag for satellites to remain in orbit. A vehicle designed to operate in the Near Space is sometimes called nearcraft, and there are two types of vehicles that usually operate in the Near Space, these include sub-orbital rockets, which make quick jumps into and out of near space, and high-altitude balloons. Regarding the high-altitude balloons, which are those of interest to us in what respects Near Space photography, the most common type is the weather balloon (or sounding balloon) which may reach altitudes of 40 km (25 miles) or more, limited by diminishing pressures causing the balloon to expand to such a degree that it disintegrates. So  these balloons are released into de Near Space and in particular into the Stratosphere.
There are references to other types of high-altitude balloons, Scientific balloons, that can remain at high altitudes for several days, but usually an high-altitude balloon is released form Earth's surface and then rises up to an altitude where it bursts and falls back into the surface, all this in some hours time. But in 2012 I think it is already possible to speak about a third type of high-altitude balloon, because in the last years there have been some many launches of this type in different parts of the world, which could be referred by hobby balloon, Near Space photography balloon or amateur balloon. These category of high-altitude balloons differ from the weather or scientific balloons in its mission which in this case is hobby-related, taking pictures, personal technological achievement, or just for fun, but in fact all  have in common (weather and scientific balloon included) that the balloon payload is carried in both cases by a weather balloon.

Now to recap, the Near Space photography hobby aims to send to the Near Space, in particular, to the Stratosphere,  a payload carried by a weather (gas) balloon which ultimately aims to take photographs of the Earth at different altitudes. Of course taking pictures is many times just one of the features of the payload, because many other things are, and can be done, in addition to taking photos, namely measuring atmospheric parameters like temperature, pressure and wind, and many other possible experiments and measures (ozone, CO2, other).

Before finalising this post it is important to refer that high-altitude balloons are usually filled either with helium or hydrogen. The helium is more expensive than hydrogen, but is also less explosive, and so, due to the latter, today the majority of balloons, especially the hobby-related amateur ones are filled in with helium. This will certainly be my choice, but caution must be taken because also with helium things can go wrong.