By Sabine Stanley, Ph.D., Johns Hopkins University
Low Earth orbit is the result of scientists’ attempts and studies to answer a very common question: Where does space begin? It includes all the layers of the atmosphere, which are distinguished by their characteristics, how gas behaves, temperature, pressure, and distance from the surface. However, the layer, with 80% of the mass, is the thinnest of all.
Even though low earth orbit includes all layers of the atmosphere, most of what people know from the atmosphere happens in the first two layers. Troposphere and stratosphere are the closest layers with the major amount of atmospheric mass. Low earth orbit somehow marks the beginning of outer space.
The Karman line was a governmental attempt to mark the boundary between Earth and outer space, but they could not agree on an altitude. Thus, it ranges from 80 to 100 kilometers. What everyone agrees on are the layers of the atmosphere and their altitudes. The first layer is the troposphere.
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What Is the Troposphere?
The lowest layer of atmosphere is called the troposphere, with an average thickness of 10 kilometers, spread above the Earth’s surface. Everything known as ‘weather’ happens in this layer, namely, winds, thunderstorms, tornadoes, hurricanes, blizzards, and cloud formation.
The convective overturning of air results in all the weather phenomena. Besides, water goes through all its phases in the troposphere: vapor, rain, and snow. The ‘overturning’ is reflected in the name of this thin but dense layer: tropos is Greek for ‘turn’, referring to the overturning of air. Now, why does air overturn here?
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Overturning in the Troposphere
The overturning – vertical mixing – happens since the highest temperature of the troposphere is at the bottom. The average temperature on the surface is about 60°F, and it decreases down to an average of −75°F at the top. About 80% of the atmosphere’s mass is concentrated here. The troposphere ends with a boundary called the tropopause, where the next layer begins.
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What is the Stratosphere?
The stratosphere is the second-lowest layer of Earth’s atmosphere, starting at 10 kilometers above the surface. Unlike the troposphere, the temperature in the stratosphere increases with altitude. At the top, the temperature is around 32°F – almost 100 degrees higher than the bottom. However, the pressure decreases to one millibar, i.e., 1000 times less than the Earth’s surface pressure. The stratosphere ends at the stratopause. Does the higher temperature mean the stratopause is warm?
Is the Stratosphere Warm?
The higher temperature in the troposphere is the result of the Sun’s ultraviolet (or UV) radiation, trapped by ozone particles (O3). The famous ozone layer, where UV is absorbed, is located here. The highest concentration of ozone is at the lowest part of the stratosphere, but O3 can also be found up to the middle of the layer.
Most of the radiated UV is absorbed by O3, breaking it into O2 and atomic oxygen (O). Next, O and O2 combine again and recreate O3. However, the absorbed energy does not create a warm environment, since the molecules are so far away from each other that collisions rarely happen. Hence, airplanes flying around this altitude need to create pressure inside the cabin and regulate the temperature.
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Airplanes in the Stratosphere
The cruising altitude for most commercial flights is around 39,000 feet or 12 kilometers above the surface. This means that airplanes fly in the lowest part of the stratosphere, above the weather and the turbulence it causes. The boundary of the two layers can be seen from the plane, as clouds do not enter the stratosphere. The air is too thin here, so airplane cabins are pressurized.
Besides the commercial planes, NASA’s SOFIA also flies in the lower stratosphere.
SOFIA: Stratospheric Observatory for Infrared Astronomy
SOFIA is a Boeing 747 with a 100-inch telescope attached. It flies at 12 kilometers, above 99% of Earth’s atmosphere, and studies the solar system in the infrared part of the light spectrum. Transient phenomena, such as eclipses and occultations, are also studied best with SOFIA.
Pluto and Saturn’s moon, Titan, was also studied by SOFIA from the stratosphere. Higher in the stratosphere, the air pressure is too low for typical airplanes to fly. Military jets and other planes that do fly at higher altitudes use their engine power to conduct the flight.
However, scientific balloons are launched to collect data. The best location to do so is McMurdo Station in Antarctica, as the South Pole vortex keeps the balloons contained in a small area and does not let them fly away into the distance.
It can be concluded that the first two layers of the atmosphere in the low Earth orbit are the ones humans make the most use of.
Common Questions about Low Earth Orbit
Low Earth orbit is the ‘circle’ around Earth’s atmosphere up to 2000 kilometers above the surface. The satellites and other human-made space objects also orbit in the low earth orbit. The end of the orbit is the beginning of space, where solar winds start, and the Earth’s atmosphere is too thin to be considered gas.
Low earth orbit extends up to 2000 kilometers above the surface. All the atmosphere layers reside in this area, with almost 80% of the mass concentrated in the lowest layer, the troposphere. The weather, winds and tornados, plane flights, and satellite orbits all occur in this 2000-kilometer-high zone before space characteristics dominate the environment.
The stratosphere is the second-lowest layer in the low Earth orbit. It starts at 10 kilometers above the surface, and even though its temperature rises through altitude due to UV radiation, its pressure decreases significantly. It contains the ozone layer and the cruising altitude for most commercial planes.
Low Earth orbit is about 2000 kilometers, which is not very high by space and orbit standards. In fact, the end of the low orbit somehow marks the beginning of space. The International Space Station orbits 400 kilometers above the Earth’s surface, and many satellites go above that. All the explorers sent to other planets pass the low Earth orbit at an early stage in their trip.