One of the greatest achievements of mankind is the accomplishments of the Apollo 11 space mission. The mission allowed humans to step foot on the surface of the Moon. While that has been a huge leap in space exploration, a study suggests that humans have not left Earth yet, atmospherically speaking. The notion of ‘no humans did leave the Earth’s atmosphere’ is correlated to the uncertain span of the exosphere, the outermost layer of our planet’s atmosphere. A study in early 2019 found evidence that the exosphere even reaches past the Moon, which indicated that we never left the Earth’s territory despite the success of Apollo 11.
The Parts of the Earth’s Atmosphere The atmosphere of our planet is divided into several layers based on specific characteristics. The troposphere is the lowest layer with a span of 10 kilometers from the Earth’s surface, above sea level. This layer includes organisms and most weather activities since clouds are formed here. The stratosphere is the second layer with a span of about 50 kilometers between the first and third layers. Here, most aircraft and similar vessels fly because of less turbulence and minimal cloud content. After that, the mesosphere spans about 85 kilometers where meteors burn. This layer also features thin air that makes breathing extremely difficult.
Above the mesosphere is the thermosphere with a span between 500 and 1,000 kilometers above sea level. This layer is responsible for absorbing the high-energy X-rays and ultraviolet rays from the Sun. So, the temperature here is extremely high, as opposed to the cold temperatures in the mesosphere. And finally, the highest and outermost layer—the exosphere. This layer has a span of hundreds of thousands of kilometers, although, no one seems to agree on its distance. Based on the details at the University Corporation for Atmospheric Research, the extent of the exosphere is between 100,000 and 190,000 kilometers above sea level.
Where Does Space Actually Begin? According to Labroots.com, the layers of the atmosphere should be the guide where space actually begins from Earth. However, the confusing span of the exosphere messes its potential as a guide. Scientists have easily determined the troposphere where weather happens, stratosphere where airlines can fly, mesosphere where meteors burn up, and the thermosphere where space stations can orbit. Yet they cannot lock down the exosphere. Space scientists have thought for the longest time that the end of the Earth’s atmosphere and the start of outer space is beyond the Karman line. This line has a span of about 100 kilometers above sea level, which is below the range of the mesosphere. Once a person goes beyond that line, they are now officially in space.
The reasoning behind that logic is what powers regulatory and legal measures in aircraft and spacecraft. Once you pass the Karman line, the atmosphere is so thin that prevents commercial airlines to sustain its flight, without reaching orbital velocity. As such, an individual requires a more specialized vessel like spacecraft to traverse this region. Despite the ruling of the Karman line, no international consensus exists that determines where space begins.
Some astrophysicists think it should be 80 kilometers above sea level due to the particular changes in gravity and orbital velocity. But others do not believe that when it comes to a technicality as long as the exosphere is concerned. A discovery by researchers at the European Space Agency and NASA’s Solar and Heliospheric Observatory shows that the exosphere is not just 190,000 kilometers. It is up to 630,000 kilometers away from Earth, exceeding the distance between Earth and the Moon of about 384,400 kilometers. They published their results in the Journal of Geophysical Research: Space Physics.
The Span of the Exosphere ‘The Moon flies through Earth’s atmosphere. We were not aware of it until we dusted off observations made over two decades ago by the SOHO spacecraft,’ explained Igor Baliukin, the lead author of the study and a scientist at Russia’s Space Research Institute. Geocorona is what their observations have found that helped determine the span of the exosphere. The Solar Wind Anisotropies instrument onboard the SOHO spacecraft revealed the merger of the Earth’s outermost atmosphere and a portion of outer space. SWAN detected the geocorona or a cloud of hydrogen atoms when the gas’ signatures registered in its sensors.
The association of hydrogen gas with outer space and exosphere is explained by water vapor. Earth, Venus, and Mars have atmospheres with hydrogen gas because somewhere on or within their surface, water exists. The cloud of hydrogen gas has been prominent when the rays from the Sun interacted with it. Their interaction resulted in a distinct wavelength of UV called Lyman-alpha, something atoms can absorb and emit. And since the observations showed the light being absorbed by the atmosphere, it can only be seen from space. Furthermore, the geocorona differs where the Earth is facing.
The dayside of the Earth shows the compression of hydrogen atoms as the Sun bombards them with light, while on the night side, the region expresses higher density. The positive part of this discovery is that the UV rays in the geocorona are negligible, compared to the real source. But the negative part is that future astronomical observations within the area of the Moon can be affected by the geocorona. ‘Space telescopes observing the sky in ultraviolet wavelengths to study the chemical composition of stars and galaxies would need to take this into account,’ explained Jean-Loup Bertaux, an author of the study.