Why the International Space Station orbits at 400 kilometers in space?

Science

03 January 2019
Article
Auteur(s): Philip Lepoutre
Have you ever wondered how high up the ISS is and why it is there?

by Philip Lepoutre

Science Editor

In this article, we will be discussing these topics, and explain how they are related to interesting phenomena like the Van Allen Belts and atmospheric drag.

The International Space Station (ISS) has been credited with being the second largest ever peaceful investment, second only to the United Nations. According to Article 2 in the Memorandum made in 1998 between NASA and the Russian Space Agency, the ISS is a vantage point from which humans can perform experiments and observations that may one day, help the human race to colonize the solar system. 

Scientists debated in the late 80s about the orbit of the ISS, they had to decide on an orbit that assured both a reduction in financial costs, but also an orbit that would protect the astronauts and allow the station to perform the functions mentioned in Article 2 of the 1998 Memorandum.  

  • Astronaut Safety 

Their priority was to protect the astronauts, so this already eliminates the possibility of an orbit at more than 600 kilometres altitude, because the radiation is just too high for human beings to endure for extended periods of time. Above the 600-kilometre mark, you enter the “Van Allen” belts, zones of energetically charged particles that have been captured by our Earth’s magnetic field. They got their named from the American physicist James Van Allen (1914-2006):

 

(Picture of James Van Allen)

Back in the 1950s, Van Allen launched a “rockoon”, a rocket lifted by a balloon above the atmosphere, and it detected the first hint of radiation at high altitudes. These zones of radiation, later known as Van Allen Belts, were the first major discovery of the Space Age, and caused serious issues to the Apollo program, because astronauts had to go pass these radiation belts to go to the Moon. Placing the ISS in an orbit that coincides with these radiation belts would be very dangerous, to both the astronauts and to the material on board. So, any orbit above 600 kilometres was disregarded.

Impact of Atmospheric Drag

Atmospheric drag is a real issue, as it has the capability of de-orbiting objects in just a few years. For instance, in early September of 2016, ground-controllers lost connection with the Chinese space station Tiangong-1, that was in Low Earth Orbit(LEO). Its orbit slowly decayed from 355km until it finally crashed into the Pacific Ocean on the 2nd of April 2018.

This plot shows the orbital height of the Chinese space station Tiangong-1 over the last year before its re-entry on April 2nd, 2018.

 

 

 

 

 

Atmospheric drag is caused by the friction of the specific object with the gas molecules from our atmosphere. Our atmosphere decreases with height, so the higher an orbit is, the less it is subjected to atmospheric drag, thus, scientists deemed that it is unreasonable to place a huge structure like the ISS at an orbit of 300 kilometres or lower in altitude, due to the constant atmospheric drag that would act upon it. Even at an altitude of 400 kilometres, the ISS is already dropping at a rate of 2 kilometres per year, so to keep it in a stable orbit, translational burns, also known as re-boosts are made.

Digital illustration of a transnational burst, also known as a re-boost.

 

 

 

 


This plot shows the orbital height of the ISS over the last year. Clearly visible are the re-boosts which suddenly increase the height, and the gradual decay in between.

 

 

 

  

  • Cost

We have seen that any orbit above 600 kilometres or below 300 kilometres is not viable. To fully determine why the ISS orbits at around 400 kilometres in altitude, we also need to take into accounts the costs of maintenance, and the cost of launches to the space station. As we increase the orbit higher and higher, the distance needed for rockets to get to the station increases, so the cost per launch increases as well. But as we go lower, the atmospheric drag increases, so placing a station in an orbit that is too low would cause tremendous maintenance costs, because of the constant re-boosts that you would have to make. Thus, in 1998, NASA decided to place the ISS into an orbit which varied from 330 to 405 kilometres in altitude.

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