We tell why the ISS is located at an altitude of exactly 400 km, and not more or less

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One of the readers sent us the following question:

The orbital altitude of the ISS is constantly changing in the range from 340 to 417 km above sea level, the station is constantly decreasing due to the influence of the atmosphere, but the orbital altitude is being maintained at a level of about 400 km.

This range of heights was chosen for a number of reasons as optimal in terms of all the pros and cons. One of the most important criteria for choosing an orbit is the cost of cargo delivery to the ISS and the very possibility of a particular spacecraft flying to a given altitude.

From a financial point of view, the lower the ISS is, the better, since flights to it will be cheaper, also earlier, when astronauts were delivered to the station by shuttles, its orbit was significantly lower than about 350 km due to the fact that the shuttle with the maximum payload in in principle, could not rise to an altitude of 400 km.

On the other hand, the lower the orbit, the stronger the friction of the ISS against the rarefied atmosphere, the station decelerates faster and falls to the Earth, so more often it is necessary to correct the orbit, which requires fuel costs, which still need to be delivered to orbit. At an altitude of 300 km, friction against the atmosphere will be too strong, too much fuel will be needed to correct the orbit, and in the event of engine failure, the station may not be repaired before it falls to Earth.

Thus, the orbit around 400 km is optimal: there are all conditions for space experiments, cargo delivery is not too expensive, and not many resources are spent on maintaining the orbit. Increasing the orbital altitude of the ISS, say, up to 600 km and above will not bring any benefit in terms of scientific experiments, but it will cost significantly more.

In addition, at an altitude of more than 500 km, the radiation level begins to grow rapidly, which is harmful to astronauts and the station’s equipment, it will be necessary to create additional protection for the station, which will again require huge financial investments. Therefore, no one thinks about raising the ISS orbit either.