To begin with, the ISS altitude is constantly changing. It ranges from 340 to 400 km above sea level. "How so? Just 340 kilometers? Yes, yesterday I went to the dacha one way, and then I drove more! Why can't you raise the station higher? " - some will be surprised. In fact, there is no need for this for two reasons: the conditions on the ISS are quite enough for carrying out the necessary experiments in zero gravity and designating the constant presence of a person in space. Another reason is related to economy: imagine how much more space powers would have to spend in order to deliver astronauts and cargo at a distance, for example, several thousand kilometers from Earth?
Why does the height of the station need to be changed all the time? The fact is that the ISS is slowed down by the rarefied atmosphere of the Earth. Yes, yes, at an altitude of 340 km, its influence is still noticeable, and air particles over time reduce the speed of the ISS, which is why it begins to approach the surface. The station's altitude would have to be adjusted less often if it were at an altitude of about 400 km, where the influence of the planet's atmosphere is minimized. Until recently, this remained impossible, as American astronauts were transported to the ISS using shuttles that could not reach an altitude of 400 km. But after their cancellation, the ISS ceiling was constantly adjusted upward: from 353 km in 2011 to 417 km in 2014. Approximately this orbital altitude is used on the ISS today.
This is interesting: did you know that time flows on the ISS differently than on Earth. How exactly - in our article "How does time go on the ISS?"
Another reason for changing the altitude of the ISS orbit is to avoid space debris. Experts 24 hours a day observe the movement of space debris in orbit, and if there is a possibility of collision, the ISS altitude is adjusted.
This is interesting: On a cloudless night with low illumination, the ISS can be seen from Earth. To do this, it is enough to know the time and date of the passage of the space station over your settlement. Information is available here