Even though they're in Low Earth Orbit, there is a just enough atmosphere to cause the orbit to decay slightly over time, and this has to be compensated for by boosting the orbit
Just to expand on this a little (since it's gaining new relevance with the lower LEO and VLEO large constellations getting planned/launched), orbital life times drop non-linearly with altitude, atmospheric drags plays a big role fast until hundreds of km above the Karman line. An oldie and simplified but goodie site here [0] gives a decent basic overview. Remember that this will also vary, the atmosphere is affected by temperature and other factors.
Particularly since the ISS is manned and mass was very restricted, its altitude is a tight balancing act between high enough to not need to boost too often for comfort/practicality and as low as possible to reduce radiation exposure and stay away from the inner Van Allen radiation belt, which can get quite low indeed on occasion based on solar activity and geographic areas. It does mean though that ISS reboosts have to happen fairly regularly.
One big area of current interest this has an impact on is orbital debris lifetimes. There's an excellent Gabbard diagram animation [1] on Reddit that's worth a look if you haven't seen it before. You can see the decay rate very visually. Having constellations be very low is valuable for passive fail-safety in case of loss of control or collisions.
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Just to expand on this a little (since it's gaining new relevance with the lower LEO and VLEO large constellations getting planned/launched), orbital life times drop non-linearly with altitude, atmospheric drags plays a big role fast until hundreds of km above the Karman line. An oldie and simplified but goodie site here [0] gives a decent basic overview. Remember that this will also vary, the atmosphere is affected by temperature and other factors.
Particularly since the ISS is manned and mass was very restricted, its altitude is a tight balancing act between high enough to not need to boost too often for comfort/practicality and as low as possible to reduce radiation exposure and stay away from the inner Van Allen radiation belt, which can get quite low indeed on occasion based on solar activity and geographic areas. It does mean though that ISS reboosts have to happen fairly regularly.
One big area of current interest this has an impact on is orbital debris lifetimes. There's an excellent Gabbard diagram animation [1] on Reddit that's worth a look if you haven't seen it before. You can see the decay rate very visually. Having constellations be very low is valuable for passive fail-safety in case of loss of control or collisions.
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0: https://www.spaceacademy.net.au/watch/debris/orblife.htm
1: https://old.reddit.com/r/space/comments/ld4vlq/gabbard_diagr...