system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why does the ISS require thrusting to maintain orbit, while the moon has been orbiting the earth for a long time without any problems.
gpt
Atmospheric drag.  The ISS is high, but there's still enough atmospheric drag to slowly lower its orbit.  Without the regular boost, the orbit would drop lower into the atmosphere and drag would keep increasing to the point the orbit gave out.