system
Given the following post, respond as if you're commenting on Reddit:
human
How do they prevent the ISS from crashing into satellites and space junk?
gpt
The ISS is hit by very small debris occasionally and has had near-misses by larger debris. Things in orbital space don't move around at random but on consistent paths. Most of these paths are not perfectly stable, being affected by the varying gravity of the Earth, the light drag of gasses in the stratosphere, and the forces of solar wind and even the pressure of light, and so their orbits can 'decay' over time, causing them to fall into the Earth after some period of time. Active satellites maintain or change these paths with periodic boosts by small on-board rockets or thrusters.  

The paths of most large objects in space are recorded and continuously tracked so that their possible locations at any time in the future can be predicted, allowing collisions to be avoided. But very small objects cannot be easily detected by radar or telescopes and so are much harder to predict. So manned spacecraft are usually armored for protection against being hit by these small objects. 

With the number of things in orbit today very large, there is a potential hazard called the Kessler Syndrome. This is when a collision between two things in orbit produce a lot of small debris that fly off in many directions and can't be tracked, colliding into other things producing even more debris, and so on. If this was severe enough, it could destroy most satellites and create a vast swarm of debris preventing manned spacecraft from safely travelling to space for decades or centuries, until all these small objects were finally swept out of orbit naturally or deliberately cleared by other means we haven't yet developed. This is what makes the militarization of space very dangerous for our future in space. You can't use or even test weapons in space without the risk of causing one of these Kessler events.