Even though a space station orbits 250 miles above the earth, it still runs into traces of atmosphere.

When air is that "thin," gas particles dont act like a fluid, so the conventional wisdom about aerodynamics that have shaped race cars and jet planes arent relevant for low earth orbiting spacecraft. Rather, the atmosphere acts like individual particles that elastically collide with the spacecraft.

After enough collisions with these particles, the space station will slow down slightly, which changes its momentum and consequently lowers its orbit. As the orbital altitude decreases the density of the atmosphere increases and enhances the slowing down of the station even more.