The space station’s original orbit had been carefully chosen. Any lower, and the thicker air would make the orbit decay too fast. Any higher, and the danger from micrometeoroids would increase. That was because the rocks zipping around in space were subject to the same slow orbital decay as Izzy herself. Which was a good thing, since it dragged them down into the atmosphere and destroyed them, leaving clear space for Izzy to sail through. Her orbital altitude of four hundred kilometers was a Goldilocks compromise between "too much orbital decay for Izzy" and "enough orbital decay to sweep the sky of dangerous rocks."