This plot shows the orbital height of the ISS over the last year. Clearly visible
are the re-boosts which suddenly increase the height, and the gradual decay
in between. The height is averaged over one orbit, and the gradual decrease
is caused by atmospheric drag. As can
be seen from the plot, the rate of descent is not constant and this variation is caused by
changes in the density of the tenuous outer atmosphere due mainly to solar activity.