The rise and fall of the ISS... in orbit, the station is constantly falling toward the earth. If not for the routine correction boosters, it would fall into the atmosphere and burn up. Fun thought for the astronauts aboard I'm sure...
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.
http://www.heavens-above.com/IssHeight.aspx