Programs

Orbital Decay

At ~350-400 kilometers altitude, the ISS is above most, but not all, of the Earth’s atmosphere, and tenuous though it is at the Station’s height, some drag is produced as atoms collide with the ISS.

The Station’s orbit decays by a few hundred meters each 24 hours. The decay is variable as solar activity causes changes in the density of the outer atmosphere.

During a period of high solar activity (solar max) the density at any given altitude is higher (there is more energy put into the molecules in the atmosphere, so they spread apart) so at any altitude there were more molecules than there would be otherwise.

Periodic reboosts are thus necessary every few months, usually undertaken by a Progress cargo ship docked to the Russian segment, or a Shuttle Orbiter when docked to the ISS. The ESA Jules Verne ATV can also provide a reboost. The Japanese plan for a transfer vehicle as well.