From: Mike Powney, of Epsom, England
To: Mark Ferring, flight director

Question: Why is it necessary to have the Shuttle/Soyuz boost the orbit of the ISS. Does this mean that without the boost the ISS' orbit would decay?

Answer: Mike,

There are many things that affect orbital decay. However, the largest contributor to orbital decay is atmospheric drag. Atmospheric drag is dependent on the density of the atmosphere at the given orbital altitude. Atmospheric density is primarily affected by solar flux and the seasons -- so it is constantly changing. Even though ISS orbits more than 200 miles above the Earth, there is enough atmosphere up there to slow it down.

The way orbital mechanics work, you raise your orbit by executing a burn to speed you up. The reboost burns for the ISS can be executed by a visiting shuttle or progress vehicle and even by the engines on the ISS' Service Module. A typical reboost may raise your orbit 6 miles or so. The reboost strategy employed by ISS depends on ISS attitude changes, visiting vehicles, etc. Just like an airplane is more (or less) aerodynamic depending on the side that is facing "forward", ISS is more or less aerodynamic depending on which attitude it is in. Certain attitudes create more atmospheric drag than others.

Also, if a visiting vehicle, such as the Space Shuttle, is planning to rendezvous with ISS in the near future, ISS will be allowed to decay to a lower orbit so the Shuttle does not have to expend as much energy to rendezvous at a higher orbit. This allows for the Shuttle to haul up more payload mass. Finally, you are correct, this does mean that without these periodic reboosts, the ISS orbit would decay and eventually de-orbit.

Phil Curell
Operations Support Officer (OSO)