|MadSci Network: Astronomy|
I cannot find a formal, written policy that is available on-line. The procedure is not particularly complex, though.
There are four active orbiters. Just like your family car, each has a maintenance cycle including times for major overhauls. As missions are planned the availability of the four shuttles is compared to the desired launch date. Normally only one shuttle is available to meet the launch date, and that one is selected.
In some cases, there is a need for a special capability unique to a certain shuttle. For example, for a time the Endeavor, the newest shuttle, could do flights lasting almost one week longer than the other three shuttles. Flights where the experiments benefited from longer time in space, i.e., efforts to grow protein crystals, used the Endeavor until the others were retrofitted and upgraded. The mission and orbiter would be scheduled around the availability of the orbiter. F
Try the links in the MadSci Library for more information on Astronomy.