The remainder of this chapter simply touches upon a few of them. Budget Trajectories are constrained by the laws of celestial mechanics, but the realities of budgets constrain the desires and needs of project science to determine the final choices. This task of balancing the monetary, the political and the physical is ordinarily resolved before most project personnel are assigned. The purpose, scope, timing and probable budget for a mission must be clearly understood before realistic spacecraft design can be undertaken. But even a final, approved and funded design may be altered when assumed conditions change during its lifetime.
Design changes are always costly.
The Galileo mission design, for example, underwent many significant and costly changes before it was finally launched. The Space Station Freedom spent tens of billions of dollars over several years prior to having comprehensive design changes imposed and becoming the International Space Station . Resource Contention Timing for many JPL missions is affected most directly by solar system geometry, which dictates optimum launch periods. If possible, it is very advantageous to fly a mission toward an area where the spacecraft will share little or none of its viewperiod with other missions. Such a study can assist project management in the selection of launch date and mission profile with the least contention for external resources, and maximized science return for the mission.