<P> Use of a parking orbit requires a rocket upper stage to perform the injection burn while under zero g conditions . Often, the same upper stage which performs the parking orbit injection is used for the final injection burn, which requires use of a restartable liquid - propellant rocket engine . During the parking orbit coast, the propellants will drift away from the bottom of the tank and the pump inlets . This must be dealt with through the use of tank diaphragms, or ullage rockets to settle the propellant back to the bottom of the tank . A reaction control system is needed to orient the stage properly for the final burn, and perhaps to establish a suitable thermal orientation during coast . Cryogenic propellants must be stored in well - insulated tanks, to prevent excessive boiloff during coast . Battery life and other consumables must be sufficient for the duration of the parking coast and final injection . </P> <P> The Centaur and Agena families of upper stages were designed for such restarts and have often been used in this manner . The last Agena flew in 1987, but Centaur is still in production . The Briz - M stage often performs the same role for Russian rockets . </P> <Ul> <Li> The Apollo program used parking orbits, for all the reasons mentioned above except those that pertain to geostationary orbits . </Li> <Li> When the Space Shuttle orbiter launched interplanetary probes such as Galileo, it used a parking orbit to deliver the probe to the right injection spot . </Li> <Li> The Ariane 5 does not use parking orbits . This simplifies the launcher since multiple restart is not needed, and the penalty is small for their typical GTO mission, as their launch site is close to the equator . An upgrade to the second stage (ESC - B) will have multiple restart capability, so future missions may use parking orbits . </Li> <Li> In a literal example of a parking orbit, the Automated Transfer Vehicle (ATV) could park for several months in orbit while waiting to rendezvous with the International Space Station . For safety reasons, the ATV could not approach the station while a Space Shuttle was docked or when a Soyuz or Progress was maneuvering to dock or depart . </Li> </Ul> <Li> The Apollo program used parking orbits, for all the reasons mentioned above except those that pertain to geostationary orbits . </Li>

When is a satellite said to be in parking orbit