Aparking orbit is a temporary orbit used during the launch of aspacecraft. A launch vehicle follows a trajectory to the parking orbit, then coasts for a while, then engines fire again to enter the final desired trajectory.

An alternative trajectory that is used on some missions isdirect injection, where the rocket fires continuously (except during staging) until its fuel is exhausted, ending with the payload on the finaltrajectory. This technique was first used by the SovietVenera 1 mission to Venus in 1961.

Geostationary spacecraft require an orbit in the plane of the equator. Getting there requires ageostationary transfer orbit with an apogee directly above the equator. Unless the launch site itself is quite close to the equator, it requires an impractically large amount of fuel to launch a spacecraft directly into such an orbit. Instead, the craft is placed with an upper stage in an inclined parking orbit. When the craft crosses the equator, the upper stage is fired to raise the spacecraft'sapogee to geostationary altitude (and often reduce the inclination of the transfer orbit, as well). Finally, acircularization burn is required to raise theperigee to the same altitude and remove any remaining inclination.^[1]

In order to reach the Moon or a planet at a desired time, the spacecraft must be launched within a limited range of times known as alaunch window. Using a preliminary parking orbit before final injection can widen this window from seconds or minutes, to several hours.^[2][3] For theApollo program's crewed lunar missions, a parking orbit allowed time for spacecraft checkout while still close to home, before committing to the lunar trip.^[3]

The use of a parking orbit can lead to a number of technical challenges. For example, during the developmentCentaur upper stage, the following problems were noted and had to be addressed:^[4]

• The injection burn occurs underzero g conditions.
• If the same upper stage which performs the parking orbit injection is used for the final injection burn, a restartableliquid-propellant rocket engine is required.
• 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, orullage rockets to settle the propellant back to the bottom of the tank.^[5]
• Areaction 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.

The Centaur andAgena families of upper stages weredesigned for restarts and haveoften been used in missions using parking orbits. The last Agena flew in 1987, but Centaur is still in production. TheBriz-M is also capable of coasts and restarts, andoften performs the same role for Russian rockets.^[6]

• The Apollo program used parking orbits, for all the reasons mentioned above except those that pertain to geostationary orbits.^[7][8]
• When theSpace Shuttle orbiter launched interplanetary probes such asGalileo, it used a parking orbit to deliver the probe to the right injection spot.^[9]
• TheAriane 5 does not usually use parking orbits.^[10] This simplifies the launcher since multiple restart is not needed, and the penalty is small for their typicalGTO mission, as their launch site is close to the equator. A less commonly used second stage, theAriane-5ES has multiple restart capability, and has been used for missions such as theAutomated Transfer Vehicle (ATV) that use parking orbits.^[11] TheAriane 6 upper stage supports multiple restarts and can be used with missions that require parking orbits.
• In a literal example of a parking orbit, theAutomated Transfer Vehicle could park for several months in orbit while waiting to rendezvous with theInternational Space Station. For safety reasons, the ATV could not approach the station while a Space Shuttle was docked or when aSoyuz orProgress was maneuvering to dock or depart.^[12]

• Astrodynamics
• Spacecraft propulsion
• Orbits

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