A parking orbit is a temporary orbit used as a staging point: a launch vehicle or spacecraft coasts there briefly before the burn that sends it to its real destination. Pausing in low orbit decouples launch timing from departure geometry — you launch when the pad is ready, then depart when physics is.
Understanding Parking Orbit
Why pausing beats launching on time
A departure burn must happen at a specific point on a specific orbit — for an interplanetary probe, the point where the burn's direction matches the required escape asymptote. Without a parking orbit, that point must be reachable directly from the pad at one instant; with one, the problem splits cleanly in two. Launch merely has to achieve the parking orbit (its plane set by lift-off time and azimuth), and the subsequent coast turns the timing problem into a waiting problem: the right departure point comes around every revolution. The technique buys minutes-to-hours of daily launch window where a direct ascent might have seconds, at the modest price of an upper stage that can restart in microgravity after a long coast — itself a non-trivial engineering requirement that shaped stage design from the earliest lunar missions onward.
Parking orbits and the junk problem
Every parked stage is a disposal decision waiting to be made. The classic failure mode litters the record: stages that delivered their payloads and died in medium or elliptical parking orbits persist for decades as large, unmanoeuvrable debris — prime targets for fragmentation, since they often retain residual propellant. Modern mitigation practice attacks this from both ends: park low (a 180 km parking orbit self-cleans within days), vent tanks and batteries (passivation) so a dead stage cannot explode, and where performance allows, reignite one last time for a controlled disposal. The parking orbit's virtue — being temporary — only holds if someone makes it so.