section_path: Phases/Leaving orbit
section_name: Leaving orbit
paragraph_text: Leaving orbit
  
  On some missions beyond LEO (Low Earth Orbit), spacecraft are inserted into parking orbits, or lower intermediary orbits. The parking orbit approach greatly simplified Apollo mission planning in several important ways. It acted as a "time buffer" and substantially widened the allowable launch windows. The parking orbit gave the crew and controllers several hours to thoroughly check out the spacecraft after the stresses of launch before committing it for a long journey to the Moon.
  
  Robotic missions do not require an abort capability or radiation minimization, and because modern launchers routinely meet "instantaneous" launch windows, space probes to the Moon and other planets generally use direct injection to maximize performance. Although some might coast briefly during the launch sequence, they do not complete one or more full parking orbits before the burn that injects them onto an Earth escape trajectory.
  
  The escape velocity from a celestial body decreases with altitude above that body. However, it is more fuel-efficient for a craft to burn its fuel as close as possible to its periapsis (lowest point); see Oberth effect.