Editing Spaceflight (section)

=== Leaving orbit ===
{{Main|Escape velocity|Parking orbit}}On some missions beyond [[Low Earth orbit|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 window]]s. The parking orbit gave the crew and controllers time to thoroughly check out the spacecraft after the stresses of launch before committing it for a long journey to the Moon.<ref name="lauwin">{{cite web |title=Apollo lunar landing launch window: The controlling factors and constraints |url=https://history.nasa.gov/afj/launchwindow/lw1.html |publisher=NASA}}</ref>[[Image:RIAN archive 510848 Interplanetary station Luna 1 - blacked.jpg|thumb|upright|Launched in 1959, [[Luna 1]] was the first known artificial object to achieve escape velocity from the Earth ''(replica pictured)''.<ref>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1959-012A |title=NASA – NSSDC – Spacecraft – Details |publisher=Nssdc.gsfc.nasa.gov |access-date=November 5, 2013}}</ref>]]

Robotic missions do not require an abort capability and require radiation minimalization only for delicate electronics, and because modern launchers routinely meet "instantaneous" launch windows, space probes to the Moon and other planets generally use direct injection to maximize performance by limiting the boil off of [[cryogenic propellant]]s. 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 as the distance from the body increases. 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]].<ref name="Escape Velocity of Earth">[http://van.physics.uiuc.edu/qa/listing.php?id=1053 Escape Velocity of Earth] {{Webarchive|url=https://web.archive.org/web/20070713212922/http://van.physics.uiuc.edu/qa/listing.php?id=1053 |date=2007-07-13 }}. Van.physics.uiuc.edu. Retrieved on 2011-10-05.</ref>