Editing Trans-lunar injection (section)

== Free return ==
{{main|Free return trajectory}}

[[File:circumlunar-free-return-trajectory.png|thumb|Sketch of a circumlunar free return trajectory (not to scale)]]
In some cases it is possible to design a TLI to target a [[free return trajectory]], so that the spacecraft will [[Loop quantum gravity|loop]] around behind the Moon and return to Earth without need for further propulsive maneuvers.<ref>{{cite book |last=Schwaninger |first=Arthur J. |url=https://ntrs.nasa.gov/api/citations/19630007117/downloads/19630007117.pdf |title=Trajectories in the Earth-Moon Space with Symmetrical Free Return Properties |publisher=[[NASA]] / [[Marshall Space Flight Center]] |year=1963 |series=Technical Note D-1833 |location=Huntsville, Alabama}}</ref>

Such free return trajectories add a margin of safety to [[human spaceflight]] missions, since the spacecraft will return to Earth "for free" after the initial TLI burn. The Apollos 8, 10 and 11 began on a free return trajectory,<ref>{{Cite web |last=Mansfield |first=Cheryl L. |date=May 18, 2017 |title=Apollo 10 |url=http://www.nasa.gov/mission_pages/apollo/missions/apollo10.html |website=NASA}}</ref> while the later missions used a functionally similar hybrid trajectory, in which a midway course correction is required to reach the Moon.<ref>{{Cite web |title=APOLLO 12 |url=https://history.nasa.gov/SP-4029/Apollo_12a_Summary.htm |website=history.nasa.gov}}</ref><ref>{{cite report |url=http://www.esa.int/esapub/bulletin/bullet103/biesbroek103.pdf |title=Ways to the Moon |page=93}}</ref><ref>{{Cite web |title=Launch Windows Essay |url=https://history.nasa.gov/afj/launchwindow/lw1.html |website=history.nasa.gov}}</ref>