Atrans-Earth injection (TEI) is a propulsionmaneuver used to set aspacecraft on atrajectory which will intersect theEarth'ssphere of influence, usually putting the spacecraft on afree return trajectory.
The maneuver is performed by arocket engine.
The spacecraft is usually in aparking orbit around theMoon at the time of TEI, in which case the burn is timed so that its midpoint is opposite the Earth's location upon arrival. Uncrewed space probes have also performed this maneuver from the Moon starting withLuna 16's direct ascent traverse from the lunar surface in 1970.
On theApollo missions, it was performed by the restartable Service Propulsion System (SPS) engine on theService Module after the undocking of the (LM)Lunar Module if provided. An Apollo TEI burn lasted approximately 150 seconds, providing a posigrade velocity increase of 1,000 m/s (3,300 ft/s). It was first performed by theApollo 8 mission on December 25, 1968.[1] It was last performed by the propulsion module ofChandrayaan-3 mission during 13 October 2023[2]
Total 17 missions have performed such a maneuver.NASA has performed it the most (10 times), followed by Soviet Union (3 times), China (3 times), and India (once). These missions are in order,
In 2004, from outside the Earth-Moon system, theStardust probe comet dust return mission performed TEI after visitingComet Wild 2.
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