Trans Earth Injection

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Trans Earth Injection (TEI) is a term describing the propulsion maneuver used to set a spacecraft on a trajectory which will intersect the Earth.

The spacecraft is usually in a parking orbit around Moon at the time of TEI (Apollo 13 being the exception). Unmanned space probes have also performed this maneuver from the Moon starting with Luna 16's direct ascent traverse from the lunar surface in 1970. In 2004, from outside the Earth-Moon system, the Stardust probe comet dust return mission performed TEI after visiting Comet Wild 2.

The maneuver is usually performed by a rather large rocket engine. On the Apollo missions it was performed by the restartable Service Propulsion System (SPS) rocket. This particular TEI burn lasted approximately 203.7 seconds, on the Lunar backside, providing a posigrade velocity increase of 1076 m/s (3531 ft/s). It was first performed by the Apollo 8 mission on December 25, 1968.

The burn is timed so that the mid-point of the TEI is opposite the Earth (or more precisely, the Earth's location upon arrival).