Transearth injection (TEI) is the burn that sends the spacecraft out of lunar orbit and back toward Earth.
- TEI is fired when the spacecraft's position and the Moon's motion give the best geometry for the return path.
- The burn adds velocity in the direction that, combined with the Moon's orbital velocity, produces a path that reaches Earth.
- Because the Moon's motion contributes, the burn itself can be smaller than it would be if the Moon were stationary.
- TEI is the last major burn of the mission. After it, the spacecraft coasts home under gravity, with only small midcourse corrections if needed.
- The return path is shaped by Earth's gravity: the spacecraft falls back toward Earth, arriving on a trajectory that reentry can handle.
Key idea: TEI is timed so the Moon's own motion helps carry the spacecraft home.