Trans-Earth Injection
Trans-Earth injection is the maneuver that sends a spacecraft away from the Moon or another body onto a return trajectory toward Earth.
What trans-Earth injection is
Trans-Earth injection is a propulsion maneuver that places a spacecraft on a path back toward Earth. In Apollo-style lunar missions, it was the burn that took the command and service module out of lunar orbit and into the trans-Earth coast phase. The maneuver is the return-leg counterpart to trans-lunar injection.
Why leaving lunar orbit takes a burn
A spacecraft in lunar orbit is bound to the Moon. To return home, it must change speed and direction enough to leave that local orbit and enter a trajectory dominated increasingly by Earth's gravity. The burn must be timed so the spacecraft departs from the right point in lunar orbit and reaches Earth at the right angle.
Apollo's service propulsion system
On Apollo lunar landing missions, the command and service module performed TEI with the service propulsion system after the lunar module had returned, docked, and been jettisoned. The burn was one of the mission's major engine events because it committed the crew to the Earth-return path.
The trans-Earth coast
After TEI, the spacecraft did not arrive instantly. It coasted for days while its trajectory was tracked from Earth. Mission controllers and the crew could perform midcourse corrections to refine the return path, manage landing-zone targeting, and keep the command module within the safe reentry corridor.
Connection to free return
A free-return trajectory can bring a spacecraft back toward Earth without a major burn at the Moon, but a lunar-orbit mission is different. Once a spacecraft enters lunar orbit, it normally needs a TEI burn to leave. That is why the reliability of the lunar-orbit departure engine was so important in Apollo planning.
Reentry corridor
TEI must set up the later Earth entry conditions. If the return angle is too shallow, a spacecraft can skip out of the atmosphere or miss the intended corridor. If it is too steep, heating and loads can exceed safe limits. The burn and later corrections therefore aim for a survivable, recoverable splashdown path.
Modern return planning
The phrase is still useful beyond Apollo. Lunar orbiters, crewed lunar missions, and some sample-return profiles must all solve the same broad problem: leaving the vicinity of another body and targeting Earth. The hardware and trajectory style can change, but the navigation logic remains familiar.
Why it matters
Trans-Earth injection is the moment a lunar mission turns homeward. It connects engine reliability, navigation, crew safety, tracking, thermal protection, and recovery planning. A Moon mission is not complete when it reaches the Moon; it also has to execute the return path precisely.