Trans-Lunar Injection
Trans-lunar injection is the engine burn or maneuver that sends a spacecraft out of Earth orbit and onto a trajectory toward the Moon.
What trans-lunar injection is
Trans-lunar injection is the maneuver that commits a spacecraft to a Moon-bound path. A spacecraft first reaches Earth orbit or a high Earth staging orbit, then performs a precisely timed burn that increases its speed and changes its path so it will encounter the Moon several days later.
Why timing matters
TLI is not just a matter of burning hard enough. The Moon is moving, Earth is rotating, and the spacecraft must leave at a geometry that brings it to the right place at the right time. A small timing or velocity error can become a large miss distance days later, so mission teams plan correction opportunities after TLI.
Apollo's Saturn V burn
During Apollo lunar missions, the Saturn V's S-IVB third stage performed the TLI burn after the spacecraft had been checked in Earth orbit. Apollo 8's approval for TLI in December 1968 was a historic step: it sent humans beyond Earth orbit for the first time.
Artemis and Orion
Modern Artemis missions use different hardware but the same basic concept. Artemis I used an upper-stage burn to send Orion toward the Moon, while Artemis II mission updates described Orion leaving Earth orbit after a translunar injection burn by its service module engine. The exact vehicle can change; the navigation job remains recognizable.
Connection to free return
A TLI burn can target a free-return trajectory, a landing-support trajectory, or another lunar transfer profile. Early Apollo planning valued free return because lunar gravity could bend the path back toward Earth if a major engine burn at the Moon failed. Later mission goals sometimes required leaving that passive-return path.
Midcourse corrections
After TLI, spacecraft normally have planned windows for small trajectory correction burns. These maneuvers clean up launch and burn errors, refine the lunar approach, and shape the return or orbit-insertion setup. TLI starts the lunar transfer, but it is rarely the last navigation action before the Moon.
Not the same as launch
Launch gets a spacecraft off Earth and into space; TLI sends it from an Earth-bound orbit or staging path toward the Moon. On some missions those events are separated by hours. The distinction matters because Earth-orbit checkout, crew readiness, engine performance, and mission rules can all be assessed before committing to the lunar leg.
Why it matters
Trans-lunar injection is one of the cleanest examples of orbital mechanics becoming a go-or-no-go mission event. It links launch performance, engine reliability, navigation, abort planning, and lunar arrival geometry. Without a successful TLI, a Moon mission remains close to Earth.