TLI

Trans-lunar injection is a propulsive maneuver that accelerates a spacecraft from Earth orbit to a trajectory capable of reaching the Moon. The spacecraft performs a burn at a specific point in its orbit, typically gaining 3.2 to 3.3 kilometers per second of velocity, to break free from Earth's gravitational dominance and enter a lunar transfer orbit. Without TLI, a spacecraft remains bound to Earth and will eventually return to the atmosphere.

The timing and location of TLI are critical. The burn must occur when the spacecraft is positioned so that its resulting trajectory passes near the lunar sphere of influence, usually calculated to arrive at the Moon after three days of coast. Ground controllers perform detailed trajectory modeling before TLI authorization. Any significant error in velocity or attitude during the burn will either result in a miss or require costly mid-course corrections using additional fuel.

Variants and execution

TLI may be performed as a single continuous burn lasting several minutes, or as a series of shorter burns depending on the spacecraft's propulsion system and mission design. Crewed missions have historically executed TLI using a single major burn; unmanned cargo vehicles and lunar landers sometimes employ multiple smaller impulses. The maneuver requires sufficient fuel reserve to complete the burn and still maintain margins for contingency operations or orbital adjustments near the Moon.

The term originated with the Apollo program and remains standard in aerospace engineering and mission planning. It specifically describes injection into a trans-lunar trajectory rather than other orbital transfers. Related terms include lunar orbit insertion (LOI), which is the complementary burn executed at the Moon to enter lunar orbit, and trans-Earth injection (TEI), which sends a spacecraft home from lunar orbit.