Trans-Lunar Injection (TLI)

A Trans-Lunar Injection, or TLI, is the engine burn that flings a spacecraft out of orbit around Earth and onto a path toward the Moon. It is the moment a mission stops circling home and starts the journey across.

Quick facts

• Starting point: a low circular “parking orbit” about 185 km up, moving at roughly 7.8 km/s (Apollo, inclined about 32.5 degrees, circling once every about 88 minutes).
• Burn power needed (delta-v): about 3.05 to 3.25 km/s for a fast Apollo-style direct transfer from low Earth orbit. Slower “low-energy” routes can cut the injection burn to roughly 1.1 to 1.3 km/s, but only when they start from a much higher orbit; leaving from the same low parking orbit takes a similar 3.2 km/s either way.
• Speed after the burn: about 10.4 km/s relative to Earth, just under Earth’s escape velocity of about 11.2 km/s, so the craft stays bound to Earth on a long ellipse rather than flying off for good.
• Apollo burn time: about 350 seconds on the S-IVB stage’s J-2 engine (about 1,033 kN of thrust).
• Travel time: about 3 days for a direct trajectory; weeks to months for low-energy transfers.
• How far it reaches: the orbit’s far point stretches out near the Moon’s distance of about 384,400 km.

How it works

A spacecraft first settles into a low, nearly circular parking orbit and coasts at about 7.8 km/s. At a carefully chosen instant, the upper-stage engine fires for several minutes. That extra speed does not lift the craft into a higher circle; instead it stretches the orbit into a long, lopsided ellipse, an egg-shaped path whose far end now reaches almost all the way to the Moon.

Think of swinging a ball on a string overhead, then suddenly letting out far more string on one side. The ball swings out much farther before curving back. The TLI burn does the same to the orbit: it pushes the high point (the apogee) out from a few hundred kilometers to nearly the Moon’s distance. The craft then coasts uphill against Earth’s gravity, slowing as it climbs, until it gets close enough that the Moon’s gravity takes over. The burn is timed at the point in the parking orbit opposite the target, so the craft arrives where the Moon will be. A TLI can also be shaped as a “free-return trajectory,” looping behind the Moon so lunar gravity swings the craft back toward Earth with no further burns.

Why it’s used

Any spacecraft going from Earth orbit to the Moon needs a TLI. Reaching a parking orbit first gives controllers time to check systems, fine-tune the path, and wait for precise alignment with the Moon before committing to the trip. For crewed missions, the free-return version adds a vital safety margin: if the spacecraft cannot brake into orbit around the Moon, the trajectory still carries the crew home on its own. Robotic missions can instead start from a high orbit and take a low-energy route, trading a much longer flight for a real propellant saving.

Notable missions

• Apollo 8, 10, and 11: the Saturn V’s S-IVB stage and J-2 engine performed TLI onto free-return trajectories to the Moon.
• Luna 1 (1959): the first TLI attempt, which missed the Moon, followed by Luna 2 (1959), the first successful lunar impact.
• Lunar Reconnaissance Orbiter (LRO) and GRAIL: NASA robotic missions injected toward the Moon via TLI.
• Chang’e 1 and Chandrayaan-2: Chinese and Indian lunar orbiters that used trans-lunar injection.
• Artemis I and Artemis II: modern crewed-capable lunar flights. On Artemis I, the SLS rocket’s Interim Cryogenic Propulsion Stage fired the TLI burn for the uncrewed Orion. On Artemis II, that upper stage raised Orion’s orbit and then separated, so Orion’s own service-module main engine performed the translunar injection.

TLI is time-critical work: the launch window and burn must match the Moon’s position closely, and any small error has to be cleaned up with mid-course correction burns. Get it right, and a few minutes of engine fire turns a craft circling Earth into one bound for the Moon.