Lunar Transfer Orbit

A Lunar Transfer Orbit, or LTO, is the path a spacecraft rides from orbit around Earth all the way out to the Moon. It is a long, stretched-out loop, set up by a single well-timed engine burn, that lets a craft coast most of the way under gravity alone.

Quick facts

• Starting point: a low Earth “parking orbit,” typically about 185 km up (circular), moving at roughly 7.8 km/s.
• The setup burn: called Trans-Lunar Injection (TLI) — adds about 3.05 to 3.25 km/s of delta-v (the engineering term for a change in speed).
• New speed: climbs from ~7.8 km/s in low orbit to about 10.4 km/s relative to Earth (roughly 37,000 km/h) right after the burn.
• Burn length: about 350 seconds — roughly 6 minutes — for Apollo and the crewed Artemis II flight.
• Far point (apogee): reaches out near the Moon’s orbital distance, about 380,000 to 400,000 km.
• Travel time: about 3 days for a fast transfer (2 to 5 days). Fuel-saving “low-energy” routes take 3 to 6 months.

How it works

A spacecraft first circles Earth in a low parking orbit at about 7.8 km/s. To reach the Moon, it must raise the far side of its orbit — the apogee, or high point — out to roughly the Moon’s distance. At exactly the right moment, the engine fires forward (prograde) for about 350 seconds, adding around 3.1 km/s. This is the Trans-Lunar Injection. The push stretches the near-circular orbit into a long, thin ellipse whose high point almost touches the Moon’s orbit.

From there the craft coasts “uphill,” trading speed for height and slowing as it climbs — much like a ball thrown skyward slows as it rises. The timing is arranged so the Moon arrives at the meeting point just as the spacecraft does. Near the Moon, lunar gravity takes over: a braking burn called Lunar Orbit Insertion settles the craft into orbit, or a “free-return” geometry uses the Moon to swing it back toward Earth with no major burn at all.

There is also a slower option. Low-energy transfers loop far out toward the Sun-Earth L1/L2 region and lean on the combined pull of Earth, Moon, and Sun — a zone called the weak stability boundary — so the craft is captured with little or no extra delta-v, at the price of a months-long flight.

Why it’s used

The LTO is the standard, fuel-efficient way to send both crewed and robotic spacecraft to the Moon. A direct flight under constant engine power would burn an enormous amount of propellant, so missions instead use one well-timed TLI burn and then coast under gravity. The fast, roughly 3-day transfer is preferred when time, crew life support, or a narrow launch window matters, as with Apollo and Artemis.

Low-energy, ballistic-capture transfers are chosen when propellant is scarce and the schedule is flexible — small probes and solar-electric craft — because they cut the lunar-orbit-insertion delta-v by roughly 18% compared with a standard transfer. Free-return versions are favored for crewed flights because, if the insertion engine fails, the Moon’s gravity automatically returns the vehicle to Earth, giving the crew a built-in safety abort.

Notable missions

• Apollo 8/10/11 and the crewed Apollo program (1968-1972): fast ~3-day transfers, with TLI fired by the Saturn V’s S-IVB stage in a burn of about 350 seconds; the early missions used free-return or hybrid trajectories for crew safety.
• Artemis I (2022): the uncrewed Orion was sent on its lunar transfer by an Interim Cryogenic Propulsion Stage (ICPS) burn of about 18 minutes, then flew out to a distant retrograde orbit around the Moon rather than a free-return path.
• Artemis II (2026): the crewed Orion was placed on a free-return trajectory by its own European Service Module main engine (a refurbished Space Shuttle engine), in a TLI burn of about 350 seconds that built its speed up to roughly 25,000 km/h, per ESA.
• Hiten (Japan, 1991): first demonstration of a low-energy, weak-stability-boundary transfer, taking about 5 months.
• GRAIL (NASA, 2011): twin lunar-gravity mappers that used a low-energy route via the Sun-Earth L1 region, taking about 3 to 4 months.
• CAPSTONE (2022) and Danuri/KPLO (2022): reached the Moon on low-energy ballistic-lunar-transfer paths using minimal propellant.

The core trade-off is always speed versus fuel: a fast transfer gives precise, predictable timing but demands large burns and a tight launch window, while a low-energy route saves propellant at the cost of a months-long, more complex journey. Choosing between them is one of the first big decisions in planning any trip to the Moon.