A Circumlunar trajectory, Trans-Lunar trajectory or Lunar free return is a type of free return trajectory

Free return trajectory

A free return trajectory is one of a very small sub-class of trajectories in which the trajectory of a satellite traveling away from a primary body is modified by the presence of a secondary body causing the satellite to return to the primary body...

 which takes a spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

 from Earth, around the far side

Far side of the Moon

The far side of the Moon is the lunar hemisphere that is permanently turned away, and is not visible from the surface of the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon...

 of the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, and back to Earth.

Background

The first spacecraft to fly a circumlunar trajectory was Luna 3

Luna 3

The Soviet space probe Luna 3 of 1959 was the third space probe to be sent to the neighborhood of the Moon, and this mission was an early feat in the spaceborne exploration of outer space...

. Circumlunar trajectories were also used by Apollo missions prior to lunar orbit insertion, to provide a free return to Earth in the event of a propulsion system malfunction on the way to the Moon. This was used on Apollo 13

Apollo 13

Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...

, when an oxygen tank rupture necessitated return to Earth without firing the Service Module engine, although a number of course corrections using the Lunar Module

Apollo Lunar Module

The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...

 were required to maintain this trajectory.

A number of manned missions were also proposed to intentionally conduct circumlunar flybys, including the Soviet Soyuz 7K-L1

Soyuz 7K-L1

The Soyuz 7K-L1 spacecraft was designed to launch men from the Earth to circle the Moon without going into lunar orbit in the context of the Soviet manned moon-flyby program in Moon race. It was based on the Soyuz 7K-OK with several components stripped out to reduce the vehicle weight...

 or Zond

Zond program

Zond was the name given to two distinct series of Soviet unmanned space program undertaken from 1964 to 1970. The first series based on 3MV planetary probe was intended to gather information about nearby planets...

 programme, and several US proposals, including Gemini-Centaur

Centaur (rocket stage)

Centaur is a rocket stage designed for use as the upper stage of space launch vehicles. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity...

and an early Apollo proposal.