Free return trajectory

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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 (for example, the Earth) is modified by the presence of a secondary body (for example, the Moon) causing the satellite to return to the primary body. This method has been used by spacecraft and was somewhat popularised by its use during the Apollo 13 mission, though this trajectory was actually a hybrid, requiring some minor adjustments. If performed correctly, a true free return trajectory is completed without the assistance of any mid-course corrections or maneuvers. The free return trajectory allows a great distance to be covered relatively directly without the use of additional fuel to return the satellite to the primary body, thus the term free. It is important to note that the satellite or spacecraft will not actually be captured by the secondary body without the use of its rocket engine to slow the craft.