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 (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 several spacecraft, most notably the Apollo 8, Apollo 10, and Apollo 11 lunar missions. Those spacecraft were launched into a free-return trajectory to allow their safe return in the event of a systems failure after launch. They all successfully inserted into orbit upon arriving at the Moon, and so did not take advantage of the free return. Due to the landing site restrictions that resulted from constraining the launch to a free return that flew by the Moon, subsequent Apollo missions, starting with Apollo 12 and including the ill-fated Apollo 13, used a hybrid trajectory that launched to a highly elliptical Earth orbit that fell short of the Moon with effectively a free return to the atmospheric entry corridor. They then performed a mid-course maneuver to change to a trans-Lunar trajectory that was not a free return. This retained the safety characteristics of being on a free return upon launch, and only departed from free return once the systems were checked out and the lunar module was docked with the command module, providing back-up maneuver capabilities. In fact, within hours of the accident, Apollo 13 used the lunar module to maneuver from its planned lunar orbit insertion trajectory to a free return trajectory. Apollo 13 was the only Apollo mission to actually complete a free return trajectory.

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". The satellite or spacecraft will not actually be captured by the secondary body without the use of its rocket engine to slow the craft.