A Lunar cycler or Earth–Moon cycler is an orbit, or spacecraft therein, which periodically passes close by the Earth and the Moon, using gravity assists and occasional propellant-powered corrections to maintain its trajectories between the two. If the fuel required to reach a particular cycler orbit from both the Earth and the Moon is modest, and the travel time between the two along the cycler is reasonable, then having a spacecraft in the cycler can provide an efficient and regular method for space transportation.
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History
Buzz Aldrin first suggested the use of cycler orbits in 1985 for the life support equipment and logistical supplies necessary for extensive manned exploration of the Moon and Mars. He also presented his thoughts on the benefits of using cycler spacecraft at the Space 88 Symposium in Albuquerque during a panel discussion on "Approaching the Construction Problems in Space." These trajectories must have the characteristic that they can be easily targeted for either launch or destination planet and that the times between encounters yield a reasonable stay time on the destination and provide for both routine and emergency return on a fairly regular basis.
Examples
One of the challenges for NASA's Interstellar Boundary Explorer (IBEX) satellite was to find a stable orbit in cislunar space, a region from outside the Earth's atmosphere to just beyond the Moon's orbit. The orbit in which the satellite was originally set could only be reliably predicted for about 3 years. Within cislunar space small changes in initial conditions could lead to very different trajectories years later. Because IBEX's original orbit was often closer to the Moon than the Earth, it was regularly performing gravity assists. The uncertainties in the initial orbit conditions would cause nearly identical trajectory predictions to eventually experience different gravity assist geometries, and the slightest change in that geometry during the encounter would cause huge differences in the subsequent satellite trajectory. Some predictions would hit the Earth and others would escape the Earth–Moon system. The new orbit keeps the apogee away from the Moon, reducing its sensitivity to gravity assists. IBEX can now predict this trajectory well beyond 11 years, allowing the satellite to easily observe the Sun's full solar cycle.