Lunar cycler
A Lunar cycler or Earth-Moon cycler is a periodic orbit that passes close to both bodies repeatedly. If the propellant required to reach a particular cycler from the Earth and from the Moon is modest and the transit time between the two bodies along the cycler is reasonable, then having a spacecraft on the cycler can provide an important component of an economical space transportation system. Cyclers may also be useful for navigation satellites (Satellite navigation).[1]
Cycler orbits are potentially useful for transporting people or materials using little propellant. Instead, they rely on gravity assist maneuvers to keep them going with occasional powered corrections (Orbital maneuver) to maintain the trajectory. These trajectories should 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.[2]
History
In 1985, Buzz Aldrin suggested the use of Cycler orbits (periodic trajectories that repeat the roundtrip transfer from home planet to destination planet) for the life support equipment and logistical supplies necessary for extensive manned exploration of the Moon and Mars (Mars cycler). 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.[3]
Example Lunar Cycler Programs
The IBEX challenge was to find an orbit that could be controlled for a long time. The original orbit could only be predicted for about 3 years. Cislunar space, which is the region outside Earth's atmosphere and extending out to just beyond the Moon’s orbit, including the Lagrangian points. Within this region, small changes in initial conditions can cause completely different trajectories years later. Because IBEX’s original orbit was often closer to the Moon than to the Earth, it was performing gravity assists regularly. The uncertainty in the initial conditions from orbit estimation knowledge and attitude re-pointing maneuvers would cause nearly identical trajectory predictions to eventually experience different gravity assist geometries, and the slightest change in that geometry during the encounter causes huge differences in the subsequent 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 and has reduced the sensitivities. IBEX can now predict this trajectory well beyond 11 years, so they can get the full Solar Cycle.[4]