Cycler

For a laboratory apparatus used to amplify segments of DNA, see Thermal cycler.
Sketch of a circumlunar free return trajectory (not to scale), plotted on the rotating reference frame rotating slightly faster than a sidereal month.

A cycler is a potential spacecraft on a closed transfer orbit that would pass close to two celestial bodies at regular intervals. Cyclers could be used for carrying heavy supplies, life support and radiation shielding.