Mars cycler A Mars cycler is an elliptical orbit (green) that crosses the orbits of Earth (blue) and Mars (red), and encounters both planets at the points where it crosses their orbits, although not necessarily on every orbit. (Not to scale.) A Mars cycler (or Earth–Mars cycler) is a kind of cycler, a spacecraft with a trajectory that encounters Earth and Mars regularly. The Aldrin cycler is an example of a Mars cycler. Cyclers are potentially useful for transporting people or materials between those bodies using minimal propellant (relying on gravity-assist flybys for most trajectory changes) and can carry heavy radiation shielding to protect people in transit from cosmic rays and solar storms. ## Earth–Mars cyclers A cycler is a trajectory that encounters two or more bodies regularly. Once the orbit is established, no propulsion is required to shuttle between the two, although some minor corrections may be necessary due to small perturbations in the orbit. The use of cyclers was considered in 1969 by Walter M. Hollister, who examined the case of an Earth–Venus cycler. Hollister did not have any particular mission in mind, but posited their use for both regular communication