A Mars cycler (or Earth–Mars cycler) is a special kind of spacecraft trajectory that encounters Earth and Mars on a regular basis. The term Mars cycler may also refer to a spacecraft on a Mars cycler trajectory. The Aldrin cycler is an example of a Mars cycler.
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A cycler trajectory encounters two or more bodies on a regular basis. 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
Cycler trajectories between Earth and Mars occur in whole-number multiples of the synodic period between the two planets, which is about 2.135 Earth years. Among the first Earth–Mars cycler trajectories calculated were VISIT 1 and VISIT 2, with cycles repeating every 7 synodic periods or about 15 Earth years.
In 1985, Dr. Buzz Aldrin presented an extension of his earlier Lunar Cycler work which identified a Mars Cycler corresponding to a single synodic period. The existence of the now-eponymous Aldrin Cycler was calculated and confirmed by scientists at JPL later that year: a single eccentric loop around the sun from Earth to the Martian orbit in 146 days, spending the next 16 months beyond the orbit of Mars, and another 146 days from the Martian orbit back to Earth.
For each Earth–Mars cycler that isn't a multiple of 7 synodic periods, an outbound cycler intersects Mars on the way out from Earth while an inbound cycler intersects Mars on the way in to Earth. The only difference in these trajectories is the date in the synodic period in which the vehicle is launched from Earth. Earth–Mars cyclers with a multiple of 7 synodic periods return to Earth at nearly the same point in its orbit and may encounter Earth and/or Mars multiple times during each cycle. VISIT 1 encounters Earth 3 times and Mars 4 times in 15 years. VISIT 2 encounters Earth 5 times and Mars 2 times in 15 years.
Some possible Earth–Mars cyclers include the following:
The most comprehensive survey of Earth–Mars cycler trajectories (to date) was conducted by Russell and Ocampo.
Theoretical usage
Aldrin proposes a pair of Mars cycler vehicles providing regular transport between Earth and Mars. One cycler would travel an outbound route from Earth to Mars in about five months. Another Mars cycler in a complementary trajectory would travel from Mars to Earth, also in about five months. Taxi and cargo vehicles would attach to the cycler at one planet and detach upon reaching the other. Aldrin details use of such a system in his science-fiction book, Encounter with Tiber.