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 is a special kind of spacecraft trajectory that encounters two or more bodies on a regular basis. Cyclers are potentially useful for transporting people or materials between those bodies using little or no propellant. Instead, they rely on gravity assist maneuvers to keep them going.
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, Buzz Aldrin theorized a so-called Aldrin Cycler corresponding to a single synodic period. The existence of such trajectories was calculated and confirmed 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.[1]
Some possible Earth-Mars Cyclers include the following:[1]
| Synodic Periods per Cycle | Solar Revolutions per Cycle | Time per Cycle | Aphelion Radius | Earth/Mars Transfer Time | Notes |
|---|---|---|---|---|---|
| 1 | 1 | 2.135 years | 2.23 AU | 146 days | Aldrin Cycler |
| 2 | 2 | 4.27 years | 2.33 AU | 158 days | |
| 2 | 3 | 4.27 years | 1.51 AU | 280 days | Aphelion inside semi-major axis of Mars orbit |
| 3 | 4 | 6.405 years | 1.89 AU | 189 days | |
| 3 | 5 | 6.405 years | 1.45 AU | 274 days | Aphelion inside semi-major axis of Mars orbit |
| 3 | 5 | 6.405 years | 1.52 AU | 134 days | Aphelion inside semi-major axis of Mars orbit |
| 4 | 5 | 8.54 years | 1.82 AU | 88 days | |
| 4 | 6 | 8.54 years | 1.53 AU | 157 days | Aphelion inside aphelion of Mars orbit |
| 5 | 4 | 10.675 years | 2.49 AU | 75 days | |
| 5 | 5 | 10.675 years | 2.09 AU | 89 days | |
| 5 | 6 | 10.675 years | 1.79 AU | 111 days | |
| 5 | 7 | 10.675 years | 1.54 AU | 170 days | Aphelion inside aphelion of Mars orbit |
| 5 | 8 | 10.675 years | 1.34 AU | 167 days | Aphelion inside semi-major axis of Mars orbit |
| 6 | 4 | 12.81 years | 2.81 AU | 87 days | |
| 6 | 5 | 12.81 years | 2.37 AU | 97 days | |
| 6 | 6 | 12.81 years | 2.04 AU | 111 days | |
| 6 | 7 | 12.81 years | 1.78 AU | 133 days | Requires minimal ballistic correction |
| 6 | 8 | 12.81 years | 1.57 AU | 179 days | Requires minimal ballistic correction |
| 6 | 9 | 12.81 years | 1.40 AU | 203 days | Aphelion inside semi-major axis of Mars orbit; Requires minimal ballistic correction |
The most comprehensive survey of Earth-Mars cycler trajectories (to date) was conducted by Russell and Ocampo[2].
Aldrin proposes a pair of Mars cycler vehicles providing regular transport between Earth and Mars.[3] 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.[4]
Aldrin details use of such a system in his science-fiction book, Encounter with Tiber.