Is it possible to establish a practical 'cycler' transportation system between Earth and Mercury? The concept involves a small, manned payload (7.5 mt or less) doing the Earth departure and Mercury Orbit Insertion burns. The crew inhabits the pre-deployed Cycler and, prior to Mercury encounter, converts water stored on the cycler into the propellants needed for the MOI burn.

Principle numbers for the concept are: Mercury's orbital period: 87.9 days; the Mercury-Earth synodic period: 115.9 days; the Earth's orbital period: 365.25 days and the cycler's orbital period. Several periods are possible, but I arbitrarily chose a 351.6 day orbit. This gives a transfer time (either way) of 175.8 days. Evidently, the match-up is not exact enough to prevent a larger delta-V than desired.

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    $\begingroup$ Would you accept a double cycler Earth to Mercury with conjunction at Venus? One problem with a direct Earth Mercury cycler that I can see is the orbit of Venus (close to half inclination to that of Mercury w.r.t. Earth) so its synodic period with Earth (about 8/5 years) and Mercury (about 6/16 years) should be considered too. One starting point: Planetary Moon Cycler Trajectories (PDF) $\endgroup$
    – TildalWave
    Commented May 1, 2014 at 15:49
  • $\begingroup$ Yes, subject to the magnitude of velocity change required at Mercury. If the cycler can put the manned vehicle in a position relative to Mercury that allows an MOI from the cycler's orbit with propellant mass at or below a 'conventional' Hohmann transfer. Thanks for the excellent link! I will definitely be reviewing it. ., . $\endgroup$ Commented May 1, 2014 at 16:09
  • $\begingroup$ Well for the cycler part ("Castle") that stays in the "free-return" trajectory, the ΔV requirements should be minimal (i.e. corrections) once the cycler orbit is established. But the "Taxi" that departs it and is later catching the cycler during one of its next legs the ΔV required would be similar to direct Hohmann transfer. So this then becomes an exercise in mass economy, how much of it can stay in cycler orbit and how much of it is required for the circularisation at Mercury. Delta-v will always stay the same, but the mass that needs to achieve it might be substantially smaller. $\endgroup$
    – TildalWave
    Commented May 1, 2014 at 17:30
  • $\begingroup$ Yes, delta-V stays the same. The difference is in the mass being accelerated. $\endgroup$ Commented May 1, 2014 at 17:33
  • $\begingroup$ At Earth, the payload being accelerated is just the crew module and the dry mass of the propulsion stage. At Mercury, the mass is exactly the same. The trick here (well, one of them. . .) is to have the MOI delta-V be at or less than the delta-V for the Earth departure - the propellants for this maneuver having been loaded from the cycler's supplies. My concern is if thye cycler's orbit meets Mercury's orbit and Mercury is not EXACTLY at the point where the cycler meets the orbit, , , Will the delta-V needed to make up the difference exceed the cycler's supply of propellant? $\endgroup$ Commented May 1, 2014 at 17:40

1 Answer 1


Usable Mercury cyclers exist.

Given the large inclination of Mercury, we want the encounter to happen along the line of apsides, thereby requiring a quasi-periodic stationary cycler. The simplest type of cycler in this family is a high periapse fly by ellipse, ideally close to a Hohmann transfer orbit. This requires that the planets' synodic period is a simple fraction of the orbital period of either of the planets. Furthermore, the orbital period of the cycler must be close to the previously stated quasi-period, divided by an integer.

Interesting quasi-periods include:

3, -17.36375
19, 10.02959
22, -7.33416
41, 2.69542

(Synodic periods, inaccuracy in degrees)

As for actual appearance of the planetary alignments, a usable launch window would in worst case appear within a decade in the Mercury-Earth system.

The 22 and the 41 synodic period cycler are the two most interesting. The 22 (7 year) orbit offers a trajectory suitable for reuse 4-5 times, at a relatively low delta-v cost. This is because the orbital period of 0.5385 years, compared to the 0.5327 years for a perfect Hohmann transfer to the ascending node, has a close to minimal vinf.

That is also the case for the even better 41 (13 year) orbit, where the same comparison is 0.61905 years to 0.61853 years for a descending node transfer. (The deviation from a Hohmann transfer is negligible.) A slight boost to a 0.63636 empty 7 year cycle every 46 years reduces the angular drift to 1.635 degrees per century, allowing stability for maybe as long as a millennium.

This type of cycler does not offer two-way transportation, so two cyclers are necessary. (One inbound, one outbound.)


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