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An Aldrin-Cycler is a vessel on an orbit on which it passes both Earth and Mars every few years without expending much fuel.

Aldrin cycler orbit

At first glance this seems like a great thing for a permanent transportation system between Earth and a future Mars colony. But when you look closer at it you notice that it zooms past Earth and Mars with a quite high relative speed. Any payload which is supposed to travel with it would first need to accelerate a lot to rendezvous with the cycler and then, at the destination, decelerate a lot to rendezvous with the planet. The energy required for that would likely be higher than for a usual hohmann-transfer from one planet to the other.

So what practical applications would an aldrin-cycler have?

(image source: http://ccar.colorado.edu/asen5050/projects/projects_2008/shupe_report/shupe_report.html)

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You are correct that it'd take more energy. Mars Vinf for a Hohmann is around 2.7 km/s. Mars Vinf for the Aldrin Cycler is almost 10 km/s. And the Aldrin Cycler orbit isn't exactly low maintenance. It's line of apsides must be rotated 50 degrees each orbit. The eccentricity and inclination of Mars' orbit also complicates things. –  HopDavid Jun 10 at 6:46

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Imagine a large vehicle that can recycle water and air, has shielding, and contains everything for a trip that would take months. This would be the cycler. The vehicle is heavy and is needed for a long trip; crew cannot survive in a small rocket.

The benefit is that when you launch your crewed rocket to catch up with the cycler, you only need to accelerate the crew and food. So the rocket is very small and cost would be much smaller that accelerating the whole cycler every time.

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One disadvantage of cycler orbits is the necessity to perform rendezvous at the very first attempt. –  Deer Hunter Mar 4 at 7:47
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@DeerHunter I don't think that is an issue. If your rocket fails you have problems either way. You can have more that one cycler in the loop, with a small delay. The last one would be an emergency. –  this Mar 4 at 11:28

One good thing about these periodic trajectories / interplanetary orbits, be it Mars Cyclers, Earth-Moon Cyclers (gravity assist UP/DOWN Escalator Orbits), or Resonant Cyclers (fixed VISIT Orbits) is that they can be maintained with a relatively small penalty for the total mass of the spacecraft, so they could be whole industry complexes processing raw goods for later delivery to the planets or moons they pass along their route, while using processing byproducts and remains for reaction mass, shielding, construction materials, or life support.

Since mass comes at a small expense, spacecraft in circulating orbits in inner Solar system where sunlight is sufficient might as well maintain whole greenhouses and gardens for oxygen and food requirements of their inhabitants. This advantage in maintaining stable orbits of large mass bodies cheaply is also briefly described in Circulating transportation orbits between earth and Mars, Friedlander et al., SAIC / JPL (PDF):

Circulating orbits share one potential advantage for manned transportation between Earth and Mars. They allow a large orbiting facility (herein called a "CASTLE") providing all power, living and work space, life support, gravity environment, and solar storm shelter to be "once-launched", thereby obviating the need to carry these massive elements repeatedly through large planetocentric ΔV maneuvers. Transportation to and from these CASTLES is carried out by smaller Space Taxis using hyperbolic rendezvous techniques.

Additionally, many of these trajectories also extend far beyond the visiting planets, several AU past Mars and into the Asteroid belt, where raw materials they would process could come from. Granted, these would still have to be delivered to cyclers while maintaining their momentum, but they would then have a long time for any needed trajectory corrections, and still deliver processed goods in time.

Alternatively, since there's small to no mass penalty assuming no loss of momentum during boarding, they could simply serve as a taxi service for interplanetary travel, providing temporary habitat with all the infrastructure required for it. Trajectories could also be maintained with large solar sails that don't use any consumables, so they could be viable means of propulsion for a very long time.

Together with Interplanetary Transport Network, all of these could serve as a low-maintenance and cheap connecting transport routes for, say, space mining industry, processing plants, space laboratories, personnel transport, and so on.

Suggested further reading:

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Any payload which is supposed to travel with it would first need to accelerate a lot to rendezvous with the cycler and then, at the destination, decelerate a lot to rendezvous with the planet.

There is some work that has been done to find optimal cycler orbits, check this work. Some of the computed orbits reduced the delta-V for inbound taxis almost to 5.9 Km/sec. Not enough to be comparable to a normal orbital transfer, but the report doesn't suggest this is a lower bound, so it could be improved in principle.

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Please feel free to add more information from the link posted to avoid link rot and to make life easier for the readers. Cheers! –  Deer Hunter Jun 17 at 7:47

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