# Could a spacecraft be put into such an orbit that it would alternately slingshot around Mars and the Earth?

I was wondering if a spacecraft could be put into such an orbit that it would alternately slingshot around Mars and the Earth. This way it it would be making a continuous trip from Earth to Mars and back and repeat.

My motivation for wondering about this is to have a large spacecraft or even space station performing a bit like the Lunar cycler, which "...can provide an efficient and regular method for space transportation..." but of course the orbital mechanics would be very different in this case.

• This is an interesting question! I've edited your wording and added a reference to the Lunar cycler to help explain what you are after. The mass doesn't matter so much as far as the orbit is concerned, in other words, a 1 ton and a 100 ton spacecraft would perform almost identically (once you get them into the same orbit). The hard part is the repeatability of the orbit, so I've focused your question on that part. Welcome to Stack Exchange! – uhoh Jun 3 '18 at 11:59
• I've seen claims although no details, that you can do this if you also slingshot Venus so you get av three way cycle – Steve Linton Jun 3 '18 at 12:00
• – JCRM Jun 3 '18 at 13:59
• Lacking familiarity with the program known as Kerbal Space Program, I have to ask if such a program would be configurable to answer the question posted? – fred_dot_u Jun 3 '18 at 16:41

As mentioned, these orbits are known as orbital cyclers or cycler trajectories and (in theory) can be found between any two bodies. They are orbits that closely approach the two bodies at regular interval without requiring large trajectory adjustments.

In fact, there will be many possible cycler trajectories for two given bodies, determined by integer multiples of their synodic period - the time between subsequent conjunctions.

For the Earth-Mars system, the most famous cycler is the Aldrin Cycler (Dr. Buzz Aldrin of Apollo fame), which corresponds to a synodic period of 1 (approximately 2.135 earth years).

An Earth-Mars cycler may be particularly useful for manned interplanetary travel. Heavy equipment such as life-support, radiation shielding and accommodation can be left on the ‘cycling’ craft and a smaller, lighter vessel can be used at either end of the journey to decrease relative velocity and enter orbit around the planet.

See this question for more detail on the usefulness of this system.

• Does that mean the cycler actually comes close to Mars on both intersections of their orbits (intersections of red and blue lines)? Or will they "meet" only every 2.135 earth years? – Everyday Astronaut Jun 4 '18 at 12:31
• @derwodamaso Yes, the simpler cyclers like the one here only meet each planet once per cycle. Cyclers can be inbound or outbound, depending if the outer planet is encountered while decreasing or increasing in orbital altitude – Jack Jun 4 '18 at 12:56
• It is important to note that these cyclers are not free return. Buzz came to my office in 1996 to try to sell me on using cyclers somehow for Mars Sample Return. He said that they were free return, based on work by his trajectory guy. Something seemed fishy there, so we at JPL looked it, and, long story short, found out that his guy was not checking the Mars swingby distance against the radius of the planet! To make cyclers free, you would need a substantially subterranean (subareanean?) closest approach of Mars, which is problematic. – Mark Adler Jun 4 '18 at 16:42
• @MarkAdler That's a fantastic story! Do you know if the same is true for the Earth swingby or is it a unique problem with Mars' mass/size? Could a body with higher surface gravity allow for free return? Either that or simply attach a frictionless tunnel boring machine to your craft I suppose... – Jack Jun 4 '18 at 18:37
• "Free return" means no or very little $\Delta V$ required each time to get back to Earth, back to Mars, back to Earth, etc. It has nothing to do with stability. A cycler orbit would always be considered "unstable", in the sense that it requires precise maneuvering to hit each swingby target in order to maintain the cycle. – Mark Adler Jun 4 '18 at 21:35