What if we moved a large asteroid to a highly elliptical orbit between Earth and Mars and then occupied it like we occupy the ISS?

It'd be a slow trip to Mars, but if we're in no rush, we'd get free transit between Earth and Mars. Sort of like a super slow postal service.

Would make a good bridge between the two nation planets once we populates the red one. Would also give free resources, maybe water and some raw materials to build with.

Possibly some good shielding from harmful solar radiation, too, if you're cool with building your asteroid abode several feet underground to block out the energetic rays.

You could keep the asteroid and Mars both permanently occupied, and if something went super wrong here or there, there would be a chance that the asteroid would be coming around in a month or two, just in time to launch from their fly ships toward here or there and document what went wrong.

They wouldn't do much to help, but they'd maybe take over once it was safe to do so and help rebuild, I guess.

To clarify, my dumb idea is not to use an asteroid like a spaceship (launching from planet to planet with huge delta V costs)... but rather to put it in a very stable orbit so that it's there for thousands and thousands and thousands and thousands of years.

...The upfront cost would be earth-shattering, yes, but the results would be fairly permanent and whatever benefit we might get out of them, we'd enjoy... practically forever?

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    $\begingroup$ Have you ever tried to calculate how many Falcon Heavy's would be needed to move a large asteroid ? $\endgroup$
    – Cornelis
    Commented Jan 20, 2021 at 10:21
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    $\begingroup$ It wouldn't be "free transit" as you would still have to rendezvous with it, and if you do that your spacecraft will get to Mars in exactly the same time whether the asteroid is there or not. $\endgroup$
    – GordonD
    Commented Jan 20, 2021 at 10:48
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    $\begingroup$ Related, but not quite a duplicate: What uses would the Aldrin-Cycler have? $\endgroup$ Commented Jan 20, 2021 at 11:38
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    $\begingroup$ We are not able to move a large asteroid to a highly elliptical orbit between Earth and Mars. This movement is a lot orders of magnitude above the movements ever done in space. It would waste much more resources than we may ever get from the asteroid. There is no free lunch and there are no free resources. There is also no free transit in space. Hitchhiking may work on Earth but not in space. To enter the ferry will need as much energy as needed for entering a transfer orbit from Earth to Mars. $\endgroup$
    – Uwe
    Commented Jan 20, 2021 at 17:00
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    $\begingroup$ @a1ex It's a valid question, but maybe we just build a permanent comfortable "transfer station" rather than use an entire asteroid. But the orbits of Mars and Earth are asynchronous as would be an orbit with a semi-major axis equal to their average orbital radii. So unfortunately it seems impossible to have such a transfer station in the right place at the right time at the right velocity at both ends of the journey. $\endgroup$
    – Roger Wood
    Commented Jan 20, 2021 at 17:38

5 Answers 5


This sounds like the cycler concept, just using a modified asteroid for the cycler. The idea is that you have the cycler carry lots of heavy life support equipment so you don't have to accelerate and decelerate all this heavy equipment each trip. However, you'd still have to accelerate and decelerate the cargo to rendezvous with the cycler, as others have pointed out. And it's only useful for transporting people and other living things: for inert cargo (which doesn't require life support), you'd be better off using whatever propulsion method you have to send the cargo directly to the destination.

  • $\begingroup$ Oh wow. Yeah, that seems the same basic idea. I never heard about cyclers, clearly. I'll have to read up on them. Cyclers are in orbit between their two bodies permanently, without ever landing, and without need for delta V from thrusters (save for occasional minor corrections), right? $\endgroup$
    – user36480
    Commented Jan 20, 2021 at 22:07

TINSTAAFL. You still have all the delta-V to deal with at both ends of the trip. The expense of "man-rating" the asteroid would be overwhelming in itself.

  • $\begingroup$ Sure! It would be the most expensive undertaking ever. I have no doubt. But if you put the asteroid in a relatively stable orbit, the expense of keeping it there would be minimal. And if the asteroid was large, you'd have lots of surface area to build on and lots of raw materials to build with. I'm not saying any of this makes sense, because I frankly don't know, but this wouldn't be like a car that breaks down just ten years after you first bought it. Sure, your upfront cost would be huge, but you'd get tens of thousands of years of asteroid ferrying? $\endgroup$
    – user36480
    Commented Jan 20, 2021 at 21:00

The orbit of such a ferry is restricted by Kepler's laws for the movements of planets and asteroids in the solar system. You can't select orbital parameters in contradicition to these Laws.

If you select transits between the ferry and Earth and Mars, you get a certain orbital period of the ferry. This period is different to the periods of Earth and Mars and the period can't be changed in the future.

If you got a pleasant transfer schedule from Earth to Mars in a certain year, you have to wait very long for the next possible transfer. All the periods of Earth, Mars and the ferry are fixed and are different. If there is no transfer possible in the next centuries there is nothing else possible than waiting.

So you get a ferry that can't be used most of the time.


One would most likely want to lasso a water-ice comet like 238P. The H2-O2 fueled rockets would carry nuclear reactors, water mining equipment, and H2O splitter. They would plug into the "gas tank" comet and use their engines to redirect the comet to Mars for a slingshot to earth. Then they could have more infrastructure ready to join near earth before another slingshot to mars. It's all cycler from there, with shuttles to join and leave the convoy.

Unlimited air, fuel, and hot showers would be a nice way to travel!


It's certainly feasible once we get adequate space infrastructure to convert an asteroid or two into a ferry between Earth and Mars. Such a ferry could be a mobile spacestead (a spacestead is essentially a large scale space station that rotates to simulate gravity and is outfitted like an environment on Earth), with a smaller version of the O'Neill cylinder design being the most practical for such purposes. The good news is that there's at least a few relatively large asteroids in close proximity to Earth and Mars (like within a few million kilometres), that would just need to be hollowed out, covered with a mesh, and have engines mounted on them.

  • $\begingroup$ It might be worth doing some maths on how much fuel is required move an mass inefficient asteroid into a Cycler orbit en.wikipedia.org/wiki/Mars_cycler and if that is less than just boosting a dedicated craft from earth $\endgroup$ Commented Mar 30 at 3:06
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    $\begingroup$ This answer came up in my review queue. I suggest either making a stronger case to support the statement "It's certainly feasible..." or making a more considered assertion, such as "Some aspects of this idea may be feasible once..." $\endgroup$
    – phil1008
    Commented Mar 30 at 6:26
  • $\begingroup$ You don't have enough evidence to support your claim. If it's so feasible, you ought to prove it. $\endgroup$ Commented Apr 3 at 17:07

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