If we pushed Mercury out of orbit for a close fly by of Venus in order to put Mercury in roughly Venus's orbit while flinging Venus to swing by Mars. Which will pull Mars's orbit a little closer to the Sun, after restabilizing with asteroid fly bys to recreate a more circular orbit, while sending Venus to Jupiter. Where Venus exchanges orbits with Callisto and sends Callisto back to Mars to get captured as a moon.

Now add volatiles to Mercury, measure how much more sunlight and dynamo/magnetic field Mars has, check Callisto for volatiles released during new placement and also more sunlight below the snow line, and wait for Venus to cool down and stabilize at Earth like temperatures.

Jumping past the extreme difficulties of pushing Mercury out of orbit, are there any issues making this scenario infeasible and would it make all affected bodies habitable for life, with more energy for the oceans underneath Callisto's ice, a larger magnetic field for Mars, Mercury, Callisto and possibly Venus, closer to Earth like temperatures for all bodies involved vs. their previous temperatures, closer to Earth like atmospheric pressure (except probably Venus), etc.

  • $\begingroup$ You HAVE to explain 'more' habitable. There is habitable and there is not habitable. Either you die or you don't. You can't be "more alive". Yes, under some circumstances live is easier than under others, but shuffling planets around will not make life easier, life on those planets would still be just as impossible on the open. Some bacteria from earth might survive on Mars btw. And in a building humans could survive on Mars, see a convincing display in the horrible movie "the Martian". So on top of explaining 'more', you even have to specify "otherwise" and 'habitable' $\endgroup$
    – Raditz_35
    Jul 13, 2017 at 15:37
  • $\begingroup$ No, it would NOT "make all affected bodies 'more' habitable for life". $\endgroup$
    – Joe
    Jul 14, 2017 at 13:39

2 Answers 2


I have absolutely no idea how this game of planetary billiards is supposed to work (pushing Mercury out to Venus would push Venus out to Mars?), but once everything shakes out, assuming I understand it correctly, here's what you'll end up with:

  • Mercury is now where Venus once was. It's still not habitable. Surface temperatures will still be boiling during daytime, and freezing during nighttime. And day/night last for 88 days on Mercury.
  • Venus is now orbiting Jupiter in Callisto's place. This makes Venus the largest moon in the Solar System by a huge margin. Chances are, the ensuing gravitational perturbations are going to mess up the orbit of at least one other Jovian moon, and potentially send it/them flying across the Solar System. And you'd better hope none of them wind up on a collision course with Earth.
  • So what of Venus itself? Well, it's still not habitable either. Venus is suffering from a runaway greenhouse effect that makes it even hotter than Mercury. Propelling it out to Jupiter will certainly cool it down, but it'll take a very long time. Even if it ends up at a habitable temperature, the atmospheric pressure would still be too high and would squash us flat, and then there's the sulfuric acid rain...
  • Mars is "a little closer to Earth". It's not going to gain any magnetic field from this because the reason Mars lacks a magnetic field is that its core has cooled down and isn't molten, and I don't think you're going to heat it back up again. It still wouldn't quite be habitable, but it might be terraformable. At the very least, the slightly closer proximity and slightly higher temperatures would make a manned mission to Mars just that bit more viable.
  • Callisto is now orbiting Mars instead of Jupiter. Well... kinda. Callisto's diameter is 71.1% that of Mars, and its mass is 16.8% that of Mars. It's even larger relative to its new parent body than Charon is to Pluto. As a result, the barycentre of the Mars-Callisto system will be outside Mars' diameter, and Mars will start "orbiting" a point outside its own radius. Meanwhile, Phobos and Deimos will probably get flung out of their orbits because Callisto is 7-8 orders of magnitude larger than them.
  • As for Callisto, it would actually be capable, in theory, of harbouring life, albeit only in its subsurface oceans because - you guessed it - there's no atmosphere on the surface.

TL;DR: This wouldn't make any of them habitable to humans, but Callisto might be habitable to some sort of life, and we'd have a slightly easier time trying to set foot on Mars.

  • $\begingroup$ If you only give Mercury say 1/5th the Earth's atmoshere, the greenhouse should be significantly reduced. My understanding is Mars core is molten but the mantle is solid which doesn't allow a dynamo to give it a magnetic field. By heating up the core/mantle with a Venus fly by and now tidal forces from a Callisto moon, my hope was to increase it's tenuous and necessary magnetic field. (And possibly the increased rotation rate given to Mars.) $\endgroup$ Jul 14, 2017 at 15:22
  • $\begingroup$ @BrooksNelson If you gave Mercury 1/5th of Earth's atmosphere you'd get nothing back. Mercury is too small to hold onto that atmosphere for long. A Venutian encounter with Mars is never going to produce enough energy to liquify any part of mars, let alone the mantle. Ditto making Callisto its moon. $\endgroup$ Jul 14, 2017 at 15:46
  • 1
    $\begingroup$ If Callisto was to actually orbit Mars, it might be enough to cause tidal heating on Mars and cause it to gain enough energy to become volcanic active again. Eventually, it might even gain it's magnetic field back. $\endgroup$ Jul 14, 2017 at 15:50

I think Your orbit computations are "a bit off".

Just to give Mercury enough energy to reach Venus orbit on a Hohmann orbit would require a tremendous amount of energy, but let's assume You get it. somehow.

Energy is preserved in "close flyby", so there's no way you can stabilize Mercury in Venus orbit (requires energy) and send Venus up the gravity well to Mars (requires even more energy).

Similar problems plague conservation of momentum.

It is also unclear how You mean to provide "a larger magnetic field for Mars, Mercury, Callisto and possibly Venus", which is necessary to preserve atmosphere in the long run and to shield living beings from cosmic radiation.

All this said: yes, if You manage to bring all planet in the "habitable zone" (Goldilock zone) and to give them suitable atmosphere then they would become "more habitable" (in the sense: "a larger number of species would be able to survive without special protection").

  • $\begingroup$ Ah, so the planets and moons may not be able to impart enough energy in these flybies to get them to the next flyby and/or orbit. Mars liquid core/solid mantle receiving a core heating Venus flyby, a large tidal heating moon, and potentially an increased rotation rate from both to increase it's magnetic field. Mercury's heated up core from the flyby with Venus. Callisto's heated up core from Venus flyby and Mars tidal forces since not tidal locked yet, increasing the salt ocean size and core differentiation for a bigger magnetic field. Venus from flybies and from Jupiter not tidally locked. $\endgroup$ Jul 14, 2017 at 15:34

Not the answer you're looking for? Browse other questions tagged .