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So assume someone wants to terraform Venus. And for extra points the decide to get her a moon too. So, how could you get Ganymede to Venus with as little effort as possible but also as fast as it can happen?

Now I know what y'all think, why not take something like Mercury or Ceres? And the answer is, because it doesn't sound as cool.

In principle, getting Ganymede to Venus shouldn't be totally impossible.

If we look at their Orbits, Venus is going around at 34.8 km/s. Where as Ganymede rolls around at 13.2 km/s as it travels with Jupiter.

The idea is that we really only need to kick Ganymede out of Jupiter's Orbit. This can be done by slowing it down to around 6 km/s. Then it will fall down and intersect with Venus. It will be pretty much impossible to just let it be captured by Venus. It is falling way faster so a few rotations around the sun will have to happen. Each one bringing it closer to Venus. Maybe some Gravity assists later it will just sort of stay in an Orbit around Venus ?

I guess if you ware patient you could create a series of epic gravity assists that will bring Ganymede into the Perfect Orbit. It will just take like forever. I was hoping to do it in about 5 Years or so.

So yeah, how?

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    $\begingroup$ This is not how orbital mechanics works. Not at all. Not even one little bit. $\endgroup$
    – AlexP
    Commented Aug 16, 2020 at 17:34
  • $\begingroup$ Clarify. I assume you mean the part in which i talked about "kicking" Ganymed out. That was pretty badly written. You of course have to accelerate Ganymed to get him out and into a Solar Orbit. Then you can slow down. the same would work by just slowing down, but Jupiters Gravity is strong and you dont have a lot of time to break. So going for the approach that first accelerates and then decelerates the Moon is probably the only way it could work. $\endgroup$
    – Erik Hall
    Commented Aug 16, 2020 at 17:38
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    $\begingroup$ I mean all the parts. I am especially fond of the handwaving of orbital insertion... But anyway, the point is that without knowing what the magitech can and cannot do, one cannot say anything of substance about how to move Ganymeded from Jovian orbit to Venusian orbit. (It is magical technology for sure, far far very far beyond what puny humans have available.) $\endgroup$
    – AlexP
    Commented Aug 16, 2020 at 17:59
  • $\begingroup$ Sick burn xD In terms of how to get Ganymed from A to B, there are a few Options. The slow and boring one is Gravity. It should be possible to force Ganymed into a Eliptical Orbit over a few 1000 years and then "kick" it out with "little" thrust if any at all. Another one is to use Thrusters on the Surface. There is Hydrogen and Oxygen there, but the scale is kind of enormes. Also, all the Heat has to go somewhere. This is faster but also more distructive. Another option may be Burning the Surface with Lasers and getting thrust that way. But again, heat. $\endgroup$
    – Erik Hall
    Commented Aug 16, 2020 at 18:05

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The idea is that we really only need to kick Ganymede out of Jupiter's Orbit. This can be done by slowing it down to around 6 km/s.

There is really a lot to unpack here.

If you pick Ganymede at its current orbit, and reduce its orbital speed from 10.880 km/s to 6 km/s, it's orbit will become lower (and possibly very excentric, depending on how you meant what you said). This will not kick it out of Jupiter's orbit, but it will mess with the Jovian system. This has the potential to throw some moons off of it, but in random directions and not very far from Jupiter's orbit. On a second pass they will be thrown into random new orbits around the sun.

But most probably you meant to slow down Ganymede (and only Ganymede) on its path around the sun, so that its perihelion is within Venus's orbit. Consider the following:

  1. The Δv cost to cause Ganymede to escape Jupiter, then put it in the same orbital plane as Venus, then cause it to intersect with Venus, all together, may be one or two orders of magnitude greater than the Δv cost for the last thing left to do: an orbital insertion around Venus.
  2. An orbital insertion means that the object being inserted will have the same average orbital speed around the Sun as the planet it's being inserted around. So Venus will have to give Ganymede a lot of Δv if it is to capture it naturally. This will take many passes, which might take millions of years (specially since most passes will not result in a flyby).
  3. On any pass Ganymede might be thrown off the intended path by Jupiter, Mars or Earth, spoiling your project.

Just decelerate Ganymede on your own on the first flyby. If you wish to see the logistics for this, just send a vessel from Jool to Eve in Kerbal Space Program (Jool is the game's equivalent to Jupiter, and Eve is the equivalent to Venus). Then consider that Ganymede is orders of magnitude more massive than a spaceship, but then again, if you are moving it you are a Kardashev level 2 or 3 civilization anyway.

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  • $\begingroup$ Thanks for the answer ! It was meant in the 2. way but what i wrote didnt make that to clear. I thought of just brute forcing it. You have to kick Ganymed out anyways so you got some sort of System that can accelerate a Moon. Kind of like a direct assent. But id imagine this would put the Moon through some bad times. Going the Natrual way seemed more doable but i have been messing around in Universe Sandbox 2 and yeah... Didnt work out that well... But then again, some Spaceprobs managed to do something sort of not similar.. $\endgroup$
    – Erik Hall
    Commented Aug 16, 2020 at 17:36
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    $\begingroup$ @ErikHall: Ganymede is close to 20 orders of magnitude more massive than space probes. This takes a Kardashev II+ civilisation. $\endgroup$
    – John Dallman
    Commented Aug 16, 2020 at 18:07
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    $\begingroup$ Ignoring the huge amount of energy, it's also worth noting that Venus doesn't have a moon for a reason. It is too close to the sun. Moons need to be far enough from their parent planet so that they are not pulled apart by tidal forces, but not so far that they get captured by their star. Venus does not have any such stable orbits. Now, if you only care about this for like a few millennia it doesn't matter, but otherwise Venus isn't where I'd go $\endgroup$
    – Michael Stachowsky
    Commented Aug 18, 2020 at 15:16

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