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My hypothetical gas giant has five major moons. Three of those moons are icy bodies with abundant water ice and volatiles. Any of these could provide fuel for translunar spacecraft, or for spacecraft headed further afield.

However, not all of these moons are created equal. One of them is much deeper into the gas giant's gravity well, and the furthest out is very small.

Considering proximity to the host planet and to the other moons, which of the three icy moons is the best place to establish a refuelling station?

The System:

Planet mass - 187.36 Earth Masses

Moon A (Icy) - Mass: 1.93 × 1021 kg, Surface Gravity: ~0.223 m/s2, Semi-major axis: 597,520km, Period: 3.8863 days

Moon B (Unsuitable) - Mass: 93% Mars, Semi-major axis: 747,800km, Period: 5.4408 days

Moon C (Unsuitable) - Mass: 88% Earth, Semi-major axis: 1,188,200km, Period: 10.8724 days

Moon D (Icy) - Mass: 1.6 × 1023 kg, Surface Gravity: ~1.428 m/s2, Semi-major axis: 1,883,220km, Period: 21.7446 days

Moon E (Icy) - Mass: 4.28 × 1019 kg, Surface Gravity: ~0.064 m/s2, Semi-major axis: 3,588,200km, Period: 57.1901 days

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  • $\begingroup$ Would you mind adding in surface gravities for these moons? That could make a big difference for whether gathering ices is even feasible on each of them. $\endgroup$ – HDE 226868 May 23 at 2:42
  • $\begingroup$ @HDE226868 Ah! Very good point, and an oversight on my part. I'll have to work them out! The three icy moons have a composition similar to moons such as Iapetus or Tethys if that helps. $\endgroup$ – Arkenstein XII May 23 at 2:47
  • $\begingroup$ Is the fuel here ice, or something else? $\endgroup$ – Halfthawed May 23 at 3:03
  • $\begingroup$ @Halfthawed I'm thinking it's either hydrogen and oxygen that's been electrolysed from water-ice, or the drives are fusion and they're extracting heavy water from the ice instead. The worldbuilding hasn't gone that far yet. $\endgroup$ – Arkenstein XII May 23 at 3:05
  • $\begingroup$ @ArkensteinXII Comets seems like a better choice than moons, then. Why not put a station far out into orbit and have freighters run comets to it? $\endgroup$ – Halfthawed May 23 at 3:10
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Use moon E for interplanetary craft. It is likely the interplanetary craft don't descend into the gravity well at all, but shuttle craft come up and meet them to refuel and exchange the personal and cargo. These interplanetary ships contain the life support equipment for long voyages, which is essentially useless mass when in the giant's gravity well, and you don't want to lift that up out of the gravity well each visit. You only lift the perishables out of the well: food, water, oxygen, fuel, etc.

Moon E is not small. You didn't specify the percentage of ice, but if it's more than a few percent you have thousands of years of fuel available. On the other hand, the low gravity of moon E is a major benefit for lifting the fuel to the interplanetary carriers.

If, on the other hand, you are talking about refueling craft that only fly between the moons, then it very much depends on where the most common journeys are made. Naturally the fuel will have to come to the major spaceports. We don't all drive to Saudi Arabia to refuel our cars.

If you are only talking about a single ship, or small colony, then it's likely that the earthlike gravity of the inner moons is a more attractive reason to place a colony there than the ease of obtaining fuel.

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The one with the least gravity

Gravity requires fuel to escape thus the lowest gravity requires the least amount of fuel to escape from means more fuel for the journey.

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    $\begingroup$ Sure, but if that moon is further into the planet's gravity well, would you therefore lose more fuel getting into a more distant orbit than if you'd refuelled at a larger moon, but further out? $\endgroup$ – Arkenstein XII May 23 at 3:03
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    $\begingroup$ Depends if you're using said planet's gravity for slingshotting $\endgroup$ – Thorne May 23 at 4:40
  • $\begingroup$ @ArkensteinXII the plant is a gas giant,.you can't land on it. Unless you are mining it, the furher you are from it the better off you are in terms of fuel. $\endgroup$ – Renan May 23 at 13:01
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The best location for the fuel Depot is wherever most of the people and infrastructure in the gas giant system are.

Without knowledge of exactly what the major propulsion system of your interplanetary spacecraft it is very difficult to make recommendations, but there are two major regimes to consider:

If the delta-V potential of your spacecraft is low enough that orbital mechanics are going to be a significant factor in your space travel, you can't afford to stop somewhere that isn't a destination. You'll be arriving with a high relative velocity (bare minimum ~40% of the orbital velocity of the moon in question, and most likely much higher than that) and you're going to need to spend delta-V to brake into orbit to get refueled.

If the delta-V potential of your spacecraft is high enough that planetary orbital mechanics are vague guidelines, then the delta-v difference to reach individual moons is mostly academic, and the best place to refuel is the place where everybody already is.

As such, my recommendation is to decide whichever of the moons is the most likely to be your major colony in the Gas Giant system, and the spaceport on/over that world is where the vast majority of refueling is going to happen.

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