# What orbital structures could be raised around a gas giant's moon?

My narrative project takes place on an habitable moon (0.7 earth masses) orbiting a gas giant 3 times the mass of jupiter at a distance of about 1003000 kms and is in a 5:2 spin-orbit resonance with it's primary (meaning that while it takes 90 hours for a revolution it takes 36 for a rotation), the orbital eccentricity 0.025.

I have been wondering what kind of orbital super-structures could be placed or could be advantageous for a space faring civilization, like you know, orbital artificial rings, space elevators, skyhooks and the like.

Especially my main concern are space elevators combined with planetary rings in geostationary orbit, do you think that they'd be feasible/useful to endeavors such as space-travel?

Ps could it be possible to build such a structure with the technology available to a civilization like the one in 'the expanse', with addition of advanced AIs, robotics and nanotechnology, before they establish permanent self-sufficient settlements on the other satellites? Assuming that by then the civilization would already have the materials needed to build such a structure?

• I don't have the math for a real answer but I can think of two things. The tidal forces may pull a ring off center from the moon unless the ring's rim is parallel to the gas giant. Also, we, on Earth, don't have the material science to pull it off (we're almost there but not quite). Aug 17 '21 at 20:27
• ShadoCat Regrettably those are excellent points Aug 17 '21 at 20:31
• Sorry. The space elevators might work. As I said, we are almost there ourselves. Maybe in a decade or two, we'll go from "we can't do it with what we have" to "who'll pay for it?". Since the planet is not tidally locked, there will be a bit of "sway" in the elevator as the upper end is pulled slightly toward the gas giant as it moves past. You just need to make sure it is built so that a harmonic doesn't build up in it (so it doesn't shake itself apart). Aug 17 '21 at 20:43
• Also, a ring parallel to the gas giant would be essentially in a polar orbit which would make it expensive to get to from the surface of the moon. Your best bet would be to have space stations in the vicinity of the upper end of the elevator and a way to deal with the fact that they will both be in constant movement relative to each other. Cheap shuttles would work as would mass driver based vehicles being shot from the elevator to the space station and back (though that might get a bit "exciting"). Aug 17 '21 at 20:47
• Yes, anything that takes energy from the harmonic protects the elevator. Aug 17 '21 at 20:47

## Space Elevator

To take advantage of the planet's rotation, the base of the space elevator must be placed at or close to the planet's equator.

Geostationary Altitude (cable disembarkation height) = $$^3\sqrt{{GM}\over{\omega^2}}$$

= 48,938 km around a world of 70% Earth's mass, with a 36-hour day.

The top of the elevator depends on a lot of design decisions. In most literature I've looked at, $$r_{end}$$ is between 2x and 4x $$r_1$$ Therefore, the top of the elevator is somewhere between 96 thousand and 200 thousand kilometers.

Interference:

Anything with a precessing orbit < $$r_{end}$$ will eventually become a collision hazard. However, as long as these are controlled craft, you can plan ahead and adjust your path to avoid the cable.

There will be ongoing "air traffic control" around the cable to monitor for hazards.

Feasibility:

The tension in the elevator cable (if made of Zylon at a density of 1,700 kg/m^3) is in the neighborhood of 100 to 400 gigaPascals (GPa). Currently, the strongest industrially available materials are much weaker than that (Zylon itself is about 6 GPa)

## Orbital Ring

A planetary ring makes a complete circle around the planet at low altitude (usually less than 100 km). It is, by definition, stationary relative to the ground. In most designs, the ring is kept in place by tethers that keep the ring centered around the planet.

Because the ring is stationary, it can be pre-planned that the path of the orbital ring does not interfere with the space elevator. The orbital ring does not have to be positioned on the equator, but it must have the planet at it's center (so no offset polar ring at 70 degrees N latitude)

Feasibility:

The compressive strength required for this construct is comparable to the space elevator in the range of a few hundred gigaPascals (~ 587 GPa for a steel ring).

This is still beyond the range of currently known materials by a few orders of magnitude.

## Skyhook

A skyhook is a heavy space station (maybe an asteroid moved into position), that lowers and raises a cable that ground operators can connect a load to at various points on the planet surface where the space station overflies.

Alternatively, the skyhook can be designed for loads to be hooked up in mid-air. The skyhook will then lift the airborne load into orbit.

It requires a lot of energy keeping the station in place, but the skyhook has been proven possible with existing, or nearly existing technology. There have been a few designs demonstrating the feasibility and materials used.

Interference:

If the orbit of the station precesses, a sky hook will become a predictable hazard for a space elevator. The elevator can be avoided by correcting the position of the heavy station. And how often the problem comes up can be done by selecting an orbit that rarely or never intersects the elevator.

A skyhook basically does what an orbital ring or space elevator do, but at a much higher operational cost. I'd imagine sky hooks would be a legacy technology in any civilization that has space elevators/orbital rings.

Skyhooks might still fit a niche for dangerous loads you don't want on the orbital ring/elevator. Or private loads that you'd like to have picked up where you are, instead of hauling the load to a ring/elevator ground station.

• Zylon up to 400 GPa ? For real? Or you misspeled carbon nanotubes and 400? If you point out how strong some material should be, seem the case, then the sentence need a fix for clarity. Also 30-40 GPa may be sufficirent, as bare minimum Aug 18 '21 at 12:21
• I used Aylin for weight. Thought I said that it didn’t have the necessary strength. Let me check. Aug 18 '21 at 12:27
• I can see how the wording is a little misleading. I’ll get it fixed. Aug 18 '21 at 12:27
• The A isn't much different from just pointing out some non-rocket launching solutions around regular planet and does not consider potencial influence of gas giant - which a bit sad, as not sure that just mention those makes it an answer to op's q. Doubt there will be strong influence but subjective perception does not work for space and has to be calculated. Aug 18 '21 at 12:31
• I’ve done very similar math for a story I’m working on right now. I can double-check the specifics, but eyeballing it, the planet is too far away from the gas giant, and the super structures too close to the planet for any splashy effects that I could think of. Aug 18 '21 at 12:50