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I am assuming here that these solar mirrors/panels are orbiting Ganymede, not flying through its atmosphere.

Nothing indicates that Ganymede has a substantial atmosphere - it is missing it now, in the show there are domes and humans use space suits, there is nothing that indicates the presence of an atmosphere.

I would expect that the destroyed satellites would stay in orbit rather than start falling immediately toward ground. AFAIK pieces should stay in orbit rather than start falling immediately like after the destruction of a powered object flying through the atmosphere.

So why these panels fall immediately rather than stay as a debris in orbit?

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    I think it's safe to rule out number 4, anyway. With the notable exception of the proto-molecule, they appear to be trying pretty hard for realistic science. Perhaps the books would provide more context, though, from someone who's read them.
    – Steve-O
    Commented Jan 15, 2018 at 14:32
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    My assumption was always that the explosions pushed some into higher orbits and some down out of orbit
    – user20310
    Commented Jan 15, 2018 at 17:34
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    I can confirm that they're called "orbital mirrors" in the book, and that they also fall down immediately there, which does seem like a contradiction.
    – tobiasvl
    Commented Jan 16, 2018 at 9:32
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    Just a small note - just because there are suits and domes doesn't mean there isn't an atmosphere. Just because there's an atmosphere doesn't mean you can breathe it, or that it's safe to expose yourself to it. Also note that the satellites would normally be under constant thrust due to reflecting the incident light (or transmitting the power in case of a solar satellite). Combined with the weak gravity of Ganymede (lower orbital speed) and whatever impulse they got when they were destroyed, it might be conceivable that they fall within mere hours. You'd need to math it out.
    – Luaan
    Commented Jan 16, 2018 at 15:36
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    The real question is: why are the mirrors are big heavy structures instead of ultra-lightweight frames holding reflective foil?
    – Harabeck
    Commented Jan 22, 2020 at 17:27

5 Answers 5

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The book, Caliban's War, doesn't shed more light (heh) on the issue.

The mirrors do, as you assume, orbit Ganymede. At least: Almost every time they're brought up in the book, they're called "orbital mirrors".

The great orbital mirrors had always been his allies, shining down on his fields like a hundred pale suns.
Caliban's War, page 21

The following passage might imply that the mirrors start falling when the power is cut (how long that takes is, like in the show, not stated):

Power to the grow lights had been cut, and the mirrors … He couldn't think about the mirrors.
Caliban's War, page 25

If there's an actual correlation between the power failure and the mirrors falling, this might mean that the mirrors normally experience orbital decay, and that constant adjustments to their course are needed to keep the orbits. When the power disappears, Ganymede's communication with the mirror array is lost, and the mirrors' orbit starts decaying until they fall down.

In order for this to happen, though, Ganymede would need to have an atmosphere. Orbital decay like this would really only occur if there was atmospheric drag. However:

The atmosphere was so thin it could pass as an industrial vacuum.
Caliban's War, page 9

I could only find one other person raising this point online. The authors and showrunners do not seem to have addressed it, and I couldn't find any articles talking about it either (although there are many articles about the show's adherence to real-life physics and the few holes that do exist, I couldn't find any about this).

All in all, this seems to be a plot hole which hasn't been caught by a lot of people.

user20310 said the following in a comment:

My assumption was always that the explosions pushed some into higher orbits and some down out of orbit

This might also be a possibility. Prax Meng often talks about that "the cascade" will kill Ganymede. He means that the moon's artificial ecosystem will collapse, piece by piece, because of the events at the beginning of the story. However, small events cascading into larger ones is a theme throughout the book (and season; episode 10 of season 2 is even called "Cascade"). It's possible the authors intended the destruction of the mirrors in orbit to trigger a Kessler cascade. However, with the low atmospheric drag, this wouldn't suddenly cause the mirrors to lose orbit entirely, like you say in your question.

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    "this wouldn't suddenly cause the mirrors to lose orbit entirely" - what worse, there are scenes (later in show) that depict mirrors falling straight down.
    – reducing activity
    Commented Jan 16, 2018 at 16:19
  • "pushed some into higher orbits and some down out of orbit" - maybe I missed it but there is no case of debris/pieces showed to be pushed upward or remaining in orbit.
    – reducing activity
    Commented Jan 16, 2018 at 16:20
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One answer would be that the term 'orbital mirror' is a historical generic term like 'tin foil' and 'kleenex'. Foil is no linger made of tin and not all tissue paper is made by Kleenex but the terms have stuck. The Mirrors didn't take long to fall so they were pretty close to Ganymede station. Otherwise there would have been plenty of time to destroy or divert them, or at least evacuate people from the most vulnerable dome areas. They could have been supported by a low power Epstein drive or perhaps some of the collected sunlight could have been used to generate thrust from vaporized ice brought up from the surface by transport ship as fuel. They wrongly assumed that any catastrophic failure would nevertheless give the operators time to divert the mirror to crash away from the station. Nobody imagined they'd get stuck in a battle zone.

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  • I really like this solution! It is unlikely to be what author intended, but I really like solutions for plotholes that fit within a story. I should rewatch and check is there anything that would disprove that idea.
    – reducing activity
    Commented Dec 1, 2018 at 21:57
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I haven't read the books, but one possibility could be that the mirrors were too low to be in geosynchronous orbit. They therefore require constant thrust to remain over the same spot on the ground, in what's sometimes called a powered orbit by Star Trek fans.

When the mirrors were destroyed, their thrusters stopped working so there was no longer anything keeping them up.

One practical problem with this explanation is that huge amount of power needed to keep the mirrors hovering could otherwise be directly used on the surface for grow lights.

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  • Actually, there might be something here. A synchronous orbit around Ganymede would be approximately 46 000 km radius (compared with Earth's c. 42 000 km) and Ganymede's much smaller gravity well would almost definitely make that at best unstable, if it's even in Ganymede's gravity well at all. So it makes sense that the mirrors are actually actively kept in position; when they lose propulsion, they will fall. (Now, they still have some orbital velocity, so they won't fall straight down, but...)
    – DavidW
    Commented Sep 11 at 14:33
  • @DavidW Intriguing thought about the high unstable orbit. However, there wouldn't be much light reflected from 46'000km away unless the mirrors were the size of a small continent.
    – Emile Cormier
    Commented Sep 13 at 13:18
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It seems obvious: misunderstood orbital mechanics. There is no way the mirrors would fall on a colony directly below. Any orbital destruction in orbit would have created a ring of debris around the moon that would have, over millennia, decayed and fallen on the planet or been ejected. Even energetic explosions that pushed the mirrors backwards on their orbital path—bringing them to ground—would have had to impose such a significant delta V backwards that they would land "somewhere" on the moon, never directly below the geostationary point where they stood. And that "somewhere" would be a long, long, long trail of debris, not a concentrated spot anyways.

It's hard to admit in a story that has been trying to follow the physics most of the time, but let's face it: the entire story is a case of dropping the ball on orbital mechanics.

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  • While this is likely true, we do try to find ways for story elements to work in-universe. Don't forget the Epstein drive pretty much rules out real-world science.
    – DavidW
    Commented Sep 11 at 0:08
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I was puzzled by the speed of descent too, and the two situations I can see affecting, are 1) kinetic energy input by CQB rounds, 2) if the surfaces have built-in tension to keep them flat, like a tarp, taught as a bow, when released, causes the spring-like tension to translate to a velocity.

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