I've asked before about my polytheists with competing moon gods. I now have a better understanding of how tides -- a very visible manifestation of lunar influence -- work with multiple moons (thanks), but now I need to disrupt it.

My story calls for a human spaceship observing this planet (presumably from orbit) to unintentionally and unknowingly interfere with the lunar power dynamic on the planet. It's ok if they would realize it if they stopped to think about it; i.e. they're allowed to be careless. But they're not malicious. Is there some mechanism by which this could happen or do I need to rethink my premises?


  • Timeframe is unspecified but I'd like there to be a credible path from "earth today" to whatever technologies are called for. I'm not looking for magic.

  • It helps my story if the effect is gradual, but something that produces an effect immediately noticable on the planet is workable.

  • The planet (of unspecified size) has at least three moons. More are acceptable. (Three are important to the plot.)

  • An effect that causes the ship to be interpreted (on the planet) as a new moon is ok, so long as it has gravitational effects on the planet as if it were a (small?) moon.

  • The natives of the planet are pre-industrial.

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    $\begingroup$ In "The Giants" Series by James P. Hogan interstellar travel is done by generating artificial black holes that then are used to generate wormholes. Doing that too close to planets could have all sorts of interesting effects... $\endgroup$ – Tim B Nov 25 '14 at 19:44
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    $\begingroup$ Not a duplicate question (worldbuilding.stackexchange.com/questions/4462/…) but similiar concepts, the (currently only) answer there talks about orbiting an orbiting death star's gravitational effect. That said I think the effect would have to be more specific like a lander using powerful retrograde gravity thrusters near a shore line or something. $\endgroup$ – Culyx Nov 25 '14 at 20:15
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    $\begingroup$ A civilization which can send a spaceship large enough to disrupt tides to a different star system must be smart enough to know about tidal forces, so they cannot be ignorant of the gravitational effects of their spaceship. We can barely send a few people to the Moon and might just will be able to send a few people to Mars in a few decades, this is significantly below the technology required for interstellar travel. The only solution which springs to my mind is of a failed sleeper ship, which runs mostly on autopilot and the crew reverted to barbarism generations ago. $\endgroup$ – vsz Nov 25 '14 at 22:46
  • $\begingroup$ The spaceship being a planet? cf., "The wanderers" by Fritz Leiber. By some reason, they do not think (or do not have time to check) that there is life in the planet. $\endgroup$ – SJuan76 Nov 26 '14 at 0:50
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    $\begingroup$ I proposed an solution to 'anti-gravity' thats very heavily in the theory range, but still has some basis in science. Basic theory is you can modify the small dimensions to increase (or decrease) the distance over which gravity functions. Not sure if it's workable for you, but thought it was worth suggesting. answer is here worldbuilding.stackexchange.com/questions/4261/… $\endgroup$ – Twelfth Nov 26 '14 at 18:58

15 Answers 15


If the effects of the tides are significant, and the spaceship inhabitants are observing the planet, it's likely that they would know that they are influencing the tides of the planet. However, if they're on some sort of commercial mission, rather than a "don't interfere/we come in peace" mission, it's highly likely that they may just not care. If that's the case, then there are numerous things that a spaceship could do that might affect the tides of the planet. Here are some examples:

The spaceship could be really big

A spaceship big enough to affect the tides would act like another moon, though tidal effects would be more variable/random if the spaceship was moving around. In this case, the spaceship would probably be visible from the planet, though cloaking fields etc. could negate this. Effects on the tides wouldn't come all at once, but rather would be a steady shift towards a new tidal equilibrium. Tides are a resonance phenomena, so it will take a few days/weeks for the tides to shift into the new pattern.

The spaceship could produce artificial gravity

Things like Alcubierre drives rely on warping space, which would likely affect the tides. Since this would be a less constant effect, it would probably cause something more like a tidal wave than a consistent change in tides. There would be a consistent effect if the spacecraft was using artificial gravity to stay at a certain non-natural orbit. For example, if a spacecraft wanted to be in a geosynchronous orbit, but at a low altitude, the craft could do this by reducing the effect of gravity from the planet on the ship. If this reduction in gravity affected the ocean under the planet, it would cause tidal effects.

The spacecraft might need to either dump things into the ocean or suck up water

A huge craft spending a few days/weeks replenishing internal water supplies by sucking up water would definitely cause tidal effects if it was using something like a gravitational tractor beam to do so. Likewise, a spaceship dumping all of its garbage would cause huge tidal waves. If they were cleaning out some internal holds by sucking up and dumping water repeatedly, this could cause alternating super low tides and tidal waves, along with massively disrupting currents on the planet.

All of these things would be very noticeable for the people in the spaceship, but they might not care. They may also not be paying any attention to the significance of changing the tides on the planet for the people below. Consider the effects of oil prospecting in the rain forest on the natives living there. The prospectors aren't really unaware of what their presence does, but they don'r necessarily think or care about the consequences. I could definitely see a space faring nation having similar attitudes towards a pre-industrial culture.

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    $\begingroup$ In order for point #3 to make any difference, it would probably have to be big enough that point #1 applies. And then we're in Star Wars territory: "That's no moon..." ;) $\endgroup$ – Mason Wheeler Nov 26 '14 at 1:23
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    $\begingroup$ I think another way to get the inhabitants to be "unaware" of the effect they are causing would be to make them unfamiliar with the problems. For example, although they would clearly be aware of tidal effects, if they do not come from a world with large bodies of water (desert planet, space stations) they would not necessarily think about how tidal forces would affect them. $\endgroup$ – Logan Pickup Nov 27 '14 at 1:09
  • $\begingroup$ For point #3, the ship could also have some sort of worm hole that is being used to ship water back to a far off planet, which may not even be aware that the ship is syphoning water from an inhabited planet. $\endgroup$ – ckersch Nov 29 '14 at 19:20

All the answers so far dealt with the ship creating gravity itself. But there's another possibility: The ship might have some sort of gravity shields (to allow to pass close nearby heavy objects like neutron stars or black holes during travel without the ship being damaged), and they might have forgotten to switch them off after arrival. Now every time they pass between one of the moons and the planet, they disturb that moon's gravity (by shielding parts of the planet from it), and thus interfere with the tides.

So, how plausible are gravity shields? Well, at least not less plausible than artificial gravitation or Alcubierre drives. There's the claim by Podklednov to have observed such an effect. There's no independent verification and great doubts about it, so this is far from accepted science, but at least it's more than we have for artificial gravity, where I'm not aware of even a claim of observation. If Podkletnov is right, a gravity shield would be made of rotating superconductors.

  • $\begingroup$ Add to it that the ship is made out of a newly discovered highly dense metal to protect the crew from radiation when out in unchartered territories. Since the ship is so dense, it does not need to be too big to cause gravitational fluctuations. $\endgroup$ – nvd Nov 26 '14 at 11:04

I'm no physicist and don't know much about gravity, but I do know quite something about tides. My hint would be to look into resonance. Some areas on earth have extremely high tidal amplitudes (for example Mont Saint Michel in France, but there are more) because the length of the basin is 1/4 of the tidal wavelength at sea. This results in a standing wave pattern, with extremely high amplitudes. Also, while most coasts experience high/low water levels twice a day, some have it once a day. Of the many tidal frequencies that are present, the once-a-day frequencies are resonant along that specific coast, resulting in one high tide and one low tide per day.

Now back to your question, a spaceship still needs to be pretty large to induce a tidal signal, but if city X lies at the head of bay Y and bay Y happens to be resonant to the (small) tidal signal of the spaceship, the resulting tidal amplitudes may still be significant. Of course this is a local effect, but may have big consequences for city X. And since it's such a local effect, it is also very difficult for the spaceship-crew to anticipate beforehand.

[1st time commenter, please be patient with my input and non-native English]

  • $\begingroup$ Thank you for this answer, and welcome to Worldbuilding! Also, don't worry about your English; it's fine. I look forward to seeing you around the site. $\endgroup$ – Monica Cellio Nov 27 '14 at 17:23
  • $\begingroup$ Nice idea! The stationary wave thing is a very interesting idea. $\endgroup$ – Rmano Nov 27 '14 at 19:12
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    $\begingroup$ @MonicaCellio Bouncing in on the local effect idea, dumping something at a critical location in the bay (unnoticed by natives unless they go dive at exactly the right place) could affect the resonance pattern. $\endgroup$ – Gilles 'SO- stop being evil' Nov 27 '14 at 21:02

Since moons affect tidal patterns on planets using gravity; I would say that's what we would have to focus on as a spaceships only means to affect tides on a distant planetary body.

Obviously, a single spaceship shouldn't have the same gravity as a moon. But let's take it a step further.

As we all know by now, the idea of "warp drive" has been a seriously considered form of FTL travel. Since your story will have a human spaceship visiting a distant planet, it would only make sense that there is some form of FTL travel (like warp drive?) available in your universe/timeline.

So let's take it a step further. Since warp drive relies on manipulating large amounts of gravitational force to manipulate space-time; maybe the spaceship could be have an early version of warp drive that didn't have all the "kinks" worked out yet.

Maybe the spaceship has to charge "gravity" up before it's ready for a warp jump to the next star system or galaxy!?

EDIT*** The "obviously" part may have been a bit misleading. So let me lay out my reasoning for that statement based on the following assumptions:

For a spaceship to be large enough to affect the tidal patterns of a distant planet (the distance, of course, would be dictated by the gravitational pull of the and the distance the object was from the planet) it would require that the spaceship to be the same mass as a moon! That would seem to be pretty huge.

Even if, the civilization was advanced enough to build a structure/spaceship that large. There are many reasons to not build ships so large for the sake of efficiency. First of all, newtons laws of force and motion would still apply to an object in space. Therefor, the greater the mass of the object they are moving (propelling the spaceship would require more energy to move a large spaceship than a small one; not to mention how much more it would take when approaching the speed of light).

  • $\begingroup$ Why do you say "Obviously, a single spaceship shouldn't have the same gravity as a moon" - if they have the technology to travel a significant distance, why wouldn't they have the technology to have a large, sustained ship the size of a moon? $\endgroup$ – corsiKa Nov 26 '14 at 22:31

I can think of two uses for gravity generation aboard a spacecraft. The first is environmental, keeping crew and components from floating around while the ship is outside of a planet's gravity well. The second is locomotion, some form of space/time warping device. The environmental system would probably be too weak and localized to fit your needs, unless the ship was truly massive (in which case its own mass might serve your purposes).
The propulsion system would probably be much too strong when engaged. Altering planetary orbits is not your goal... you just want to move a little water. So that leaves the propulsion system at rest. Perhaps, as well behaved explorers, your humans turned off their warp drive well outside of the solar system and have drifted in using more conventional propulsion. But even with these precautions, the warp drive itself, even when turned off, emits a powerful field, approximating the strength and scope of a small moon. The environmental system would therefore serve two purposes... keeping the crew's feet on the ground and helping them resist the gravity of their engine.

As for any of this being feasible, given what we know about gravity, I am not qualified to say. As far as I know, we can feel gravity, measure it, depend on it and even occasionally defy it... but we have yet to figure out how to create it sans planetary masses.


The other answers have, unfortunately, all been too kind. You cannot affect the tides in any sort of spaceship of non-ridiculous size, you cannot move a spaceship of ridiculous size, and the technical demands for hypothetical singularity-based drives are (in addition to not being widely accepted as consistent with modern physics) so far past what we have now that there's no credible path there from "earth today".

However, very much science fiction is written without a credible path from "earth today", or in blatant contradiction with physics. So don't let that stop you! It just makes the science fiction a bit less hard.

Anyway, the thing to realize about tides is that the forces involved are immense. The tides dissipate 3-4 TW of energy on the Earth, or about 1/4 of the total power usage of all humans on the planet. Accidentally imparting anywhere near that much energy to a planetary system seems hard to do unawares (only really possible with antimatter-based energy sources; you would need to waste an extra gram of antimatter per three minutes to equal the tidal dissipation on Earth). Doing so via something other than gravity (which is extraordinarily hard to manipulate) without causing huge visible effects also seems unlikely.

The bottom line is that orbiting in a spaceship is a pretty low-impact activity, while changing tides is a big one. It would be more plausible if one of the premises were changed (e.g. one of the moons is actually the spaceship, and they are maneuvering it to begin a million-year journey to a different solar system; or the spaceship isn't there just to quietly observe but to perform massive resource extraction, which they think they can manage by huge-scale seafloor mining with drones, which is largely true except for the (gradual) change on the tides).


I'm going to answer the part about "unknowingly".

If your civilization regularly uses spaceships near planets, it is wholly implausible that they do not know that spaceships affect tides. Thus, you need something extra happening that circumvents this conventional wisdom.

Some possibilities are

  • It is well-known that you're not supposed to operate this type of spaceship near planets precisely because of how they disturb the planet. Everybody follows the rule, and so common knowledge waters down to "don't do it, that's the rule". Your observers have decided to break this rule, because they have an urgent need to observe the planet and aren't satisfied with any method that adheres to the rule. Your observers are so caught up in trying to rationalize breaking the rule that they haven't yet given any honest thought as to why the rule exists.

  • The ship is malfunctioning. It's not supposed to be emitting a gravitational field, but the Einstein couplings are leaking. These are important for correctly containing and directing the immense gravitational fields that are built into the interstellar engine, but the leakage doesn't impair their ability to maintain planetary orbit, and for whatever reason the crew simply haven't had a reason to run a diagnostic on their gravity engines, and so haven't yet noticed the leak.

  • The spaceship is alien, and humans don't yet have practical experience with the technology. This may be the first ship of this type that humans have ever piloted near a planet. In retrospect, it's obvious that the ship could emit enough of a gravitational disturbance to affect tides, but nothing has prompted the observers to even consider the question. Maybe nobody has ever considered the question yet, or any of the many reasons why it wouldn't be common knowledge yet. ("Of course it would affect the tides, that's obvious! Why would I need to tell that to anybody?")


Let's suppose that the ship enters the system at a speed near the speed of light and they have a problem braking. The apparent (relativistic) mass of the ship could be quite high, and if they pass near enough one of the moons they can change the orbit the amount you need to disrupt it. They can manage to solve the problem later and come back to observe the planet, maybe not realizing they have caused such a mess (although this is quite naïve...).

If the effects are, for example, on the farthest moon and the main effect is to change the eccentricity, I can imagine that the effects could be somehow graduals, when the changed moon orbit interact with the others...

As noted in comments, it's true that the time of interaction will be short, but you can suggest that the orbit of that moon was not so stable anyway (n-body systems are really complex --- as far as I know Earth's orbit isn't really stable, it's changing a bit) and that the "nudge" can move it to another kind of quasi stable condition... like this:

Two orbits

Please note: I'm a layman on orbital mechanics --- my field is electronics. But there are oscillators that can behave like that, so maybe it's not impossible in orbital configurations.

This is not at all an original idea --- I can remember somewhere similar in a SF book I read, but I can't remember which one. Maybe something in The Algebraist (Iian M. Banks?)

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    $\begingroup$ Hey, welcome to Worldbuilding! This is a good answer; I only have one quibble with it: If they're going so fast, they won't be near the system long enough to influence it. Impulse is defined as $I=F \Delta t=\Delta p$; you'll have a greater gravitational force but a much shorter amount of time. $\endgroup$ – HDE 226868 Nov 26 '14 at 19:13
  • $\begingroup$ Thanks! Yes --- that's true. But if you combine it with a near-to-unstable orbit... probably not really solid on the physics-math side, but sufficient for the suspension of disbelief. $\endgroup$ – Rmano Nov 26 '14 at 19:18
  • $\begingroup$ True; I had assumed that the orbit was pretty stable. $\endgroup$ – HDE 226868 Nov 26 '14 at 19:19
  • $\begingroup$ I don't know a lot about the effects of unstable orbits (ooh, I feel a question coming on :-) ), but if a moon with an unstable orbit is consistent with people (well, let's call them "people" even if not humans) living on the planet, that works for me. $\endgroup$ – Monica Cellio Nov 26 '14 at 21:50
  • $\begingroup$ Maybe the planet in question is so large that they would have to be travelling at near light speeds to stay in a stable orbit. $\endgroup$ – Get-HomeByFiveOClock Nov 27 '14 at 4:57

Someone left the artificial gravity on.

The Alcubierre drive works by manipulating space-time so that you ride a 'wave' of gravity forward at any speed. You might use this same technology to create a more static artificial gravity over a given area.

When in deep space, your ship can produce artificial gravity by emulating a large mass some distance below the ship, and letting otherwise-natural gravity take over from there. The mass of our own moon is about 7*10^22 kg. To get 1 g from that naturally, you'd need to place the mass 162 miles below the ship. This is really the furthest you'd need to put it: The mass and distance would be much lower if the ship were closer to the planet than our own moon is, or if the tides don't need to be as strong. You could either make this place of artificial mass be something like a point mass (a tiny black hole) which might be visible and have little particles streaking around it, or something subtler, like an even density field over a sphere or other shape. If you want the center of mass to be contained within the ship instead of below it, you can have it malfunction so that the gravity is stronger outside the ship instead of inside, like it should be.

Normally this gravity is switched to a local-only mode (it uses significantly more energy, but creates a gravity effect limited to the ship) when you approach a planet or another ship to avoid dangerous interference, but due to a malfunction or mistake, it was left on this time.

The sudden introduction of this gravity source will slowly begin affecting the tides, and when they realize it's happening they can get it fixed. It will also pull the other moons and planet, but you can engineer it so that this is a negligible or significant thing, based on whether the ship is nearer to the planet or the moons.


All the current answers are god, but they require the ship to act like a mass that is comparable in size to the planet. That seems problematic. Maybe the ship could instead investigate the lightest of the many moons (somewhat asteroid sized) from a low orbit around it, which slightly alters that moons (presumably somewhat instable) orbit, which causes it to influence a slightly bigger moon in a domino effect, etc. etc. (maybe there is even some slingshotting involved). Finally, after (many) years the "main" moon changes its orbit - maybe it even happens that the "second in command" moon god gets promoted.

I'm not too sure about the premise of an easily influenced mini-moon, however. If the system has so far been stable enough to survive asteroid impacts, the mini-moon orbits are likely in rather stable orbits and in some resonance with the bigger moons.


Is it important that the humans didn't intend to disrupt the lunar orbits, or merely that they didn't intend to affect the indigenous culture?

While it would be difficult for a spacefaring race to not be aware of their impact on lunar orbits (they're going to chart them just to make sure they don't run into anything, at the very least), it might be that they've believed in science long enough that they've forgotten that their ancestors used to worship celestial bodies. If they haven't run into a lot of other races, and if they don't see a sea of artificial satellites in orbit, they might genuinely be surprised that the "unsophisticated" natives are even capable of noticing a change in lunar orbits. Yeah, sure, they might affect the tides, but ecologically, what's important is that the tides happen, not that they happen on schedule, amirite?

  • $\begingroup$ That's a good point; my concern is really about affecting the indigenous peoples. $\endgroup$ – Monica Cellio Nov 21 '16 at 21:24

What about the ship having some kind of spacetime warping alcubierre drive? This would affect spacetime, thus behave gravity like. To not disturb star systems, standard procedure is to disengage the drive far enough from other gravity wells. But this time some slight miscalculation caused it to be disengaged a bit nearer to the planet.

This in itself is normally not a big deal, and everyone aves it of, as the distance is just a security measure, but this time due to, well, lets call it "gravitational coupling" or so, it took the warping field quite a while longer to wear of, and it affected the outer moon, but just a bit, mostly unnoticed.

Even this would normally be not a big deal, but N body systems are notoriously unstable, and this gave that moon a little kick into the wrong direction. Due to complex interaction between the three (or even more moons if you want), they start disturbing each other, changing orbits, and if you like could even collide or shoot of one of theirs... this would be gradually noticeable, probably on the timescale of months or even years.


Quite simply, you can't.

The key part here is "unknowingly". If you have a structure large enough (simply massive) or advanced enough (gravity shields or internal gravity or something) to actually interfere with the tides, then it is undoubtedly being piloted by a very advanced species that has a full understanding of tides and is aware of the effects on a planet.

Our incredibly primitive culture had a keen knowledge of the tides for thousands of years, despite having the ability to reach space for a couple dozen. By the time a species has the ability to build a structure advanced enough to effect tides, they will have mastered the simulation capabilities far beyond what is necessary to account for tidal forces.

Whether or not they care is an entirely different matter.


Somebody left the tractor beam on or a control system accidentally malfunctioned activating the tractor beam. While this comes close to magical space technology effectively every other answer to this question invokes one form of magic space technology (Charles Stross TM) or other. This concept is modest by comparison, apart from the ludicrous energy requirements which have been discussed in at least one answer, with hurricane-force handwaving in the other answers. Mine is a gentle breeze in contrast.

No-one seems to have considered the possibility of a quantum gravity space-drive inadvertently leaking excess gravitons into nearby space. That could make it look like there was a new moon. if they have interstellar travel, there is every reason to expect science will solved the problem of quantum gravity and developed technology based on it. What better technology than a space-drive.


Too long for a comment.

TL;DR Whatever you do, that effect has to be pretty strong.

Explanation: If you have three moons, the tides (depending on the moons' trajectories) might be very chaotic, nowhere near the pre-industrial capabilities of tidal forecasts. We have several possibilities:

  1. Somehow, the tides were very important and that one domain was much more developed than any other.
  2. The forecasts are rather coarse.
  3. The forecasts are done only for places which have physical properties that make the flow easy to predict.
  4. They don't use forecasts, only observe the tides after the fact.

With each of these there are some problems:

  1. Flow dynamics (including resonating systems) is one really hard domain that we still work on. I think it's highly improbable for them to be able to do accurate tidal forecasts. But hey, it's your story. If you want to follow this path, perhaps you could have placed (a few hundreds years before the actual story is set?) some out-of-their-time character (crashed spaceship?) with only one survivor who had taught them how to calculate the right numbers for their planet.
  2. The effect of the spaceship has to be visible over the chaos and their coarse tidal forecasts. Any such effect would influence other things (perhaps more noticable).
  3. The effect has to get through the thing that makes the tides predictable. Similarly, any such effect would influence other things.
  4. Knowing what should have happen is easier, but still rather hard. As the tides were chaotic to begin with, when something unpredictable happens it would be most likely taken as "oh bother, what we though isn't quite right, there some new factor we missed...". It takes a whole new level of reasoning to arrive at "our results were quite good, but diverged, however, our current results are still consistent if we would assume this new huge blob of mass somewhere here near our planet...".

I hope this helps $\ddot\smile$


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