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There are a lot of large starships in sci-fi but their influence on a solar system is rarely deeply explored and kept fairly limited. For example, in Eve-Online fluff, titans are considered massive enough to alter tides on nearby planets but that's about the full extent of their effect. (I'm assuming that bit might be exaggerated since they aren't even remotely close to the size of the moon.) The question I have though is, at what mass would a starship being in close proximity cause more than just a small problem. (i.e. Creation of rogue planets, undesirable tidal/tectonic effects, rendering a planet uninhabitable or collision of a starship with a planet)

The rules for this are fairly simple:

  • Bigger = objectively better. More mass = more ship. Whoever has the largest vessel and the most of them wins hand over fist over their opponent. (Obviously not true in reality but we're using sci-fi logic here)
  • All engineering/practical issues are considered solved. Funny issues such as multiple day trips to travel from end to end of the vessel as well as tensile strength and thrust issues from such a large ship are not being considered.
  • Destruction of the ship or the system is considered as a failure by both sides in the conflict as they seek to take and hold the system instead.
  • Mass negation by exotic matter/etc is not possible

As a wild guess, I would think one solar mass would easily destabilize any star system's orbits. An additional wrinkle I foresee would be that you could have a much smaller vessel, but the total fleet mass of both sides combined could also be sufficient to pose a threat to the systems existence. I suppose at that point you would have an effective mass "budget". I'm interested to hear what you guys think.

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    $\begingroup$ Altering tides is already a titanic effect considering the volume of water involved on a Earth-like planet. One would expect that a ship of sufficient mass to alter tides entering (or leaving) close orbit around a planet would most likely also trigger severe earthquakes as the planet adjusts to being flexed. $\endgroup$ – GrumpyYoungMan Dec 18 '20 at 21:44
  • $\begingroup$ @GrumpyYoungMan Titanic in reality, yes. What I'm asking for here however, that would be on the extreme small impact end of the spectrum. I didn't write their fluff so I can't speak on how accurate that would be for a ship of the mass they stated. $\endgroup$ – Razmode Dec 18 '20 at 22:32
  • $\begingroup$ In a structure massive enough, amount of pressure near its center would be enormous. Should we consider that a "solved engineering issue"? $\endgroup$ – Alexander Dec 18 '20 at 23:46
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    $\begingroup$ Can a ship have an extremely low density, like a plasma? Can a big ship be modular, so it splits into multiple mass-distributing subcomponents (perhaps only coming together briefly for FTL jumps and firing BFG weapons)? $\endgroup$ – DWKraus Dec 19 '20 at 0:09
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    $\begingroup$ The problem is a little like that of ships and boats. Boats can come ashore, bigger vessels get close but can't come ashore, bigger still and eventually they can't get into some ports and have to anchor at sea. It all depends on how close you would allow them to come. $\endgroup$ – Slarty Dec 19 '20 at 15:52
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Impractically Huge

There's two ways to look at this: size and mass. Mass is the easier one, so we'll look at it first.

Mass

Our own moon affects more than just tides. It can cause earthquakes, has an (admittedly slow) effect on Earth's rotation, and even has an effect on the Earth's shape. A ship with mass equal to Luna would have some moderate to nasty effects on a planet — and that's ignoring what effects it would have on any moons already orbiting the planet.

I've not done the math, but I could easily believe that a ship having even a third the mass of Luna entering Earth's orbit would have some nasty effects — everything from flooding to earthquakes to destabilizing Luna's own orbit. And who knows what side effects that would have? Everything from screwing up shipping to confusing appropriately dressed witches dancing in a forest glade. Yuck.

But think about it — the moon is solid,Citation Needed which means a ship equaling its mass would be gi-honking-normous! Anywhere from a third to three times the size by volume depending on exactly what's inside that ship. Which brings us to the other problem.

Size

Could you imagine how our world would react if an object 3X the diameter of the moon passed between the Earth and the sun? Cats and dogs, living together! Mass hysteria! Now, I'm going to grant you that other than causing problems with solar panels should the ship be in geosynchronous orbit, this wouldn't have a long-term effect on the planet.

But let's say the ship is the diameter of the Earth. The ship has a whole lot of hollow, so it might only represent 10% (or maybe 0.001%) of the Earth's mass — but the size would cause all kinds of problems if the planet it's orbiting is utopic and enlightened and decided to rely on solar power for 90% of its energy. No silly geosynchronous orbit for this ship! No siree! That ship's under power, so it's simply plunked itself into Earth's #1 Lagrange point and it's simply blocking the sun!

I like this solution — it has that wonderful James Bond implausibility that makes it absolutely worthy of a really good super villain.

However...

However, to be frank, I personally find ships large enough to affect a planet in any way to be unreasonable, even unbelievable. What's the point? It's an engineering complexity that requires the proverbial power of a black hole to move around when a small fleet of ships can quite literally provide the same firepower (aka, "consequence in battle") for a fraction of the price and engineering headaches. Which is why the several articles over the last couple of years postulating the possible discovery of mega-structures in space make me smirk. From an engineer's perspective, why would anyone build one? There's always a more practical way to solve the problem. But that's just my opinion.

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    $\begingroup$ You build that large because you’re moving a whole society to go visit another society. If you’re talking generations to get between worlds, then scale is the only thing that makes sense. $\endgroup$ – SRM Dec 19 '20 at 17:02
  • $\begingroup$ @JBH The main premise here is that you have two universal superpowers that can field an absolute absurd amount of fleet tonnage, so much so that even a small fraction of their total force can obliterate what they wanted to take by accident. Somewhat similar to modern combat where the objective is destroyed through just the use of conventional weapons. Sure they can build much smaller ones, but they can field so many that they would be equal in mass easily. $\endgroup$ – Razmode Dec 19 '20 at 18:29
  • $\begingroup$ @srm, Gratefully, that's incorrect. Depending on what density you're willing to live with, the entire population of the Earth today would fit in the space occupied by Tunisia (read down in the article to see my point). Moving an entire society requires nowhere near the size of ships I described. In fact, from a mass perspective, they require nowhere near a fraction of the mass I described. $\endgroup$ – JBH Dec 20 '20 at 16:06
  • $\begingroup$ @Raznarok I'm not one to use the word "intelligent" when describing government too often, but the assumption that governments would bother building such large and worthless objects boggles the mind. The first one who did it would cause the other one to build a veritable beehive of smaller ships to wipe it (easily) out. So, while I could believe a mind-bendingly massive fleet of ships. I can't believe the dreadnoughts I described. Even government isn't that silly. Keep in mind, aircraft carriers are surrounded by a defensive fleet for a reason. $\endgroup$ – JBH Dec 20 '20 at 16:10
  • $\begingroup$ @JBH If you aren’t taking enough “blank space” for generations to build their own lives not planned by the original designers, I have no confidence that the ship will survive to reach its destination. Packing everyone into Tunisia means regimented lives that sentience will rebel against at some point. $\endgroup$ – SRM Dec 20 '20 at 17:58
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It's ALREADY too big for safety

A spaceship large enough to

are considered massive enough to alter tides on nearby planets

to the same level that the Earth's Moon does, is already altering the planet's orbit by 43537km change in semi-major orbital axis per day, if the ship remains on the same side as the planet. Sure that is only 0.02% of the orbit, but it is already enough to start altering climate. From just one day, of one ship being near enough the planet to cause Moon-level tides.

Do that for a year, and the orbit's eccentricity goes up so much that the planet experiences greater seasons due to ellipticity of orbit, than it does from axial tilt.

Edit: Remember that the distance between your ship and the planet has a HUGE impact. If your ship is at 10 times the distance, it needs to have 100 times the mass to cause an identical disruption. Similarly if you put your ship in close orbit around the planet (poor satellites get squished!) then it needs a lot smaller mass to "raise tides" to the same extent.

Moral of the story: If your ship is very massive, stay far away from planets, moons, non-propelled spacestations, and other valuable real estate that cannot compensate for distortions in its orbit.

P.s. It would have MUCH greater effect on the Moon's orbital shape. And woe to any satellite owners, even 10 minutes of such a large ship in the area would destabilize geostationary orbits.

Which leads to a curious story hook: Go visit the Alien homeplanet in your SuperMegaFortress ship. "We love peace! We will stay here for negotiations until we are satisfied with the contract! No, this big ship is not here to threaten you, it is merely needed for the comfort of our diplomatic staff."
Meanwhile, just by being there, not firing a single weapon, you are dooming their planet to death by warping its orbit out of the habitable zone.

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Given that this ship can be built already in space since it would be nearly impossible to such thing take off, things can get pretty huge. The real limiting factor here is gravity, as the bigger it gets, the more it weighs, and the more it weighs, the higher the gravity affecting its surface.

Systems would not be much of a issue. They can be made to work under higher gravity. Even if some technology requires less g to work, it can be moved to deeper inside the ship. Systems could only be the problem when you are already in a planetary scale and its surface g is enough to crush any sensor but even then you can keep smaller ships around to guide the bigger one.

All engineering/practical issues are considered solved

I don't know if you include it collapsing under its own weight or smaller ships getting in and out such high gravity structure as a solved problem, but it is the real problem you should be worrying about, here is the formula to the gravitational acceleration in the surface.

mG/r² or 4πrdG/3. Where m is total mass, d density, r radius and G = 6.674×10−11 m3⋅kg−1⋅s−2

I don't think a starship would be made significantly bigger than the native species planet gravitational acceleration, outside maybe national pride. Also I want to point out that to reach such sizes it probably should be built as a sphere to make the structure more uniform, and that core could be hollow if the superficial layer is able to sustain itself structurally. The hollow core would reduce overall gravity and provide nearly unlimited storage space. Another idea could be have many layers in multiple depths to provide appropriate gravity to many species.

Edit

So what you are asking is how big of a ship could build without messing the star system it is inside? Definitely not big enough. Even if it was as big and heavy as Jupiter, it would not do much damage unless we get really close to the planets. Next to the sun, any gravitational field the ship can produce is negligible.

And the sizes we can actually achieve would be even smaller. Even considering the resources to build it can be brought from across the universe, either the core would collapse under its own weight and become a simple mass of metal/building material if it is massive, or it would collapse due to lack of structure if it is hollow. You could cheat it a little by building it in the surface of a planet and adding thrusters to make it movable, or having a tiny core that work as a supermagnet to give structure to the shell. Neither of these is big or heavy enough to become a problem.

A fleet of those might be dangerous if they stay clustered, mostly for themselves, but also for some small planet ""near by"". In this case they may as well fight near the system they are attacking/defending, it is not like real armies preferred fight in cities instead of taking the battle to open terrain.

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  • $\begingroup$ When I say system I am referring to the particular star system they would be fighting within. $\endgroup$ – Razmode Dec 19 '20 at 0:22
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In practice, Earth is a kind of spaceship. Solar system is OK-ish with it.

Anything smaller is less practical (see Mars / Moon).

Anything bigger is less practical either. Human bodies and the rocket technology tolerate less gravity and can even benefit of it one way or another, but more gravity or deeper gravity well quickly becomes unacceptable).

I think that as a space habitat gradually grows, it will gradually transition from a structure held by its structural strength into a structure where things are just piled up one way or another and held by gravity (and the oldest and deepest parts falling out of use).

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Mass, applied the right way, can be a problem already when it's tiny compared to earth.

Take, let's say a tenth of the mass of the moon. That's an enormous space ship already. It is optimized for a parallel start of all of it's millions of rocket warheads, so it is shaped like an enormous pancake rocket holder grid.

Park it between sun and earth in that Lagrange point, wait, see the ice age coming.

It has also a mirror surface as radiation and laser protection. Park it so that it shines light on earth and gleefully see hurricanes forming.

Move it to half the earth-moon distance but not into orbit. Instead use the engines to keep it where it is and see earth decelerating or accelerating in response: gravitational pull is there but your ships don't move because they use their magic FTL engines to stand still in relation to earth. If there is a pull and one of the bodies doesn't move, the other one starts to move. The effect will be tiny first but can build up over time, moving earth to another eccentricity or to another orbit. Very few changes here are enough to be catastrophic. But okay, this costs enormous amounts of fuel.

So our pancake decides to move into low orbit at last, providing earth with a 2-hours day-night cycle and the moon-earth system with a third body, just to make the gravity equations more difficult to solve.

The point of all of this is, as ships are mostly hollow, their size becomes mind boggling when their mass even reaches a small percentage of a planetoid like the moon. And that size can do more than just gravity effects.

If you want to talk about gravity, I think even if you don't build one enormous but many small ships you will have an effect. All mass on an orbit needs to be added for the orbital calculations, if I remember correctly. So if you build 1000 ships each with 1% the mass of moon and you put them all in the same solar orbit as earth, even if it is not nearby, they will affect earth. Can't tell you how, though, that I better leave to people who know how to calculate this kind of stuff.

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