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I want to know what kind of effect a large circular space station would have on the planet if it went completely around the equator in one giant circle? Would it impact gravity or anything? And would the planet have any effect on the space station?

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    $\begingroup$ It seems that this question would fit better with the scope of the Space Exploration or Physics sites on this network... $\endgroup$ – DarthDonut Jul 13 '18 at 7:02
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    $\begingroup$ @DarthDonut Just because a question might be on topic elsewhere doesn't make it off topic here. $\endgroup$ – Tim B Jul 13 '18 at 8:55
  • $\begingroup$ @TimB I never said that the question is off topic ;) $\endgroup$ – DarthDonut Jul 13 '18 at 12:45
  • $\begingroup$ Arthur C. Clarke's 3001 explores this concept. $\endgroup$ – user535733 Jul 13 '18 at 12:49
  • $\begingroup$ A more recent example that actually goes into a lot of what it would take to do this is Neal Stephenson's Seveneves (You will have to get through 2/3rds of the book before it gets there though.) $\endgroup$ – Darrel Hoffman Jul 13 '18 at 12:55
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A ring is not stable, so it would require an attitude control system for stationkeeping (this was the problem discovered by MIT students concerning Larry Niven's Ringworld (source)).

It would not have any significant effect on the planet, except they would see a light ribbon in the sky if the station is large and near enough.

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    $\begingroup$ @Infrisios If your ring is anything except absolutely perfectly centered, one side will be closer to the center of the earth. Because gravity affects objects by the inverse square of the distance, that side will feel more pull of gravity, and accelerate. That will cause that side to move closer, and mechanical forces in the ring will push the other side further from the center of the earth. This will increase the imbalance, increasing how much force pulls the near side of the ring nearer. The key is that the ring is almost a rigid body - mechanical forces can let one part push on the other. $\endgroup$ – Cort Ammon - Reinstate Monica Jul 13 '18 at 7:45
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    $\begingroup$ @Infrisios planetary rings are made of separated particles, so the fact that one particle comes nearer the planet does not significantly alter any other (astronomy.ohio-state.edu/~pogge/Ast161/Unit6/rings.html ). While a sphere is in indifferent equilibrium, a ring is unstable - any variation from the starting position will pull it farther. There's a nice paper here for the "ring around the sun" case (the planetary case is qualitatively the same, downscaled): brannenworks.com/GE253/ringworld.pdf $\endgroup$ – LSerni Jul 13 '18 at 7:45
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    $\begingroup$ @Infrisios To put it simply individual objects in orbit (even if gathered together in the shape of a ring) can go into an elliptical orbit and recover from variations. Solid rings can't. $\endgroup$ – Tim B Jul 13 '18 at 8:57
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    $\begingroup$ Yes but it'd have to be actively stabilised and if it ever stops being maintained then you're looking at a catastrophe on an apocalyptic scale for the world below. Read: Everything on the equator being destroyed and the world experiencing a small nuclear winter. $\endgroup$ – Ruadhan Jul 13 '18 at 10:52
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    $\begingroup$ @JollyJoker It would have to be so far from rigid that one part could overtake another and esentially fly on a different orbit (this is what the particles in a real ring system do). Simply putting some elastic bands between the elements wouldn't help - once you'd reached the elastic limit you'd be back to rigid again. $\endgroup$ – Oscar Bravo Jul 13 '18 at 12:30
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There are serious stability issues as addressed in other answers.

The solution though is to tether the ring. You build a number of space elevators along the equator, and you attach the ring to those space elevators.

The tension in the cables keeps the ring in place and you now have an excellent launching point and can even use it for travel around the planet.

Note that we are not yet able to build space elevators, but we're getting closer all the time.

The space station would have virtually no effect on the planet beyond possibly being visible as a line in the sky. It just doesn't have enough mass.

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    $\begingroup$ While i agree the tethering could work in theory, it would only do so if the ring was in geosynchronous orbit, anything below that and the tensile stresses would rip the tethers to pieces $\endgroup$ – Blade Wraith Jul 13 '18 at 11:52
  • $\begingroup$ Well yes. That's the point of it. Although possibly you might build it just outside geosynchronous so as to keep a modest amount of tension in the system. $\endgroup$ – Tim B Jul 13 '18 at 15:35
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The principle of an orbit is that one object orbits another. Each could be considered a particle at its centre of mass for the sake of defining its location.

The centre of mass of a planet is in its core, the centre of mass of a satellite is harder to calculate but it's somewhere in the middle of the object itself. The centre of mass of a ring is in its centre.

In your case the centre of the ring is in the centre of planet, there's no orbit. Each object, planet and ring satellite, has no gravitational effect on the other.

This is covered by Shell Theorem, but the short version is, your full ring space station is not practically possible.

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  • $\begingroup$ I was about to write the same, but was checking for a similar question posted some time ago... $\endgroup$ – L.Dutch - Reinstate Monica Jul 13 '18 at 7:10
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    $\begingroup$ This is not actually the case: the shell theorem applies to spherical shells, not donuts. I made the same instinctive error not long ago, possibly in the very post @L.Dutch seems to remember. $\endgroup$ – LSerni Jul 13 '18 at 7:48
  • $\begingroup$ @LSerni, it does and it doesn't, life is never so simple but this question needed a simplified answer. It's a classic case of having to throw out everything you learned in high school science to be able to move on. $\endgroup$ – Separatrix Jul 13 '18 at 7:51
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    $\begingroup$ It is possible but requires stabilization. $\endgroup$ – Tim B Jul 13 '18 at 8:57
  • $\begingroup$ I'm not sure I understand the "it does and it doesn't, " comment. The shell theorem doesn't apply (a ring is not a shell!) and it doesn't remain stable. Period. A ring will crash into the planet unless kept permanently under control. $\endgroup$ – Mark Olson Jul 13 '18 at 12:16

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