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Could a group of satellites be put into orbit such that they form a 2d shape that moves across the sky? I have tried to research satellite constellations but I can't figure out if they just work together or actually move in a synchronised way.

I know (or assume) that satellites could be put into a line, and I can see that two satellites that are "very far apart" would need to be orbiting on different planes and hence not be moving parallel when viewed from earth. But can multiple satellites be far enough apart that they are resolved as different points by a human (and can hence form a shape) but still be close enough that they can practically follow what appears to be the same orbit from earth?

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  • $\begingroup$ The keywords "relative motion" are what I'd use to find things like the CW equations to get bounds for relative motion for satellites (under some constraints) and develop some intuition on the topic $\endgroup$ Commented yesterday
  • $\begingroup$ having slept and done a bit more searching, there seems to be research on "bounded relative motion" with relevance to some kind of satellite formations that I haven't had time to look deeply into. If there was a chance for some kind of "I don't need the shape to be constant, I just need to always have at least one satellite close-ish to this part of a shape in the sky" I think that research would be where the answer would lie. $\endgroup$ Commented yesterday
  • $\begingroup$ oh and welcome to Space Exploration SE :) $\endgroup$ Commented yesterday

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Satellites don't need to be "very far apart" to be orbiting different orbital planes. Every orbital plane comes through the center of Earth, exactly. Satellites, that are moving parallel to each other in circular orbits with one 10cm "left" from the other will have that satellite 10cm right on the other side of Earth (unless they collide, 1/4 of orbit away).

You could totally have a constellation that converges to form an image for a moment at one point in time, and repeat twice every orbit, the image on the other side of Earth mirrored, but it would "squish" into a line in between these, satellites left of the central axis of the image transitioning to right and vice versa.

Of course you could maintain the formation propulsively, the satellites opposing the gravity, and using high-efficiency low-thrust ion engines they could stay in one formation for weeks, or even months, but as soon as they switch the thrusters off, they'll return to following their individual orbital planes.

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    $\begingroup$ Perfect alignment would occur at some specific instant, but I think it's undersold a bit as a "moment" - I'd expect the constellation to be resolvable for several minutes by observers on the ground, over a range of hundreds of miles. The orbital paths will appear virtually parallel to anyone on the ground, although they must cross. $\endgroup$ Commented yesterday
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    $\begingroup$ @NuclearHoagie True, and in between the image would be at different degree of squished, but likely still recognizable most of the time. $\endgroup$ Commented yesterday
  • $\begingroup$ Okay so to clarify, my understanding that fixed orbits wouldn't work is correct, but it would be feasible to maintain a shape for a decent amount of time using existing technologies. But using something like a solar sail to maintain the shape indefinitely wouldn't work? $\endgroup$ Commented yesterday
  • $\begingroup$ @BugCatcherNakata I'm not expert enough to fully answer, but consider: a solar sail works by being a large, lightweight thing getting pushed by the solar wind. Earth's atmosphere doesn't suddenly end at a certain height, but rather becomes very very thin - but at least at low satellite orbits it's still enough to decay an orbit without a solar sail over a few years or decades. My gut feeling is that unless you put them into very very high orbits, the atmospheric drag on the sail would far outweigh the benefit it gives. If you could even "steer" its effect enough to make it help at all... $\endgroup$ Commented 23 hours ago
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    $\begingroup$ @Syndic Rules of thumb: Karman line, beginning of space: 100km. Most LEO satellites - 200-400km. Solar sail starts making sense: 1000km. Also, Van Allen belts begin at 650km so your solar sats need to be extra radiation-hardened. $\endgroup$ Commented 23 hours ago
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There isn't really such a thing as "parallel" orbits in the Euclidean sense. On a sphere, parallel lines converge 90 degrees around the sphere.

Suppose you set up a series of satellites to spell out EAT AT JOE'S over Springfield, Illinois. Well, okay, let's make it simpler and just write JOE. For convenience, let's assume the letters are oriented with "up" being north, J in the east and E in the west.

If we consider a single satellite on this orbit, we can find the two extreme points of the orbit: the northernmost point over Springfield; the southernmost point at the antipode, about halfway between South Africa and Australia. There will be two equator crossings, one off the coast of Ghana and one close to Tonga.

So now we run into our problem. If we think about the word JOE made up of a few dozen satellites, the northmost satellite in the J over Springfield (at latitude, say, 40.0001 N) has to become the southmost satellite in the Indian ocean (40.0001 S), and the bottom of the J (39.9999 N) has to become the northmost point (39.9999 S). The letter has to flip upside-down. As the satellite array approaches Africa, the word JOE is going to squish down into a line.

Obviously, if two satellites are precisely aligned east-west and at the same altitude, then logically, they have to pass through each other at the equatorial crossings in order to swap places, which would be a collision. You could fix that by having some of the satellites operate slightly higher or lower, but then you run into the problem that some of the orbits are slightly faster than others, so your letters would smear and come apart over time. So you'd need to carefully position your satellite "pixels" so that any given north-south line contains, at most, only one satellite, all at exactly the same altitude (and it's more complicated than that, because the earth's gravity isn't completely consistent, so it's going to require maintenance -- stationkeeping -- to retain the shape).

So that should answer your question: Any shape you come up with on any set of orbital paths (no matter how close together) has to invert on the opposite side of the planet, which means it will look as intended over one particular area of the planet, be upside-down on the opposite side, and everywhere else it'll be some degree of squished.

Realistically, the point where it "looks right" will move over time based on the difference between the satellites' orbital period and the earth's rotational period, but for this example I'm ignoring that part of the problem. If you put the satellites in a geosynchronous orbit, they'd stay put on the longitude but flatten and then invert every 12 hours (and also move north and south along the longitude line), so in that case it would really be that the image looks correct at a specific time every day and flattened or upside-down the rest of the time.

And that's how satellite constellations work -- they're not a consistent shape that spins around the planet, but a series of interlocking orbits carefully designed so that none of them bump into any of the others. That isn't that hard, you just have to make sure each inclination has a unique altitude, or at least a unique altitude over any given point; we can use slightly non-circular orbits to create completely orthogonal orbits, but that's getting a bit more complicated than I want to go into here.

GPS, for example (from Satellitemap.space): GPS satellites

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  • $\begingroup$ Another way to get around the collision problem would be to slightly rotate the image, so that no satellite is north/south of another. $\endgroup$ Commented yesterday
  • $\begingroup$ @MatthewJensen I said "you'd need to carefully position your satellite 'pixels' so that any given north-south line contains, at most, only one satellite". Rotating your image slightly is one way to accomplish that, but depending on the image, that could still create vertical alignments, so you just have to analyze and plan each image individually $\endgroup$ Commented 16 hours ago

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