I’m building a binary planet system, and I want to know where I could place large groups of satellites. I’m worried centrifugal force and coriolis effects will prevent a satellite from occupying any sort of sort of low earth orbit unless it’s a polar orbit between the near and far sides. Apart from what orbits can’t be used, I wonder what places might be the most stable and practical, like Lagrange points and such. One idea I had was where the centrifugal force allows satellites to orbit around the axis running between the 2 planets, but shifted toward the far side. For context, the major body is the size of Venus, and is habitable. The minor body is the size of mars, and is not. The planets centers are separated by about 7 earth radii, are tidally locked to one another, and have an orbital period of 30 hours. The barycenter is very close to the major body’s surface
-
1$\begingroup$ Because the planet orbit every 30 hours, maybe low orbits would just require some stationkeeping. Not sure tho $\endgroup$– Sean HolmCommented Jun 16, 2022 at 1:55
-
$\begingroup$ FYI, you're asking for solutions to the 3 body problem, which is generally unstable, so your question probably doesn't have a good answer in science unless it is a particular special case. $\endgroup$– MathaddictCommented Jun 16, 2022 at 15:27
-
1$\begingroup$ Now this is a good worldbuilding question. Thank you! $\endgroup$– JBHCommented Jun 17, 2022 at 4:05
-
$\begingroup$ @Mathaddict an artificial satellite shouldn't have enough gravity for it to come anything close to the 3 body problem $\endgroup$– Sean HolmCommented Jun 19, 2022 at 2:56
2 Answers
Unless the planets are less than a few diameters apart, low orbits should be reasonably stable. The highest we consider LEO is about 1/4 of an earth radius above sea level (the lowest are about 1/32). Depending on the spacing and coorbital rate, tidal forces might need to be accounted for.
Also stable are the L4 and L5 points (or the L5 and L4 points if you're on the other planet). These will be far more distant, and consequently more expensive to get to. Further, they'll get crowded pretty quickly if you send hundreds of satellites up there.
The only other stable orbits, barring niche solutions using resonance to enforce stability (which will depend extremely on the planets' masses, distance between them, and coorbital rate) would be well outside the coorbit. These are probably not as useful as the low orbits, and would be extremely expensive to get to.
The other Lagrange points, as well as wide internal orbits, could be maintained only by constant fine adjustment, so these would have to have a lifetime supply of fuel. It's not an insurmountable problem; that's how the James Webb keeps itself at the earth-sun L2 point, so it can be accomplished.
-
$\begingroup$ The planets are very close: the question says that the orbital period is 30 hours. $\endgroup$– AlexPCommented Jun 16, 2022 at 21:01
-
$\begingroup$ Assuming Earth mass and size, that's still 5 diameters between centers. I think stable low orbit is still feasible with tidal forces accounted for. The 2nd planet's CM is still almost 9 times farther away, making its gravitational pull on the low orbiting satellite nearly 81 times less. $\endgroup$ Commented Jun 17, 2022 at 14:42
-
$\begingroup$ Please read the context at the bottom for the system specifications $\endgroup$ Commented Jun 19, 2022 at 11:55
The most stable place for a satellite to orbit in a binary planet system is actually not around either of the two planets, but rather around the system's barycenter. This is because the gravitational pull from both planets cancel each other out at this point, making it much easier for a satellite to maintain its orbit.
There are also several Lagrange points in a binary planet system, which are points where the gravitational forces of the two planets cancel each other out. These points are not as stable as the barycenter, but they can still be used as locations for artificial satellites.
Finally, it is also possible to place satellites in polar orbits around either of the two planets. These orbits are not as stable as the other options, but they can be used if necessary.
-
1$\begingroup$ "Low-eccentricity quasi-satellite orbits are highly unstable, but for moderate to high eccentricities such orbits can be stable." en.wikipedia.org/wiki/Co-orbital_configuration#Quasi-satellite - If you're ok with it being useless 90% of the time, give it high eccentricity and have it orbit both bodies, +1. $\endgroup$– MazuraCommented Jun 18, 2022 at 2:20
-
$\begingroup$ This awnser only applies to the scenario where the planets are of equal mass. The only reason the barycenter seems viable then is because the L1 point happens to overlap with the barycenter here. The L1 point is unstable because any drift will make a feedback loop pulling the satellite toward one of the planets. It’s more like balancing a ball on a peak than placing it in a pit. This would be fine for a satellite, but my planets are not of equal mass, More like 8:1. Resembling the Pluto-Charon system, the barycenter is really close to the major planet, so the barycenter is not stable $\endgroup$ Commented Jun 19, 2022 at 11:53
-
1$\begingroup$ The barycenter may not be a stable place to park a satellite, but that doesn't mean that orbits traversing the barycenter are necessarily unstable. $\endgroup$ Commented Jun 20, 2022 at 16:38