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The barycenter of Pluto and Charon is in space between the two bodies, close to Pluto. It is the center point of the animation below. The image is roughly to scale. Note that the two bodies are mutually tidally locked, with the same sides always facing each other.

animation of orbits of Pluto and Charon

Charon is 17,536 km from the system barycenter, and 19571 km from the center of Pluto. Its surface gravity is 0.288 m/s2. It has a mean radius of 606 km.

Pluto has a surface gravity of 0.62 m/s2 and a mean radius of 1,187 km. The atmosphere of Pluto, though very thin, is also an issue over time. At the surface it is 1 Pa, 0.001% the pressure of Earth's atmosphere (right now, while it is close to the sun). Above the bottom layer of the atmosphere, its scale height is estimated at 50 to 60 km.

Would it be difficult to put an orbiter in a stable orbit around Pluto? Or around Charon? What kind of station-keeping might be involved?

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    $\begingroup$ Putting a probe in orbit around Pluto gets more complicated due to the atmosphere. It's tenuous, but it extends far above the planet (much further than Earth's atmosphere, IIRC). $\endgroup$ – Hobbes Mar 10 '16 at 7:34
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    $\begingroup$ @LocalFluff - now i've switched out the movie for a gif animation. $\endgroup$ – kim holder Mar 10 '16 at 18:54
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    $\begingroup$ @BrianLynch right, right, right... sorry, not thinking. Let me fix that. $\endgroup$ – kim holder Mar 11 '16 at 2:53
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    $\begingroup$ @uhoh Hm. Orbiter missions are for long-term observation of a planet. Historically they are put into a low orbit for the sake of more detailed observation. So I'd say it has to be in orbit around just Pluto, not the whole group of objects. (Or around Charon, if someone wants to look at that.) The data rate from that distance is low, and we know from previous probes that they often last much longer than their planned missions. So let us say 50 years. $\endgroup$ – kim holder Mar 11 '16 at 14:42
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    $\begingroup$ It seems likely to me that one can find inclined retrograde orbits which don't sync with the periodicity between Pluto and Charon and thus don't accumulate any substantial disturbance beyond economical station keeping fuel consumption. And orbits which are highly eccentric and get close by now and then. Pluto has several ancient moons, so there obviously exist stable orbital windows there. $\endgroup$ – LocalFluff Mar 12 '16 at 14:41
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Let us try to see what happens if we place the satellite in an orbit with and altitude of about 320 km over Pluto, so we get above most of the atmosphere, and get a nice round number for the orbital radius at 1500 km. Let us also normalize Pluto's gravitational acceleration on the satellite to "1".

Then, when closest to Charon, the satellite is 18000 km away from it, making its gravitational influence $\frac{1}{1180}$ of Pluto's, dragging in the opposite direction. When farthest from Charon, its gravitational influence is down to $\frac{1}{1610}$, dragging in the same direction as Pluto. We then have a total gravitational influence in the range 0.99915 to 1.00062.

That does not say a lot about stability, but:

Let us compare that to a system we are more familiar with. What orbital radius has a satellite in the Earth-Moon system with a similarly large range? That happens to be at around 87000 km. That is about twice as high as GEO. Given the very low impact of the Moon on GEO satellites, I would expect that to be pretty stable. Same so for the low Pluto orbit.

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  • $\begingroup$ Are there yet any explanations to why all the small moons of Pluto rotate chaotically? Is it because of the "wobbly dance"? It doesn't seem to have been expected although masses and distances were well known shortly before the flyby. Whatever is causing chaotic rotation could maybe cause unstable orbits too. But there they are since billions of years. $\endgroup$ – LocalFluff Mar 10 '16 at 14:01
  • $\begingroup$ Why do not neglect atmosphere for the first assumption, make the sat orbit circular, use known coordinates and look then at the tidal generating potential of charon? Just some thoughts... I dont have the time to work on that right know... $\endgroup$ – ben Mar 10 '16 at 14:30
  • $\begingroup$ @jarvis You could of course do that, but to transfer the problem to the Earth-Moon system is far easier. $\endgroup$ – Hohmannfan Mar 10 '16 at 14:49
  • $\begingroup$ Of course, I was just thinking of other ways of handling this problem .. :) $\endgroup$ – ben Mar 10 '16 at 15:25
  • $\begingroup$ @LocalFluff, good question, though suited more for Astronomy than here. $\endgroup$ – Hobbes Mar 12 '16 at 13:44

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