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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, 2016 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, 2016 at 18:54
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    $\begingroup$ @BrianLynch right, right, right... sorry, not thinking. Let me fix that. $\endgroup$
    – kim holder
    Mar 11, 2016 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, 2016 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, 2016 at 14:41

2 Answers 2

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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, 2016 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, 2016 at 14:30
  • $\begingroup$ @jarvis You could of course do that, but to transfer the problem to the Earth-Moon system is far easier. $\endgroup$
    – SE - stop firing the good guys
    Mar 10, 2016 at 14:49
  • $\begingroup$ Of course, I was just thinking of other ways of handling this problem .. :) $\endgroup$
    – ben
    Mar 10, 2016 at 15:25
  • $\begingroup$ @LocalFluff, good question, though suited more for Astronomy than here. $\endgroup$
    – Hobbes
    Mar 12, 2016 at 13:44
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Totally by accident I've just happened to run across the 2014 open access paper A peculiar stable region around Pluto with the abstract below.

The purpose of the paper was to see if stuff might already be in some long-lived orbits around the system, things that New Horizons might pass near and photograph, or perhaps even collide with.

I don't know if New Horizons looked or not, but if you wanted to put something there in the system that would remain stable for a while, this three-body orbit will last quite a while. To that end I've just asked Did New Horizons look for "sailboats" in the Pluto-Charon system's sailboat region? Did it pass through it or avoid it?

Abstract:

Giuliatti Winter et al. found several stable regions for a sample of test particles located between the orbits of Pluto and Charon. One peculiar stable region in the space of the initial orbital elements is located at a = (0.5d, 0.7d) and e = (0.2, 0.9), where a and e are the initial semimajor axis and eccentricity of the particles, respectively, and d is the Pluto–Charon distance. This peculiar region (hereafter called the sailboat region) is associated with a family of periodic orbits derived from the planar, circular, restricted three-body problem (Pluto–Charon–particle). In this work, we study the origin of this stable region by analysing the evolution of such family of periodic orbits. We show that they are not in resonances with Charon. The period of the periodic orbit varies along the family, decreasing with the increase of the Jacobi constant. We also explore the extent of the sailboat region by adopting different initial values of the orbital inclination (I) and argument of the pericentre (ω) of the particles. The sailboat region is present for I = [0°, 90°] and for two intervals of ω, ω = [−10°, 10°] and (160°, 200°). A crude estimative of the size of the hypothetical bodies located at the sailboat region can be derived by computing the tidal damping in their eccentricities. If we neglect the orbital evolution of Pluto and Charon, the time-scale for circularization of their orbits is longer than the age of the Solar system for bodies smaller than 500 m in radius.

and

Final Comments

[...]The relevance of the sailboat region for the New Horizons spacecraft is addressed in Giuliatti Winter et al. (2014). In this work, we verified that the nominal trajectory of the New Horizons passes near the region of the sailboat region trajectories and we also identified the location of the densest regions, which corresponds to the highest probable location of particles of the sailboat region.

>Figure 3. The set of periodic orbits, in the synodic frame, for different values of CJ presented in Fig. 2. The barycentre is located at 0, the origin of the coordinate system. The large and small black dots indicate the location of Pluto and Charon, respectively.

The set of periodic orbits, in the synodic frame, for different values of CJ presented in Fig. 2. The barycentre is located at 0, the origin of the coordinate system. The large and small black dots indicate the location of Pluto and Charon, respectively.

>Figure 7. A sample of periodic (in black) and quasi-periodic (in yellow) orbits, in the synodic frame... Pluto is represented at the position (−0.1, 0) and Charon at (0.8, 0).

Figure 7. A sample of periodic (in black) and quasi-periodic (in yellow) orbits, in the synodic frame, for (a) CJ = 2.786 and 2.936, (b) CJ = 3.016 and 3.056, and (c) CJ = 3.116 and 3.224. Pluto is represented at the position (−0.1, 0) and Charon at (0.8, 0). The quasi-periodic orbits presented here are those with the largest amplitude of oscillation which correspond to the largest islands in the Poincaré surface of section (Fig. 2).

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