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In this answer to If we could send an orbiter to the Pluto-Charon system, could we put it in a stable orbit? I wrote:

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.

and included from that paper

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.

and the image below.

Question: Did New Horizons pass through or nearby this "sailboat region"? Did it spend any time looking for small objects that may have collected in it, or trying to avoid hitting any of them? Did it pass through this region or did it stay way from it during its flyby?


>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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tl;dr: They choose to not use calculated sailboat-avoiding trajectories and instead flew right through it because "Fortunately no hazardous dust was observed"


This paper by Guo et al. "TRAJECTORY MONITORING AND CONTROL OF THE NEW HORIZONS PLUTO FLYBY" says:

The estimated flyby distance from Pluto center was 13,673.8 km, about 21.3 km smaller than the desired value of 13,695 km

Three other trajectories were planned to avoid this "sailboat" region:

[..]In the years leading up to the Pluto flyby, scientists speculated that the discovery of new moons of Pluto could indicate the existence of dust rings surrounding Pluto. To avoid damage from dust particles, alternative Safe Haven By Other Trajectories (SHBOT) were considered as backup. [...] The SHBOT trajectory was selected to go through the region where no dust was expected or the probability of dust being there would be very low predicted by orbit dynamics analysis.

  • SHBOT1: 17531 km close approach dist.
  • SHBOT3: 21615 km close approach dist.
  • Deep Inner SHBOT: 4000 (+/-300) km close approach dist.

JPL's Solar System Dynamics Planetary Satellite Mean Orbital Parameters gives a planetocentric Charon mean distance of 19591 km (and negligible eccentricity of 0.0002). Using the given 0.9 distance separation means the unit scale in these plots are 21700 km. I added all of these to the image below to get a rough visualization: Pluto close approach options

However, they choose to not use these SHBOTs and fly right through this region because:

Fortunately no hazardous dust was observed

It's unclear to me whether New Horizons made these observations, but that same paper does say this:

The highest priority concern for the SHBOT trajectory was spacecraft safety, with achieving science objectives being second. The science observations are degraded and some measurements would have been partially or fully lost due to the SHBOT trajectory lacking the necessary [New Horizons] trajectory geometry and conditions

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    $\begingroup$ @uhoh fair, I guess I didn't make it clear that they were avoiding it in my answer, will update with emphasis $\endgroup$ – BrendanLuke15 Apr 25 at 23:07

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