Table 1
Orbital elements and masses of the Pluto satellites.
| Parameter | Charon | Styx | Nix | Kerberos | Hydra |
|---|---|---|---|---|---|
| a∕RP (a) | 16.51 | 36.92 | 41.55 | 48.99 | 55.00 |
| a∕RP (b) | 16.50 | 35.70 | 40.98 | 48.61 | 54.47 |
| a∕RP (c) | – | 35.91 | 40.99 | 48.64 | 54.49 |
| e × 10−3 (a) | 0.58 | 36.70 | 16.78 | 6.82 | 12.22 |
| e × 10−3 (b) | 0.05 | 0.01 | 0.00 | 0.00 | 5.54 |
| e × 10−3 (c) | – | 5.8 | 2.0 | 3.3 | 5.9 |
inc  (a) |
112.90 | 112.86 | 112.92 | 112.57 | 113.09 |
inc  (b) |
0.0 | 0.0 | 0.0 | 0.4 | 0.3 |
inc  (c) |
– | 0.81 | 0.13 | 0.39 | 0.24 |
M  (a) |
356.76 | 18.79 | 49.94 | 326.33 | 32.45 |
 (a) |
293.25 | 348.42 | 293.25 | 293.25 | 293.25 |
 (a) |
227.40 | 227.42 | 227.45 | 227.09 | 227.16 |
| Mass (kg) (b) | 1.59 × 1021 | 1.49 × 1015 | 4.49 × 1016 | 1.65 × 1016 | 4.79 × 1016 |
| Errors (km) (b) | 5 | 514 | 23 | 135 | 29 |
| Δa (km) (a) | 0.043 | 1761 | 1086 | 637 | 443 |
| Δe (a) | 7.13 × 10−5 | 0.034 | 0.025 | 0.018 | 0.016 |
Δi  (a) |
7.26 × 10−4 | 0.08 | 0.12 | 0.88 | 0.58 |
| Δa (km) (d) | 1. × 10−3 | 1792 | 1147 | 616 | 437 |
| Δe (d) | 1.8 × 10−7 | 0.041 | 0.031 | 0.015 | 0.015 |
Δi  (d) |
~0.00 | 0.08 | 0.12 | 0.88 | 0.58 |
Notes. We use RP = 1188 km and mP = 1.303 × 1022 kg. M, ω, and Ω are the mean anomaly, argument of pericentre, and longitude of the node, respectively. (a)Osculating orbital elements from the JPL Horizons site setting the ecliptic as reference plane, at epoch 1 January 2021. (https://ssd.jpl.nasa.gov/?horizons). (b)Averaged mean orbital elements from Brozović et al. (2015), derived based on 200 yr of orbital integration. The epoch for the elements is JED 2451544.5. Errors correspond to in-orbit uncertainties; the mass of Styx has a solution with a null value, but we adopted a 1σ value. (c)Orbital elements from Showalter & Hamilton (2015), obtained fitting a precessing ellipse. The epoch is Universal Coordinate Time (UTC) on 1 July 2011. Orbital element variations (Δa, Δe, and Δi) correspond to those extracted from JPL ephemerides(a) and to N-body integrations considering a three-body problem(d) in a 40 yr time span.
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