Constraints on Charon's Orbital Elements From the Double Stellar Occultation of 2008 June 22

Abstract

Pluto and its main satellite, Charon, occulted the same star on 2008 June 22. This event was observed from Australia and La Réunion Island, providing the east and north Charon Plutocentric offset in the sky plane (J2000): X = + 12,070.5 ± 4 km (+ 546.2 ± 0.2 mas), Y = + 4,576.3 ± 24 km (+ 207.1 ± 1.1 mas) at 19:20:33.82 UT on Earth, corresponding to JD 2454640.129964 at Pluto. This yields Charon's true longitude L = 153.483 ± 0.• 071 in the satellite orbital plane (counted from the ascending node on J2000 mean equator) and orbital radius r = 19,564 ± 14 km at that time. We compare this position to that predicted by (1) the orbital solution of Tholen & Buie (the "TB97" solution), (2) the PLU017 Charon ephemeris, and (3) the solution of Tholen et al. (the "T08" solution). We conclude that (1) our result rules out solution TB97, (2) our position agrees with PLU017, with differences of ΔL = + 0.073 ± 0.• 071 in longitude, and Δr = + 0.6 ± 14 km in radius, and (3) while the difference with the T08 ephemeris amounts to only ΔL = 0.033 ± 0.• 071 in longitude, it exhibits a significant radial discrepancy of Δr = 61.3 ± 14 km. We discuss this difference in terms of a possible image scale relative error of 3.35 × 10 −3 in the 2002-2003 Hubble Space Telescope images upon which the T08 solution is mostly based. Rescaling the T08 Charon semi-major axis, a = 19, 570.45 km, to the TB97 value, a = 19636 km, all other orbital elements remaining the same ("T08/TB97" solution), we reconcile our position with the re-scaled solution by better than 12 km (or 0.55 mas) for Charon's position in its orbital plane, thus making T08/TB97 our preferred solution.

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