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. 2006 Jan 5;439(7072):52-4.
doi: 10.1038/nature04351.

Charon's size and an upper limit on its atmosphere from a stellar occultation

B Sicardy  1 , A Bellucci, E Gendron, F Lacombe, S Lacour, J Lecacheux, E Lellouch, S Renner, S Pau, F Roques, T Widemann, F Colas, F Vachier, R Vieira Martins, N Ageorges, O Hainaut, O Marco, W Beisker, E Hummel, C Feinstein, H Levato, A Maury, E Frappa, B Gaillard, M Lavayssière, M Di Sora, F Mallia, G Masi, R Behrend, F Carrier, O Mousis, P Rousselot, A Alvarez-Candal, D Lazzaro, C Veiga, A H Andrei, M Assafin, D N da Silva Neto, C Jacques, E Pimentel, D Weaver, J-F Lecampion, F Doncel, T Momiyama, G Tancredi
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Charon's size and an upper limit on its atmosphere from a stellar occultation

B Sicardy et al. Nature. .

Abstract

Pluto and its satellite, Charon (discovered in 1978; ref. 1), appear to form a double planet, rather than a hierarchical planet/satellite couple. Charon is about half Pluto's size and about one-eighth its mass. The precise radii of Pluto and Charon have remained uncertain, leading to large uncertainties on their densities. Although stellar occultations by Charon are in principle a powerful way of measuring its size, they are rare, as the satellite subtends less than 0.3 microradians (0.06 arcsec) on the sky. One occultation (in 1980) yielded a lower limit of 600 km for the satellite's radius, which was later refined to 601.5 km (ref. 4). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, R(C) = 603.6 +/- 1.4 km (1sigma), and a density of rho = 1.71 +/- 0.08 g cm(-3). This occultation also provides upper limits of 110 and 15 (3sigma) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.

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