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

B. SICARDY; A. BELLUCI; E. GENDRON; F. LACOMBE; S. LACOUR; J. LECACHEUX; E. LELLOUCH; S. RENNER; S. PAU; F. ROQUES; T. WIDEMANN; F. COLAS; F. VACHIER; R. VIERA MARTINS; N. AGEORGES; O. HAINAUT; O. MARCO; W . BEISKER; E. HUMMEL; C. FEINSTEIN; H. LEVATO; ET AL.

NATURE

Año: 2006 vol. 439 p. 52 - 54

0028-0836

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 600km for the satellite´s radius, which was later refined to 601.5km (ref. 4). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, RC = 603.6 +/- 1.4km (1σ), and a density of ρ = 1.71 +/- 0.08gcm-3. This occultation also provides upper limits of 110 and 15 (3σ) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.