title: Charon's size and an upper limit on its atmosphere from a stellar occultation.
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.
authors.affiliation: Observatoire de Paris, LESIA, 92195 Meudon cedex, France. bruno.sicardy@obspm.fr
authors.forename: B
authors.initials: B
authors.lastname: Sicardy
authors.forename: A
authors.initials: A
authors.lastname: Bellucci
authors.forename: E
authors.initials: E
authors.lastname: Gendron
authors.forename: F
authors.initials: F
authors.lastname: Lacombe
authors.forename: S
authors.initials: S
authors.lastname: Lacour
authors.forename: J
authors.initials: J
authors.lastname: Lecacheux
authors.forename: E
authors.initials: E
authors.lastname: Lellouch
authors.forename: S
authors.initials: S
authors.lastname: Renner
authors.forename: S
authors.initials: S
authors.lastname: Pau
authors.forename: F
authors.initials: F
authors.lastname: Roques
authors.forename: T
authors.initials: T
authors.lastname: Widemann
authors.forename: F
authors.initials: F
authors.lastname: Colas
authors.forename: F
authors.initials: F
authors.lastname: Vachier
authors.forename: R Vieira
authors.initials: RV
authors.lastname: Martins
authors.forename: N
authors.initials: N
authors.lastname: Ageorges
authors.forename: O
authors.initials: O
authors.lastname: Hainaut
authors.forename: O
authors.initials: O
authors.lastname: Marco
authors.forename: W
authors.initials: W
authors.lastname: Beisker
authors.forename: E
authors.initials: E
authors.lastname: Hummel
authors.forename: C
authors.initials: C
authors.lastname: Feinstein
authors.forename: H
authors.initials: H
authors.lastname: Levato
authors.forename: A
authors.initials: A
authors.lastname: Maury
authors.forename: E
authors.initials: E
authors.lastname: Frappa
authors.forename: B
authors.initials: B
authors.lastname: Gaillard
authors.forename: M
authors.initials: M
authors.lastname: Lavayssière
authors.forename: M
authors.initials: M
authors.lastname: Di Sora
authors.forename: F
authors.initials: F
authors.lastname: Mallia
authors.forename: G
authors.initials: G
authors.lastname: Masi
authors.forename: R
authors.initials: R
authors.lastname: Behrend
authors.forename: F
authors.initials: F
authors.lastname: Carrier
authors.forename: O
authors.initials: O
authors.lastname: Mousis
authors.forename: P
authors.initials: P
authors.lastname: Rousselot
authors.forename: A
authors.initials: A
authors.lastname: Alvarez-Candal
authors.forename: D
authors.initials: D
authors.lastname: Lazzaro
authors.forename: C
authors.initials: C
authors.lastname: Veiga
authors.forename: A H
authors.initials: AH
authors.lastname: Andrei
authors.forename: M
authors.initials: M
authors.lastname: Assafin
authors.forename: D N
authors.initials: DN
authors.lastname: da Silva Neto
authors.forename: C
authors.initials: C
authors.lastname: Jacques
authors.forename: E
authors.initials: E
authors.lastname: Pimentel
authors.forename: D
authors.initials: D
authors.lastname: Weaver
authors.forename: J-F
authors.initials: JF
authors.lastname: Lecampion
authors.forename: F
authors.initials: F
authors.lastname: Doncel
authors.forename: T
authors.initials: T
authors.lastname: Momiyama
authors.forename: G
authors.initials: G
authors.lastname: Tancredi