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