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 densities². 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 radius³, 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 (1), and a density of = 1.71 0.08 g cm^-3. This occultation also provides upper limits of 110 and 15 (3) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.

1. Observatoire de Paris, LESIA, 92195 Meudon cedex, France
2. Université Pierre et Marie Curie, 75252 Paris cedex 5, France
3. Observatoire de Paris, IMCCE, 75014 Paris, France
4. European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
5. International Occultation Timing Association, European Section, 30459 Hannover, Germany
6. Facultad de Ciencias Astronómicas y Geofísicas, Observatorio Astronómico & Instituto de Astrofísica de La Plata, CONICET, Paseo del Bosque 1900 La Plata, Argentina
7. Complejo Astronómico, El Leoncito, CP J5402DSP, San Juan, Argentina
8. Gene Shoemaker Observatory, Casilla 21, San Pedro de Atacama, Chile
9. Planétarium de Saint-Etienne, 42100 Saint-Etienne. France
10. Association des Utilisateurs de Détecteurs Electroniques (AUDE), France, c/o F. Colas, 45, Av. Reille, 75014 Paris, France
11. Campo Catino Austral Observatory, Casilla 21, San Pedro de Atacama, Chile
12. Università di Tor Vergata di Roma, Via della Ricerca Scientifica n.1, 00133, Rome, Italy
13. Observatoire de Genève, CH-1290 Sauverny, Switzerland
14. Observatoire de Besançon, BP1615, 25010 Besançon cedex, France
15. Observatório Nacional, 20921-400, Rio de Janeiro, Brazil
16. Observatório do Valongo/UFRJ, CEP 20080-090, Rio de Janeiro, Brazil
17. Observatório CEAMIG-REA, CEP 31545-120, Belo Horizonte, MG, Brazil
18. Observatório Astronômico Christus, Universidade de Fortaleza, rua João Carvalho, 630, CEP 60140-140 Fortaleza, Brazil
19. Observatoire Aquitain des Sciences de l'Univers, 33270 Floirac, France
20. Observatorio Astronómico, Universidad Nacional de Asunción 2169, Paraguay
21. Observatorio Astronómico Los Molinos, Facultad de Ciencias, 11400 Montevideo, Uruguay

Correspondence to: B. Sicardy^1,2 Correspondence and requests for materials should be addressed to B.S. (Email: bruno.sicardy@obspm.fr).

Received 2 September 2005 | Accepted 17 October 2005

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