New Moons Around Pluto Jau-Shian Liang Department of Physics, NTHU 2005/11/24

Introduction H. A. Weaver (Applied Physics Laboratory, Johns Hopkins University) and S. A. Stern (Southwest Research Institute) report the discovery of two new satellites of Pluto. The new satellites were detected using the images obtained by Hubble Space Telescope in May, 2005

Pluto and Charon Average distance from Sun: 39.53AU Revolution period: years Rotation Period: days days Radius: km km Density: ~2g/cm 3 1-2g/cm 3 Brightness: 13.6 magnitude 15.5mag Geometric albedo: 0.55,variable 0.35 Color(V-I): 0.93magnitude 0.83mag Known surface ices: CH 4, N 2, CO, ? H 2 O, ? Atmosphere: confirmed ? Date of discovery:

Observation Mission: Hubble Space Telescope Instrument: ACS/WFC ACS/HRC Filters: F606W (V) F475W (B), F555W (V) Date: May 15/18, 2005 various dates in 2002 and 2003

(288 sec)(3800 sec)

S/2005 P1 V= / S/2005 P2 V= / S/N > 35 S/2005 P1 S/2005 P2 S/2005 P1

Orbit Unique orbits cannot be calculated from only two Hubble measurements. The measured positions are consistent with nearly circular orbits in the orbital plane of Pluto I (Charon).

S/2005 P 1 a = 64’700 ± 850 km P = 38.2 ± 0.8 days S/2005 P 2 a = 49’400 ± 600 km P = 25.5 ± 0.5 days

A re-examination of older Hubble images

Approximate Diameter Assumed Reflectivity Approximate Diameter 4% (like the darkest known KBOs) 160 km 15% (like many KBOs) 80 km 35% (like Charon) 52 km

The team plans to make follow-up Hubble observations in February to confirm that the newly discovered objects are truly Pluto's moons. If confirmed, the discovery of the two new moons could offer insights into the nature and evolution of the Pluto system, Kuiper Belt Objects with satellite systems, and the early Kuiper Belt.

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