Portia (moon) Saturday, June 20, 2009

Discovery
Discovered by	Stephen P. Synnott / Voyager 2
Discovery date	January 3, 1986
Orbital characteristics
Semi-major axis	66,097.265 ± 0.050 km[1]
Eccentricity	0.00005 ± 0.00008[1]
Orbital period	0.5131959201 ± 0.0000000093 d[1]
Inclination	0.05908 ± 0.039° (to Uranus' equator)[1]
Satellite of	Uranus
Physical characteristics
Dimensions	156 × 126 × 126 km[2]
Mean radius	67.6 ± 4 km[2][3][4]
Surface area	~57,000 km²[5]
Volume	~1,300,000 km³[5]
Mass	~1.7 × 1018 kg[5]
Mean density	~1.3 g/cm³ (assumed)[3]
Equatorial surface gravity	~0.023 m/s2[5]
Escape velocity	~0.058 km/s[5]
Rotation period	synchronous[2]
Axial tilt	zero[2]
Albedo	0.08 ± 0.01 [6] 0.07[3][4]
Temperature	~64 K

Portia (pronounced /ˈpɔrʃə/ POR-shə) is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 1986-01-03, and was given the temporary designation S/1986 U 1.^[7] The moon is named after Portia, the heroine of William Shakespeare's play The Merchant of Venice. It is also designated Uranus XII.^[8]

Portia is the second-largest inner satellite of Uranus after Puck. The Portian orbit, which lies inside Uranus' synchronous orbital radius, is slowly decaying due to tidal deceleration. The moon will one day either break up into a planetary ring or hit Uranus.

It heads a group of satellites called Portia Group, which includes Bianca, Cressida, Desdemona, Juliet, Rosalind, Cupid, Belinda and Perdita.^[6] These satellites have similar orbits and photometric properties.^[6]

Little is known about Portia beyond its size of about 140 km,^[2] orbit,^[1] and geometric albedo of about 0.08.^[6]

In the Voyager 2 images, Portia appears as an elongated object whose major axis points towards Uranus. The ratio of axises of the Portia's prolate spheroid is 0.8 ± 0.1.^[2] Its surface is grey in color.^[2] Observations with Hubble Space Telescope and large terrestrial telescopes found water ice absorption features in the spectrum of Portia.^[6][9]