Portia (moon)

Portia
Discovery
Discovered by	Stephen P. Synnott / Voyager 2
Discovery date	January 3, 1986
Orbital characteristics
Semi-major axis	66,097.265 ± 0.050 km
Eccentricity	0.00005 ± 0.00008
Orbital period	0.5131959201 ± 0.0000000093 d
Inclination	0.05908 ± 0.039° (to Uranus' equator)
Satellite of	Uranus
Physical characteristics
Dimensions	156 × 126 × 126 km
Mean radius	70 ± 4 km
Surface area	~57,000 km²
Volume	~1,300,000 km³
Mass	~1.7×1018 kg
Mean density	~1.3 g/cm³ (assumed)
Surface gravity	~0.023 m/s2
Escape velocity	~0.058 km/s km/s
Rotation period	synchronous
Axial tilt	zero
Albedo	0.08 ± 0.01
Temperature	~64 K

Portia is a closer moon to Uranus. It was found from the images taken by Voyager 2 on 1986-01-03, and was given the designation S/1986 U 1.^[5] The moon is named after Portia, the heroine of William Shakespeare's play The Merchant of Venice. It is also designated Uranus XII.^[6]

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

It heads a group of moons called Portia Group, which includes Bianca, Cressida, Desdemona, Juliet, Rosalind, Cupid, Belinda and Perdita.^[4] These moons have similar orbits and photometric properties.^[4]

Little is known about Portia beyond its size of about 140 km,^[2] orbit,^[1] and geometric albedo of about 0.08.^[4]

In the Voyager 2 images, Portia appears as a stretched 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.^[4]^[7]