Portia (moon)

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Portia
Uranus-Portia-Cressida-Ophelia-NASA.gif
Discovery
Discovered by Stephen P. Synnott / Voyager 2
Discovery time January 3, 1986
Orbit
Longest distance from the center of its orbital path
("semi-major axis")
66,097.265 ± 0.050 km[1]
How egg-shaped its orbit is
("eccentricity")
0.00005 ± 0.00008[1]
How long it takes to complete an orbit 0.5131959201 ± 0.0000000093 d[1]
Angle above the reference plane
("inclination")
0.05908 ± 0.039° (to Uranus' equator)[1]
What it orbits Uranus
Size and Other Qualities
Measures 156 × 126 × 126 km[2]
Average distance from its center to its surface 70 ± 4 km[2]
Area of its surface ~57,000 km²[3]
Volume inside it ~1,300,000 km³[3]
Mass ~1.7×1018 kg[3]
Average density ~1.3 g/cm³ (assumed)
Gravity at its surface ~0.023 m/s2[3]
Slowest speed able to escape into space
("escape velocity")
~0.058 km/s km/s[3]
How long it takes to turn around one time synchronous[2]
Angle at which it turns
(in relation to its orbit)
zero[2]
How much light it reflects 0.08 ± 0.01 [4]
Avg. surface temp. ~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]

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