Perdita (moon)

Perdita /ˈpɜːrdətə/ is an inner satellite of Uranus. Perdita's discovery was very complicated, as the first photographs of Perdita were taken by the Voyager 2 spacecraft in 1986, but it was not recognized from the photographs for more than a decade. In 1999, the moon was noticed by Erich Karkoschka and reported.[3][8] But because no further pictures could be taken to confirm its existence, it was officially demoted in 2001.[9] However, in 2003, pictures taken by the Hubble Space Telescope managed to pick up an object where Perdita was supposed to be, finally confirming its existence.[7][10]

Perdita

Discovery
Discovered by	Erich Karkoschka / Voyager 2
Discovery date	May 18, 1999 (in images dating back to January 18, 1986)
Designations
Designation	Uranus XXV
Pronunciation	/ˈpɜːrdətə/[1]
Adjectives	Perditean /pɜːrdəˈtiːən/[2]
Orbital characteristics
Mean orbit radius	76,417 ± 1 km[3]
Eccentricity	0.0012 ± 0.0005[3]
Orbital period (sidereal)	0.638021 ± 0.000013 d[3]
Inclination	0.0 ± 0.3° (to Uranus' equator)[3]
Satellite of	Uranus
Physical characteristics
Mean radius	13.3±0.7 km[4]
Surface area	~2220 km2[lower-alpha 1]
Volume	~9850 km3[lower-alpha 1]
Mass	~(4.93–8.87)×1015 kg[lower-alpha 1]
Mean density	0.5–0.9 g/cm3[5][6]
Surface gravity	~0.002–0.003 m/s2[lower-alpha 1]
Escape velocity	~0.007–0.009 km/s[lower-alpha 1]
Synodic rotation period	synchronous[3]
Axial tilt	zero[3]
Albedo	0.08 ± 0.01 (assumed)[7]
Temperature	~64 K[lower-alpha 1]

Following its discovery in 1999, it was given the temporary designation of S/1986 U 10.[8] It was named Perdita (Latin for 'lost') after the daughter of Leontes and Hermione in William Shakespeare's play The Winter's Tale. The moon is also designated Uranus XXV.[11]

Discovery image of Perdita taken by Voyager 2 on 23 January 1986. The location of the moon is indicated by the arrow on the upper right.

The moon orbits between Belinda and Puck. The above-mentioned Hubble measurements prove that Perdita does not follow a direct Keplerian motion around Uranus. Instead, it is clearly caught in a 43:44 orbital resonance with the nearby moon Belinda, and from this resonance it has been determined that Belinda's mass is 26 times that of Perdita.[12] It is also close to an 8:7 resonance with Rosalind.[3][7]

Perdita belongs to the Portia group of satellites, which also includes Bianca, Cressida, Desdemona, Portia, Juliet, Cupid, Rosalind and Belinda.[13] These satellites have similar orbits and photometric properties.[13] Little is known about Perdita apart from its orbit,[3][7] radius of 13.3 km[4] and geometric albedo of 0.08.[13][7]