Cressida (moon)

Cressida /ˈkrɛsədə/ is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 9 January 1986, and was given the temporary designation S/1986 U 3.^[9] It was named after Cressida, the Trojan daughter of Calchas, a tragic heroine who appears in William Shakespeare's play Troilus and Cressida (as well as in tales by Geoffrey Chaucer and others). It is also designated Uranus IX.^[10]

There is also an asteroid called 548 Kressida.

Cressida

Enhanced Voyager 2 image of Cressida
Discovery
Discovered by	Stephen P. Synnott / Voyager 2
Discovery date	January 9, 1986
Designations
Designation	Uranus IX
Pronunciation	/ˈkrɛsədə/[1]
Named after	Χρησίδα
Adjectives	Cressidian /krɛˈsɪdiən/[2]
Orbital characteristics[3]
Semi-major axis	61 766.730 ± 0.046 km
Eccentricity	0.00036 ± 0.00011
Orbital period (sidereal)	0.463569601 ± 0.000000013 d
Inclination	0.006 ± 0.040° (to Uranus' equator)
Satellite of	Uranus
Physical characteristics
Dimensions	92 × 74 km[4][note 1]
Volume	263800 km3 ± 38.0%[5][a]
Mass	(1.839±0.212)×1017 kg[5] (2.5±0.4)×1017 kg[6]
Mean density	0.70+0.44 −0.21 g/cm3[5][a] 0.86±0.16 g/cm3[6][b]
Synodic rotation period	synchronous[4]
Axial tilt	zero[4]
Albedo	0.08±0.005[7] 0.07[8]
Temperature	~65 K[c]
Only two dimensions are known; the third dimension is unknown.

Cressida belongs to the Portia group of satellites, which includes Bianca, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, and Perdita.^[7] These satellites have similar orbits and photometric properties.^[7] Other than its orbit,^[3] size of 92 × 74 km,^[4] and geometric albedo of 0.08,^[7] little is known about it.

In Voyager 2 imagery Cressida appears as an elongated object, with its major axis pointing towards Uranus. The ratio of axes of Cressida's prolate spheroid is 0.8 ± 0.3.^[4] Its surface is grey in color.^[4]

Cressida orbits close to a 3:2 resonance with the η ring, one of the rings of Uranus. Perturbations of the ring's shape provide a way to measure the mass of Cressida, which in 2024 was found to be (1.839±0.212)×10¹⁷ kg.^[5] Cressida is one of the few small satellites of Uranus for which the mass has been directly measured.^[5][6]

Cressida may collide with Desdemona within the next 100 million years.^[11]

See also

• Moons of Uranus

Notes

1. The third dimension has been assumed to equal the smaller known dimension.
2. The third dimension has been assumed to equal the geometric mean of the two known dimensions.
3. Calculated on the basis of other parameters.