Cressida (moon)

Voyager 2 Stephen P. Synnott

Cressida (also Uranus IX) is the fourth and one of the smaller of the 27 known moons of the planet Uranus.

Discovery and designation

Cressida was discovered on January 9, 1986 by the astronomer Stephen P. Synnott on photographic images of the Voyager 2 spacecraft. The discovery was announced on 16 January 1986 by the International Astronomical Union ( IAU); the moon first received the provisional designation S/1986 U 3

Cressida is a tragic heroine of William Shakespeare's tragedy Troilus and Cressida. She is a daughter of the Trojan priest Calchas, who has moved to the side of the Greeks, because he foresaw the fall of Troy. Cressida has to change by Calchas in exchange for the Trojan army commander Antenor to the Greeks, and is unfaithful, contrary to their previous affirmations of love Troilus, deeply hurt that this leaves behind.

Cressida also appears in tales of Geoffrey Chaucer and others.

All the moons of Uranus are named after characters from Shakespeare or Alexander Pope. The first four moons discovered Uranus ( Oberon, Titania, Ariel, Umbriel ) were named after suggestions from John Herschel, the son of Uranus discoverer William Herschel. Later, the tradition of naming was retained.

Web properties

Orbit

Cressida orbiting Uranus on a prograde, nearly perfectly circular orbit at an average distance of about 61,767 kilometers (about 2,417 Uranus radii ) from its center, ie 36,208 km above the cloud tops. The orbital eccentricity of 0.00036, the orbit is inclined 0.006 degrees from the equator of Uranus.

Cressida is the second innermost moon of the Portia Group, which also includes Bianca, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda and Perdita belong. These moons have similar orbits and similar spectral properties.

The orbit of the next inner moon Bianca is an average distance of 2601 km Cressida orbit, the moon of the next outer Desdemona only 892 km.

Cressida is in the midst of two Uranus rings, inside of the current ε ( epsilon) - ring which is removed on average about 10,618 km from the Cressida orbit, and the inner edge of the outer ν ( Ny ) dust ring in the 4333 km distance.

Cressida Uranus rotates in 11 hours, 7 minutes and 32.41 seconds. Since this is faster than the rotation of Uranus goes Cressida seen from Uranus from the west, and set in the east.

It moves within a critical distance, near the Roche limit, in a descending orbit around the planet and will eventually be torn apart as a result of tidal forces into a ring or plunge into Uranus ' atmosphere or burn up.

Rotation

It is believed that Cressida synchronously rotates and its axis having an inclination of 0 °.

Physical Properties

Cressida has an average diameter of 79.6 km. In the shots of the Voyager 2 probe Cressida appeared as an elongated object with dimensions of 92 × 74 × 74 km, the longitudinal axis is aligned with Uranus.

Their average density is much lower than the density of the Earth at 1.3 g/cm3 and points out that the moon is composed mainly of water ice. It has a very low albedo of 0.08, i.e., 8 % of the incident solar light is reflected from the surface. It is thus a very dark celestial bodies. On its surface, the acceleration due to gravity is 0.013 m/s2, corresponding to about 1 ‰ of the earth. The average surface temperature of between -184 ° Cressida is on and -209 ° C ( 89-64 K ) is estimated. In the spectrum of the surface of Cressida appears gray.

Otherwise, not much is known about this moon since images of the probe came at a great distance and therefore have a low resolution.

It is possible that Cressida collide within the next 100 million years with the Desdemona nearly 900 km away.

Research

Since the flyby of the Voyager 2 spacecraft, the Uranus system of ground-based observations as well as the Hubble Space Telescope has been studied intensively. The orbital parameters of Cressida could be clarified.