Margaret (moon)

Margaret is the only known prograde irregular satellite of the moons of Uranus. It was discovered by Scott S. Sheppard, et al. in 2003 and given the provisional designation S/2003 U 3.^[8]

Margaret

Discovery image of Margaret taken by the Subaru Telescope in August 2003
Discovery
Discovered by	Scott S. Sheppard David C. Jewitt
Discovery date	August 29, 2003[1][2]
Designations
Designation	Uranus XXIII
Pronunciation	/ˈmɑːrɡərət/[3]
Adjectives	Margaretian /ˌmɑːrɡəˈrɛtiən/[4]
Orbital characteristics
Mean orbit radius	14,345,000 km[5][6]
Eccentricity	0.6608[6] (mean)
Orbital period (sidereal)	1687.01 d
Inclination	57° (to the ecliptic)[5]
Satellite of	Uranus
Physical characteristics
Mean radius	10 km (estimate)[7]
Surface area	~1,300 km2 (estimate)
Volume	~4,200 km3 (estimate)
Mass	~5.5×1015 kg (estimate)
Mean density	~1.3 g/cm3 (assumed)
Surface gravity	~0.0023 m/s2 (estimate)
Escape velocity	~0.0085 km/s (estimate)
Synodic rotation period	?
Axial tilt	?
Albedo	0.04 (assumed)[7]
Temperature	~65 K (estimate)

Confirmed as Uranus XXIII, it was named after the servant of Hero in William Shakespeare's play Much Ado About Nothing.^[1] The name was also chosen to match the name of Sheppard's mother.^[9]

Orbit

Animation of discovery images showing Margaret's motion relative to background stars and galaxies

Irregular satellites of Uranus

Margaret stands out as the only known prograde irregular satellite of Uranus. The diagram illustrates the orbital parameters of Margaret, unique among the irregular satellites of Uranus, with inclination on the vertical axis and the eccentricity of the orbits represented by the segments extending from the pericentre to the apocentre.

Margaret's inclination of 57° is close to the limit of stability. The intermediate inclinations 60 < i < 140 are devoid of known moons due to the Kozai instability.^[10] In this instability region, solar perturbations at apoapse cause the moons in this region to acquire large eccentricities that lead to collisions or ejection over 10 million to a billion years. Margaret's periapsis precession period (P_w) is almost 1.6 million years long.^[6] Margaret itself may be ejected from the Uranian system in the far future.^[11]

Margaret's orbit is subject to solar and planetary perturbations; thus, its orbital elements are variable over short timescales. Over a timescale of 8,000 years, the average orbital eccentricity of Margaret is 0.68. In 2010, its eccentricity grew to 0.81,^[12] temporarily giving Margaret the most eccentric orbit of any moon in the Solar System, though Nereid's average eccentricity of 0.75 is greater.^[6]

See also

• Moons of Uranus