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Shepherd Satellites of Uranus

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Back to: Uranus

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Shepherd Satellites
The discovery of two shepherd satellites has advanced our understanding of the structure of the Uranian rings. The moons, Cordelia (1986U7) and Ophelia (1986U8), are seen here on either side of the bright epsilon ring; all 9 of the known Uranian rings are also visible. The epsilon ring appears surrounded by a dark halo as a result of image processing; occasional blips seen on the ring are also artifacts. Lying inward from the epsilon ring are the delta, gamma and eta rings; the beta and alpha rings; and finally the barely visible 4, 5 and 6 rings. The rings have been studied since their discovery in 1977. (Courtesy NASA/JPL)

Introduction
Uranus is the seventh planet from the Sun and is the third largest in the solar system. It was discovered by William Herschel in 1781. It has an equatorial diameter of 51,800 kilometers (32,190 miles) and orbits the Sun once every 84.01 Earth years. It has a mean distance from the Sun of 2.87 billion kilometers (1.78 billion miles). The length of a day on Uranus is 17 hours 14 minutes. Uranus has at least 15 moons. The two largest moons, Titania and Oberon, were discovered by William Herschel in 1787.

The atmosphere of Uranus is composed of 83% hydrogen, 15% helium, 2% methane and small amounts of acetylene and other hydrocarbons. Methane in the upper atmosphere absorbs red light, giving Uranus its blue-green color. The atmosphere is arranged into clouds running at constant latitudes, similar to the orientation of the more vivid latitudinal bands seen on Jupiter and Saturn. Winds at mid-latitudes on Uranus blow in the direction of the planet's rotation. These winds blow at velocities of 40 to 160 meters per second (90 to 360 miles per hour). Radio science experiments found winds of about 100 meters per second blowing in the opposite direction at the equator.

Uranus is distinguished by the fact that it is tipped on its side. Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history. Voyager 2 found that one of the most striking influences of this sideways position is its effect on the tail of the magnetic field, which is itself tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet. The magnetic field source is unknown; the electrically conductive, super-pressurized ocean of water and ammonia once thought to lie between the core and the atmosphere now appears to be nonexistent. The magnetic fields of Earth and other planets are believed to arise from electrical currents produced in their molten cores.

Uranus' Rings
In 1977, the first nine rings of Uranus were discovered. During the Voyager encounters, these rings were photographed and measured, as were two other new rings and ringlets. Uranus' rings are distinctly different from those at Jupiter and Saturn. The outermost epsilon ring is composed mostly of ice boulders several feet across. A very tenuous distribution of fine dust also seems to be spread throughout the ring system.

There may be a large number of narrow rings, or possibly incomplete rings or ring arcs, as small as 50 meters (160 feet) in width. The individual ring particles were found to be of low reflectivity. At least one ring, the epsilon, was found to be gray in color. The moons Cordelia and Ophelia act as shepherd satellites for the epsilon ring.

Uranus Statistics

Discovered by William Herschel
Date of discovery 1781

Mass (kg) 8.686e+25
Mass (Earth = 1) 1.4535e+01
Equatorial radius (km) 25,559
Equatorial radius (Earth = 1) 4.007
Mean density (gm/cm^3) 1.29

Mean distance from the Sun (km) 2,870,990,000
Mean distance from the Sun (Earth = 1) 19.1914
Rotational period (hours) 17.9
Orbital period (years) 84.01
Mean orbital velocity (km/sec) 6.81

Orbital eccentricity 0.0461
Tilt of axis 97.86°
Orbital inclination 0.774°

Equatorial surface gravity (m/sec^2) 7.77
Equatorial escape velocity (km/sec) 21.30

Visual geometric albedo 0.51
Magnitude (Vo) 5.52
Mean cloud temperature -193°C
Atmospheric pressure (bars) 1.2
Atmospheric composition:
Hydrogen 83%
Helium 15%
Methane 2%

Rings of Uranus

The following is a summary of the rings of Uranus.

Name Distance* Width Thickness Mass Albedo
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1986U2R 38,000 km 2,500 km 0.1 km ? 0.03
6 41,840 km 1-3 km 0.1 km ? 0.03
5 42,230 km 2-3 km 0.1 km ? 0.03
4 42,580 km 2-3 km 0.1 km ? 0.03
Alpha 44,720 km 7-12 km 0.1 km ? 0.03
Beta 45,670 km 7-12 km 0.1 km ? 0.03
Eta 47,190 km 0-2 km 0.1 km ? 0.03
Gamma 47,630 km 1-4 km 0.1 km ? 0.03
Delta 48,290 km 3-9 km 0.1 km ? 0.03
1986U1R 50,020 km 1-2 km 0.1 km ? 0.03
Epsilon 51,140 km 20-100 km < 0.15 km ? 0.03

*The distance is measured from the planet center to the start of the ring.

Uranus Moon Summary

The following table summarizes the radius, mass, distance from the planet center, discoverer and
the date of discovery of each of the moons of Uranus:

Radius Mass Distance
Moon # (km) (kg) (km) Discoverer Date
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Cordelia VI 13 ? 49,750 Voyager 2 1986
Ophelia VII 16 ? 53,760 Voyager 2 1986
Bianca VIII 22 ? 59,160 Voyager 2 1986
Cressida IX 33 ? 61,770 Voyager 2 1986
Desdemona X 29 ? 62,660 Voyager 2 1986
Juliet XI 42 ? 64,360 Voyager 2 1986
Portia XII 55 ? 66,100 Voyager 2 1986
Rosalind XIII 27 ? 69,930 Voyager 2 1986
Belinda XIV 34 ? 75,260 Voyager 2 1986
Puck XV 77 ? 86,010 Voyager 2 1985
Miranda V 235.8 6.33e+19 129,780 G. Kuiper 1948
Ariel I 578.9 1.27e+21 191,240 W. Lassell 1851
Umbriel II 584.7 1.27e+21 265,970 W. Lassell 1851
Titania III 788.9 3.49e+21 435,840 W. Herschel 1787
Oberon IV 761.4 3.03e+21 582,600 W. Herschel 1787

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