Astronomy

Uranus Quicktime Movie Sample

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 and orbits the Sun once every 84.01 Earth years. It has a mean distance from the Sun of 2.87 billion kilometers. 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. 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 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

Characteristic	Measurement
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	Percent
Hydrogen	83%
Helium	15%
Methane	2%

Views of Uranus

Uranus

This view of Uranus was acquired by Voyager 2 in January 1986. The greenish color of it atmosphere is due to methane and high-altitude photochemical smog.

Hubble Tracks Rotation of Uranus

This view of Uranus was acquired by NASA’s Hubble Space Telescope and reveals a pair of bright clouds in the planet’s southern hemisphere, and a high altitude haze that forms a "cap" above the planet’s south pole. This is just one view of a sequence of three that can be obtained by selecting the above gif image. Hubble’s new view was obtained on August 14, 1994, when Uranus was 2.8 billion kilometers from Earth. These atmospheric details were only previously seen by the Voyager 2 spacecraft, which flew by Uranus in 1986. Since then, detailed observations of Uranus’s atmospheric features have not been possible because the planet is at the resolution limit of ground-based telescopes. Hubble’s Wide Field Planetary Camera 2 observed Uranus through a filter that is sensitive to light reflected by a pair of high altitude clouds. This makes a high altitude haze over Uranus’ south polar region clearly visible, along with a pair of high altitude clouds or plume-type features that are 4,300 and 3,100 kilometers across, respectively. Two additional Hubble Telescope images can be found here.

Uranus, Rings and Satellites

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.

Uranus’ Rings

The 9 known rings of Uranus are visible here. The somewhat fainter, pastel lines seen between the rings are artifacts of computer enhancement. Six narrow-angle images were used to extract color information from the extremely dark and faint rings. The final image was made from three color averages and represents an enhanced, false-color view. The image shows that the brightest, or epsilon, ring at top is neutral in color, with the fainter 8 remaining rings showing color differences between them.

Uranus Family

This montage of images of the Uranian system was prepared from an assemblage of images taken by the Voyager 2 spacecraft during its Uranus encounter in January 1986. This artist’s view shows Ariel in the forefront, Uranus rising behind, Umbriel off to the left, Miranda in the foreground to the right, Titania fading in the distance at the far right, and Oberon in its distant orbit at the top.

Rings of Uranus

The following is a summary of the rings of Uranus.

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

Moon	Number	Radius (km)	Mass (kg)	Distance (km)	Discoverer	Date
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