Uranus (planet)

Uranus

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Discovery
Discovered by	William Herschel
Discovered on	March 13, 1781
Orbital characteristics
Mean radius	km
Eccentricity	0.04716771
Revolution period	84y 3d 15.66h
Synodic period	369.7 days
Avg. Orbital Speed	6.8352 km/s
Inclination	0.76986°
Number of satellitess	27
Physical characteristics
Equatorial diameter	51,118 km
Surface area	8,130,000,000 km2
Mass	25 kilogram>kg
Mean density	1.29 g/cm3
Surface gravity	m/s2
Surface Gravity (Earth = 1)	0.887
Rotation period	retrograde motion
Axial tilt	97.86°
Albedo	0.51
Escape Speed	21.29 km/s
Cloudtop avg. temp	55 K
Surface temp	min mean max K 68 K N/A K
Atmospheric characteristics
Atmospheric pressure	kPa
Hydrogen	83%
Helium	15%
Methane	1.99%
Ammonia	0.01%
Ethane	0.00025%
Acetylene	0.00001%
Carbon monoxide Hydrogen sulfide	trace

Uranus (pronounced "YOOR-ə-nus", or "yər-AYN-us") is the seventh planet from the Sun. It is a gas giant and the third largest by diameter. It was named after the Greek god Ouranos.

Table of contents

1 Physical characteristics 1.1 Composition 1.2 Axial tilt 1.3 Magnetic Field 2 Exploration of Uranus 3 The moons of Uranus

Physical characteristics

Composition

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rockyy core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. Uranus' cyan color is due to the absorption of red light by atmospheric methane.

Axial tilt

One of the most distinctive features of Uranus is its axial tilt of almost ninety degrees. Consequently, for part of its orbit one pole faces the Sun continually whilst the other pole faces away. At the other side of Uranus' orbit the orientation of the poles towards the sun is reversed, and at the two sections of its orbit between these two extremes the Sun rises and sets around the equator normally.

At the time of Voyager 2's passage in 1986, Uranus' south pole was pointed almost directly at the Sun. Note that the labelling of this pole as "south" is actually in some dispute. Uranus can either be described as having an axial tilt of slightly more than 90°, or it can be described as having an axial tilt of slightly less than 90° and rotating in a retrograde direction; these two descriptions are exactly equivalent as physical descriptions of the planet but result in different definitions of which pole is the North Pole and which is the South Pole.

One result of this odd orientation is that the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles, although the underlying mechanism which causes this is unknown. The reason for Uranus' extreme axial tilt is also not known. It is speculated that perhaps during the formation of the planet it collided with an enormous protoplanet, resulting in the skewed orientation.

It appears that Uranus' extreme axial tilt also results in extreme seasonal variations in its weather. During the Voyager 2 flyby, Uranus' banded cloud patterns were extremely bland and faint. Recent Hubble Space Telescope observations, however, show a more strongly banded appearance now that the Sun is approaching Uranus' equator. By 2007 the Sun will be directly over Uranus' equator.

Magnetic Field

Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60° with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. Neptune has a similarly displaced magnetic field, suggesting that this is not necessarily a result of Uranus's axial tilt. The magnetotail is twisted by the planet's rotation into a long corkscrew shape behind the planet. The magnetic field's 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.

This crescent view of Uranus was recorded by Voyager 2 on Jan 25, 1986, as the spacecraft left the planet behind

Exploration of Uranus

Uranus was the first planet to be discovered that was not known in ancient times, although it had been observed on many previous occasions but was always dismissed as simply another star. (The earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri).

Sir William Herschel discovered the planet in 1781, and originally named it Georgium Sidus (George's Star) in honour of King George III of England. French astronomers started calling it Herschel before German Johann Bode proposed the name Uranus, after the Greek god. This name did not come into common usage until around 1850.

NASA's Voyager 2 is the only spacecraft to have visited the planet. Launched in 1977, Voyager made its closest approach to Uranus on January 24, 1986 before continuing on its journey to Neptune.

The moons of Uranus

Uranus has 27 known moonss. The first two were discovered by William Herschel on March 13, 1787, and named, by his son, after characters from Shakespeare's A Midsummer Nights Dream, as Titania and Oberon. Two more moons discovered by William Lassell in 1851 were named Ariel and Umbriel; Gerard Kuiper discovered the moon Miranda in 1948. All moons of Uranus are named after characters from Shakespeare or Alexander Pope.

The flyby of the Voyager 2 space probe in January 1986 led to the discovery of a further 10 moons, and another satellite S/1986 U 10 was later found after studying old Voyager photographs. Eleven additional moons have since been identified using telescopes.

Uranus has a faint planetary ring system, composed of dark particulate matter up to 10 metres in diameter. This ring system was discovered in March 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink, using the Kuiper Airborne Observatory.

Uranus' natural satellites

Name	Diameter (km)	Mass (kg)	Mean orbital radius (km)	Orbital period
Cordelia	40	4.5 × 1016 ?	49,770	0.335034 days
Ophelia	42	5.4 × 1016 ?	53,790	0.376400 days
Bianca	51	9.3 × 1016 ?	59,170	0.434579 days
Cressida	80	3.43 × 1017 ?	61,780	0.463570 days
Desdemona	64	1.78 × 1017 ?	62,680	0.473650 days
Juliet	93	5.57 × 1017 ?	64,350	0.493065 days
Portia	135	1.68 × 1018 ?	66,090	0.513196 days
Rosalind	72	2.54 × 1017 ?	69,940	0.558460 days
S/2003 U2*	10	Unknown	74,800	0.618 days
Belinda	80	3.57 × 1017 ?	75,260	0.623527 days
S/1986 U10*	20	Unknown	76,420	0.638 days
Puck	162	2.89 × 1018 ?	86,010	0.761833 days
S/2003 U1*	10	Unknown	97,734	0.923 days
Miranda (moon)>Miranda	472	6.6 × 1019	129,390	1.413479 days
Ariel (moon)>Ariel	1158	1.35 × 1021	191,020	2.520379 days
Umbriel (moon)>Umbriel	1170	1.17 × 1021	266,300	4.144177 days
Titania (moon)>Titania	1578	3.52 × 1021	435,910	8.705872 days
Oberon (moon)>Oberon	1523	3.01 × 1021	583,520	13.463239 days
S/2001 U3*	12	Unknown	4,276,000	-266.6 days**
Caliban	98	7.3 × 1017 ?	7,231,000	-579.7 days**
Stephano	20	6 × 1015 ?	8,004,000	-677.4 days**
Trinculo	10	Unknown	8,504,000	-759.0 days**
Sycorax	190	5.4 × 1018 ?	12,179,000	-1288.3 days**
S/2003 U3*	11	Unknown	14,345,000	1694.8 days
Prospero	30	2.1 × 1016 ?	16,256,000	-1977.3 days**
Setebos	30	2.1 × 1016 ?	17,418,000	-2234.8 days**
S/2001 U2*	12	Unknown	20,901,000	-2823.4 days**

^*Awaiting confirmation and naming

^**negative orbital periods indicate a retrograde orbit around Uranus (opposite to the planet's rotation)

Sources: [1], [1]

Note: These sources give no information on the masses for the small satellites; some data have changed greatly so it is unlikely that these old values are correct.

Some asteroids share the same names as moons of Uranus: 171 Ophelia, 218 Bianca, 593 Titania, 666 Desdemona, 2758 Cordelia.