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Uranus
Uranus, named after the father of the Roman god Saturn, is the seventh planet in the Solar System and third
of the gas giants. It is the third largest planet by diameter, yet fourth most massive.
Uranus has the distinction of being the first planet discovered in modern history. Actually, its discovery as a
planet almost did not happen. In 1781, the astronomer William Herschel was charting the stars found in the
Gemini constellation when he observed a disk-like object. His initial conclusion was that he had discovered a
comet and reported his findings as such to the Royal Society of England. However, Herschel was puzzled
when he calculated the object's orbit. Instead of the more elliptical path occurring with comets, he found that it
was much more circular. This observation, which was confirmed by other astronomers at the time, led
Herschel to conclude that he had, in fact, discovered a new planet. Shortly thereafter, it was widely accepted
that Herschel had discovered an unknown planet.
As a result of his discovery, Herschel was given the privilege of naming the new planet. The name he chose
was Georgium Sidus, which is Latin for Georgian Planet. He opted for this name to honor then king of
England, George III. This name, however, was not widely accepted, and as a result others began to suggest
names. The name Uranus was put forth in the tradition of naming planets after deities in Roman mythology.
Over time the scientific community accepted this as the planet's name.
At present, the only planetary mission to visit Uranus is Voyager 2. This lone encounter, which occurred in
1986, provided a large amount of data and discoveries. The spacecraft took thousands of pictures of Uranus
and its moons and rings. Although the images of the planet showed little other than the uniform blue-green
color seen from Earth-based telescopes, other images revealed the presence of ten previously unknown
moons and two new rings. At this time, no future missions are scheduled for Uranus.
ATMOSPHERE
Due to its stark blue appearance, the Uranian atmospheric patterns have been much more difficult to observe
than, say, those of Jupiter or even Saturn. Fortunately, the Hubble Space Telescope has provided much more
insight into the structural nature of Uranus' atmosphere. Through more advanced imaging technologies than
Earth-based telescopes or Voyager 2, Hubble has shown that there are latitudinal bands much like those
found on the other gas giants. Additionally, the winds associated with these bands can blow in excess of 576
km/hr.
The reason behind the monotonous atmospheric appearance is the composition of the top-most layer of the
atmosphere. The visible cloud layers are composed primarily of methane, which absorbs those visible
wavelengths corresponding to the color red. Thus, the reflected wavelengths are those of blue and green.
Beneath this outer methane layer, the atmosphere is composed of roughly 83% hydrogen (H 2) and 15%
helium with trace amounts of methane and acetylene. This composition is similar to that of the other gas
giants. Uranus' atmosphere is drastically different in another regard, though. Whereas Jupiter and Saturn's
atmospheres are primarily gaseous, Uranus' contains much more ice. This indicates that the Uranian
atmosphere is extremely cold. In fact, at approximately -224° C, its atmosphere is the coldest found in the
Solar System. What is even more interesting is data indicates that this extreme temperature is constant
globally, occurring even on the side that is not sunlit.
INTERIOR
Uranus' interior is thought to consist of two layers: a core and mantle. Current models suggest that the core is
primarily composed of rock and ice and is approximately .55 times the mass of the Earth. The planet's mantle
is believed to be 8.01 x 1024 kg, or about 13.4 times the mass of the Earth. Furthermore, the mantle is
composed of water, ammonia and other volatile elements. What distinguishes Uranus' mantle from those of
Jupiter and Saturn is that it is icy, though not in the traditional sense. Instead, the ice is very hot and thick. The
mantle is 5,111 km thick.
What is most surprising about Uranus' interior and one of the most distniguishing features with respect to the
other gas giants is that it does not emit more energy than it receives from the Sun. Considering that even
Neptune, which is very similar in size to Uranus, produces approximately 2.6 times the amount of heat that it
receives from the Sun, scientists are very intrigued by the low heat that Uranus generates. There are two
popular theories for this phenomenon. The first says that Uranus was struck by a large body, dispersing into
space most of the heat that planets normally retain from their formations. The second theory claims that there
is some barrier preventing the internal heat from making its way to the planet's surface.
ORBIT & ROTATION
When Uranus was discovered it expanded the radius of the known Solar System by almost a factor of two.
What this means is that, on average, Uranus' orbit is about 2.87 x 109 km. The consequence of such an
enormous distance is that it takes sunlight around two hours and forty minutes to reach Uranus—that is almost
twenty times as long as it takes sunlight to reach the Earth! This huge distance also means that a year on
Uranus lasts almost 84 Earth years!
At 0.0473, Uranus' orbital eccentricity is just slightly less than that of Jupiter's .0484, making it the fourth most
circular orbit of all the planets. The result of Uranus' fairly small orbital eccentricity is that the difference
between its perihelion of 2.74 x 109 km and aphelion of 3.01 x 109 km is just 2.71 x 108 km.
Perhaps the most interesting thing about Uranus is how odd its rotation is compared to all of the other planets'.
The axis of rotation for every planet other than Uranus is roughly perpendicular with their orbital plane.
However, Uranus' axis is tilted almost 98°, which effectively means that Uranus rotates on its side. The result
of this is that Uranus' North Pole points at the Sun for half of its year, while the South Pole points at the Sun
the other half of its year! In other words, it is daytime on one Uranian hemispheres, while it is night time on the
other for 42 Earth years at a time. Furthermore, due to this extreme rotation, Uranus does not have days like
on other planets—that is, the Sun doesn't rise and set like on other planets.
The cause for this highly unusual axial tilt is theorized to be the effect of a large body striking Uranus with such
force that it essentially knocked the planet over on its side.
RINGS
Although Saturn's rings have been well known for some time, it wasn't until 1977 that the planetary rings
surrounding Uranus were discovered. The reason behind this is twofold: their distance from the Earth and their
low reflectivity of light. Nonetheless, the Voyager 2 spacecraft identitfied two more on its fly-by mission in
1986, followed by the Hubble Space Telescope discovery of two additional rings in 2005. The total number of
known rings currently sits at thirteen, the largest and brightest of which is the epsilon ring.
Uranus' rings differ from those found around Saturn in both the size of its particles and the particles'
composition. First, the particles making up Saturn's rings are small, with few larger than several meters in
diameter, whereas Uranus' rings contain many bodies up to twenty meters in diamter. Second, the particles of
Saturn's rings are largely composed of ice. Uranus', however, are composed of both ice and considerable dust
and debris.
Uranus Facts
Discovery Date: 1781
Volume: 6.83 x 1013 km3
Orbital Eccentricity: 0.0473
Discoverer:
William Herschel
Density: 1,270 kg/m3
Mean Orbital Distance from the
Sun: 2.87 x 109 km
Number of Moons: 27
Surface Gravity: 8.87 m/s2
Perihelion: 2.74 x 109 km
Rings: Yes
Length of Day:
17 hours 14 minutes
Time it takes light to reach
Uranus from the Sun:
2 hours 40 minutes
Length of Year:
84 Earth Years
Mass: 8.68 x 1025 kg
Mean Diameter:
50,724 km
Mean Circumference:
159,354 km
Aphelion: 3.01 x 109 km
Axial Tilt: 97.8°
Uranus as seen by the Hubble space telescope
Seasons of Uranus
Earth compared to Uranus in size
Moons of Uranus
Presently, Uranus is known to have 27 moons. Usually, moons are named after mythological figures; however,
Uranian moons are named after characters taken from the literary works of either William Shakespeare or
Alexander Pope. Although some of Uranus' moons were discovered shortly after the planet's discovery, most
were unknown until the mid-twentieth century.
The classification of Uranus' moons can be broken down into three groups: the inner moons, the large moons,
and the irregular moons. The thirteen inner moons are satellites that, as their names suggest, orbit more
closely to Uranus than the others. The official outer boundary for the inner moons is the orbit of the large moon
Miranda. The inner moons can be characterized as small (diameter not exceeding 160 km), composed of half
water ice and half rock and reflecting very little light. Also, the orbits of these moons are so highly disordered
that scientists do not currently know how they avoid colliding with one another.
The group of large moons consists of Miranda, Ariel, Umbriel, Titania and Oberon. The largest of these,
Titania, has a diameter of 1,578 km, which is about half of the diameter of Earth's moon. These moons are
similar in character. All are about half rock and half ice, except for the smallest of these, Miranda, which is
predominantly ice.
The irregular moons, which orbit at a considerably farther distance than the inner and larger moons, have
highly eccentric and retrograde orbits. These moons are thought to be captured asteroids.
The first two moons to be discovered, Titania and Oberon, were spotted by Sir William Herschel on January
11, 1787, six years after he had discovered the planet itself. Later, Herschel thought he had discovered up to
six moons (see below) and perhaps even a ring. For nearly 50 years, Herschel's instrument was the only one
with which the moons had been seen. In the 1840s, better instruments and a more favorable position of
Uranus in the sky led to sporadic indications of satellites additional to Titania and Oberon. Eventually, the next
two moons, Ariel and Umbriel, were discovered by William Lassell in 1851.The Roman numbering scheme of
Uranus's moons was in a state of flux for a considerable time, and publications hesitated between Herschel's
designations (where Titania and Oberon are Uranus II and IV) and William Lassell's (where they are
sometimes I and II). With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from
Uranus outward, and this finally stuck. In 1852, Herschel's son John Herschel gave the four then-known
moons their names.
No other discoveries were made for almost another century. In 1948, Gerard Kuiper at the McDonald
Observatory discovered the smallest and the last of the five large, spherical moons, Miranda. Decades later,
the flyby of the Voyager 2 space probe in January 1986 led to the discovery of ten further inner moons.
Another satellite, Perdita, was retroactively discovered in 1999 after studying old Voyager photographs.
Uranus was the last giant planet without any known irregular satellites, but since 1997 nine distant irregular
moons have been identified using ground-based telescopes. Two more small inner moons, Cupid and Mab,
were discovered using the Hubble Space Telescope in 2003. The moon Margaret was the last Uranian moon
discovered as of 2012, and its details were published in October 2003.
SPURIOUS MOONS
After Herschel discovered Titania and Oberon on January 11, 1787, he subsequently believed that he
observed four other moons; two on January 18 and February 9, 1790, and two more on February 28 and
March 26, 1794. It was thus believed for many decades thereafter that Uranus had a system of six satellites,
though the four latter moons were never confirmed by any other astronomer. Lassell's observations of 1851, in
which he discovered Ariel and Umbriel, however, failed to support Herschel's observations; Ariel and Umbriel,
which Herschel certainly ought to have seen if he had seen any satellites beside Titania and Oberon, did not
correspond to any of Herschel's four additional satellites in orbital characteristics. Herschel's four spurious
satellites were thought to have sidereal periods of 5.89 days (interior to Titania), 10.96 days (between Titania
and Oberon), 38.08 days, and 107.69 days (exterior to Oberon). It was therefore concluded that Herschel's
four satellites were spurious, probably arising from the misidentification of faint stars in the vicinity of Uranus
as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell.
Largest moons of Uranus
Ring system of Uranus
Moons around Uranus
Tables of Moons
The Uranian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their
surfaces to have collapsed into a spheroid are highlighted in light blue and bolded. Irregular moons with
prograde orbits are shown in light grey, those with retrograde orbits in dark grey.
Key
‡
Major
moons
Orde
Label
r
[e]
♠
Retrograde moons
Name
[d]
Uranian moons
Mass SemiPronunciatio
Diameter (×101 major
Image
n
8
(km)[f]
axis
(key)
[g]
kg)
(km)[35]
Orbital Inclinatio Eccentricit Discover
Discoverer
period
n
y
y
[2]
[36]
(d)[35][h]
(°)[35]
year[2]
kɔrˈdiːliə
40 ± 6
(50 × 36)
0.044 49,770
0.335034 0.08479° 0.00026
1986
Ophelia
ɵˈfiːliə
43 ± 8
(54 × 38)
0.053 53,790
0.376400 0.1036°
0.00992
1986
VIII
Bianca
biːˈɒŋkə
51 ± 4
(64 × 46)
0.092 59,170
0.434579 0.193°
0.00092
1986
4
IX
Cressida
ˈkrɛsɨdə
80 ± 4
(92 × 74)
0.34
61,780
0.463570 0.006°
0.00036
1986
5
X
Desdemon ˌdɛzdɨˈmoʊ
a
nə
64 ± 8
(90 × 54)
0.18
62,680
0.473650 0.11125° 0.00013
1986
6
XI
Juliet
ˈdʒuːli.ɨt
94 ± 8
0.56
(150 × 74)
64,350
0.493065 0.065°
0.00066
1986
7
XII
Portia
ˈpɔrʃə
135 ± 8
1.70
(156 × 126)
66,090
0.513196 0.059°
0.00005
1986
8
XIII
Rosalind ˈrɒzəlɨnd
72 ± 12
0.25
69,940
0.558460 0.279°
0.00011
1986
9
XXVI
Cupid
I
ˈkjuːpɨd
~18
0.003
74,800
8
0.618
0.0013
2003
10
XIV
Belinda
bɨˈlɪndə
90 ± 16
0.49
(128 × 64)
0.623527 0.031°
0.00007
1986
11
XXV Perdita
ˈpɜrdɨtə
30 ± 6
0.638
0.0012
1999
1
VI
Cordelia
2
VII
3
75,260
0.018 76,400
0.1°
0.0°
Terrile
(Voyager
2)
Terrile
(Voyager
2)
Smith
(Voyager
2)
Synnott
(Voyager
2)
Synnott
(Voyager
2)
Synnott
(Voyager
2)
Synnott
(Voyager
2)
Synnott
(Voyager
2)
Showalter
and
Lissauer
Synnott
(Voyager
2)
Karkoscha
Puck
ˈpʌk
162 ± 4
2.90
86,010
0.761833 0.3192°
0.00012
1985
ˈmæb
~25
0.01
97,700
0.923
0.0025
2003
12
XV
13
XXVI Mab
14
V
‡
Miranda mɨˈrændə
15
I
‡
Ariel
16
II
‡
Umbriel ˈʌmbriəl
17
III
‡
Titania
18
IV
‡
Oberon ˈoʊbərɒn
19
XXII
♠
Francisco frænˈsɪskoʊ
20
XVI
♠
Caliban
21
XX
22
ˈɛəriəl
tɨˈtɑːnjə
471.6 ± 1.4
(481 × 468
× 466)
1,157.8 ±
1.2
(1162 ×
1156 ×
1155)
0.1335°
65.9
129,390
± 7.5
1.413479 4.232°
0.0013
1948
Kuiper
1,353
191,020
± 120
2.520379 0.260°
0.0012
1851
Lassell
1,172
266,300
± 135
4.144177 0.205°
0.0039
1851
Lassell
1,576.8 ± 1. 3,527
435,910
2
± 90
8.705872 0.340°
0.0011
1787
Herschel
1,522.8 ± 5. 3,014
583,520
2
± 75
13.46323
0.058°
9
0.0014
1787
Herschel
2003[i]
Holman et
al.
Gladman et
al.
Gladman et
al.
Holman et
al.
Nicholson
et al.
Sheppard
and
Jewitt
Holman et
al.
Kavelaars
et al.
Holman et
al.
1,169.4 ±
5.6
~22
0.007
4,276,000 −266.56 147.459° 0.1459
2
ˈkælɨbæn
~72
0.25
7,230,000 −579.50 139.885° 0.1587
1997
♠
Stephano ˈstɛfənoʊ
~32
0.022 8,002,000 −676.50 141.873° 0.2292
1999
XXI
♠
Trinculo ˈtrɪŋkjʊloʊ
~18
23
XVII
♠
Sycorax ˈsɪkəræks
~150
24
XXIII Margaret ˈmɑrɡərɨt
~20
0.005 14,345,00
1694.8
4
0
25
XVIII ♠Prospero ˈprɒspəroʊ
~50
0.085
26
XIX
27
XXIV
♠
Setebos
ˈsɛtɨbʌs
~48
♠
Ferdinan
ˈfɜrdɨnænd
d
ka
(Voyager
2)
Synnott
(Voyager
2)
Showalter
and
Lissauer
~20
0.003
8,571,000 −758.10 166.252° 0.2200
9
12,179,00
2.30
−1283.4 152.456° 0.5224
0
51.455°
0.6608
16,418,00
−1992.8 146.017° 0.4448
0
17,459,00
0.075
−2202.3 145.883° 0.5914
0
0.005 20,900,00
−2823.4 167.346° 0.3682
4
0
2001
1997
2003
1999
1999
2003[i]
Exploration of Uranus
The exploration of Uranus has, to date, been solely through telescopes and NASA's Voyager 2 spacecraft,
which made its closest approach to Uranus on January 24, 1986. Voyager 2 discovered 10 previously
unknown moons, studied the planet's cold atmosphere, and examined its ring system, discovering two new
rings. It also imaged Uranus' five large moons, revealing that their surfaces are covered with impact craters
and canyons.
A number of dedicated exploratory missions to Uranus have been proposed, but as of 2012 none have been
formally approved.
Voyager 2
Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 81,500 kilometers
(50,600 mi) of the planet's cloud tops. This was the probe's first solo planetary flyby, since Voyager 1 ended its
tour of the outer planets at Saturn's moon Titan.
Uranus is the third largest planet in the Solar System. It orbits the Sun at a distance of about 2.8 billion
kilometers (1.7 billion miles) and completes one orbit every 84 years. The length of a day on Uranus as
measured by Voyager 2 is 17 hours, 14 minutes. 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. Given its odd orientation, with its polar regions exposed to sunlight or darkness for long periods,
scientists were not sure what to expect at Uranus.
The presence of a magnetic field at Uranus was not known until Voyager's arrival. The intensity of the field is
roughly comparable to that of Earth's, though it varies much more from point to point because of its large offset
from the center of Uranus. The peculiar orientation of the magnetic field suggests that the field is generated at
an intermediate depth in the interior where the pressure is high enough for water to become electrically
conducting.
Voyager 2 found that one of the most striking influences of the sideways position of the planet 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.
Radiation belts at Uranus were found to be of an intensity similar to those at Saturn. The intensity of radiation
within the belts is such that irradiation would quickly darken (within 100,000 years) any methane trapped in the
icy surfaces of the inner moons and ring particles. This may have contributed to the darkened surfaces of the
moons and ring particles, which are almost uniformly gray in color.
A high layer of haze was detected around the sunlit pole, which also was found to radiate large amounts of
ultraviolet light, a phenomenon dubbed "electroglow". The average temperature of the atmosphere of the
planet is about 59 kelvins. Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the
same temperature at the cloud tops.
Voyager found 10 new moons, bringing the total number to 15 at the time. Most of the new moons are small,
with the largest measuring about 150 kilometers (about 90 miles) in diameter.
The moon Miranda, innermost of the five large moons, was revealed to be one of the strangest bodies yet
seen in the Solar System. Detailed images from Voyager's flyby of the moon showed huge oval structures
termed coronae flanked by faults as deep as 20 kilometers (12 mi), terraced layers, and a mixture of old and
young surfaces. One theory holds that Miranda may be a reaggregation of material from an earlier time when
the moon was fractured by a violent impact.
The five large moons appear to be ice-rock conglomerates like the satellites of Saturn. Titania is marked by
huge fault systems and canyons indicating some degree of geologic, probably tectonic, activity in its history.
Ariel has the brightest and possibly youngest surface of all the Uranian moons and also appears to have
undergone geologic activity that led to many fault valleys and what seem to be extensive flows of icy material.
Little geologic activity has occurred on Umbriel or Oberon, judging by their old and dark surfaces.
All nine previously known rings were studied by the spacecraft and showed the Uranian rings to be distinctly
different from those at Jupiter and Saturn. The ring system may be relatively young and did not form at the
same time as Uranus. Particles that make up the rings may be remnants of a moon that was broken by a highvelocity impact or torn up by gravitational effects. Voyager 2 also discovered two new rings.
Proposed Missions
A number of missions to Uranus have been proposed. Scientists from the Mullard Space Science Laboratory
in the United Kingdom have proposed the joint NASA–ESA Uranus Pathfinder mission to Uranus. A call for a
medium-class (M-class) mission to the planet to be launched in 2022 was submitted to the ESA in December
2010 with the signatures of 120 scientists from across the globe. The ESA caps the cost of M-class missions
at €470 million.
Another mission to Uranus, dubbed HORUS, was designed by the Applied Physics Laboratory of Johns
Hopkins University. The proposal is for a nuclear-powered orbiter carrying a set of instruments, including an
imaging camera, spectrometers and a magnetometer. The mission would launch in April 2021, arriving at
Uranus 17 years later. The minimum duration of the HORUS mission is two years.
In 2009, a team of planetary scientists from NASA's Jet Propulsion Laboratory advanced possible designs for
a solar-powered Uranus orbiter. The most favorable launch window for such a probe would be in August 2018,
with arrival at Uranus in September 2030. The science package may include magnetometers, particle
detectors and, possibly, an imaging camera.
In the 2011 decadal survey for the future potential of planetary exploration, the United States National
Research Council recommended a Uranus orbiter and probe. However, this mission is considered to be lowerpriority than future missions to Mars and the Jovian system.
Image of Uranus as Voyager 2 leaves for Nepture.