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[16.0] Missions To Uranus

v2.0.0 / chapter 16 of 20 / 01 jul 16 / greg goebel

* Uranus was the first planet to be discovered that was not known to the ancients. After it was found in 1781, it remained very mysterious until it was visited by Voyager 2 in 1986. This chapter outlines what is now known about Uranus.

Uranus


[16.1] THE DISCOVERY OF URANUS
[16.2] URANUS BEFORE THE SPACE AGE
[16.3] VOYAGER 2 AT URANUS
[16.4] THE URANIAN MOONS
[16.5] THE URANIAN RING SYSTEM
[16.6] URANUS STATISTICS & COMMENTS

[16.1] THE DISCOVERY OF URANUS

* The invention of the telescope early in the 17th century had led to discovery that the planets were other worlds and not just bright points of light in the sky. This revelation was something of a shock. Late in the 18th century, the telescope provided another shock, revealing that there were other planets in the solar system whose existence had remained unknown and unsuspected.

Sir William Herschel's observations of the night sky led him to conduct an all-sky survey, cataloging every star of 4th magnitude or brighter, completing the work in 1781. During the course of this work, on 13 March 1781, he spotted what he thought was a comet. Herschel had many contacts among the British scientific establishment. The news of his discovery spread through this network and reached Nevil Maskelyne, the English astronomer royal. Maskelyne helpfully suggested that the object might be a planet, since it did not seem to have the fuzzy "coma" or a tail, as might be expected from a comet.

The German astronomer Johann Elert Bode, associated with the Bode-Titius "law" that led to the discovery of the asteroids, began to track the movements of Herschel's object, and by November 1781 Bode had calculated its orbit. It was clearly a planet, with an orbit that placed it twice as far from the Sun as Saturn. Herschel estimated the disk of the planet as 54,700 kilometers across, more than four times the diameter of the Earth. This was a surprisingly good estimate, compared to the correct modern value of 51,200 kilometers, and a tribute to Herschel's outstanding observational skills.

The discovery brought Herschel considerable public attention. There was skepticism that an amateur could have made such an important discovery, but to their credit, Maskelyne and other members of the formal scientific establishment inspected Herschel's telescopes and proclaimed them the finest instruments they had ever seen. In those days, of course, there was less distinction between professional and amateur, but even today amateurs play a significant role in the astronomical sciences.

* The next question was, of course: what should the new planet be named? Herschel showed little interest in the issue. The French mathematician de Lalande suggested that the planet named "Herschel". Bode suggested that the planet be named "Uranus", reflecting the succession of planets in distance from the Sun: Jupiter's father was Saturn, and Saturn's father was Uranus.

Herschel remained silent on the matter, and the Royal Society decided to nudge him to make a decision, encouraging him to believe that King George III would grant him a stipend of 200 pounds a year if Herschel named the planet after the king. Although Herschel was prosperous at the time, he was still aware of where his best interests lay, and wrote a high-flown letter to King George proposing that the planet should be named "Georgium Sidus (George's Star)". Herschel did in fact receive the stipend.

King George was not extremely popular, and the name wasn't popular either. In fact, Herschel stated in a letter to Bode that he did not believe the name "Georgium Sidus" would be generally accepted, an admission that Herschel had only selected the name out of convenience. The names "Herschel", the "Georgian Planet", and "Uranus" were to remain in competing use for the following decades, the name "Uranus" only winning out after Herschel's death in 1822.

* After the discovery of the new planet, Herschel continued his observations, and as a further demonstration of his observing skills found two moons of the world on 11 January 1787. These moons would quickly named "Oberon" and "Titania". On 24 October 1851, the British brewer / astronomer William Lassell discovered two more moons of Uranus, which he named "Umbriel" and "Ariel". There matters stood until 16 February 1948, when the Dutch-American astronomer Gerard P. Kuiper discovered a small fifth moon of Uranus, which he named "Miranda".

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[16.2] URANUS BEFORE THE SPACE AGE

* Not much was known about the planet Uranus before the space age. Its orbit around the Sun could be determined with precision, giving an average radius of 2.87 billion kilometers, or 19.2 astronomical units, and a period of 84.1 Earth years. The mass of the planet was known, about 14.5 Earth masses, and there were good estimates of its diameter of about 51,200 kilometers. This gave an average density of 1.3 grams per cubic centimeter, a little more than the density of water, 1 gram per cubic centimeter. Spectroscopic studies revealed traces of methane in the planet's atmosphere, and infrared measurements suggested a temperature at the higher regions of the atmosphere of about 60 degrees Kelvin.

As seen from Earth, Uranus has the apparent size of about four seconds of arc, comparable to a golf ball at a distance of 2.5 kilometers. The minimum useful resolution of an old-technology Earth-based telescope is about a second of arc due to atmospheric distortions, and so there was no way at the time to obtain much detail about the planet from Earth-based observations.

Uranus appeared to rotate with a period of about 11 hours, but since it almost certainly did not have a solid surface and had no identifiable surface markings, that figure was highly uncertain. In fact, although some astronomers occasionally thought they saw patterns such as bands or spots, such markings were invariably faint, uncertain, and impossible to duplicate. As far as anybody could tell, Uranus was entirely featureless.

The orbits of its five known satellites had also been carefully plotted out, and proved to be essentially circular, in a common plane and in the same direction. Oberon was the outermost moon, followed by Titania, Umbriel, Ariel, and finally Miranda, the innermost moon. Almost nothing else was known of the moons, since they were hardly more than pinpoints of light in even the biggest telescopes. They were clearly more than small pieces of "space junk", but certainly not as large as the Earth's Moon.

One thing the moons did reveal from the plane of their orbits around the planet was that the spin axis of Uranus is 98 degrees to the ecliptic, almost parallel to the plane of its orbit, or in other words Uranus orbits on its "side". During Uranian "summers" the Sun is almost directly above the planet's north pole, while during Uranian "winters" the Sun is almost directly above the planet's south pole. Astronomers suggested this odd orientation was due to a cosmic collision between the planet and a large body in the distant past.

* Following Kuiper's discovery of Miranda, no major discoveries were made about Uranus for a quarter of a century. In 1973, a British astronomer named Gordon Taylor, of the Royal Greenwich Observatory, calculated that Uranus would move in front of, or "occult", a star on 10 March 1977, and provided details of where on Earth the occultations would be visible. Such an occultation was of great interest to astronomers, since the changes in brightness of the star as it passed behind the planet could reveal more details about Uranus' size and atmosphere.

With plenty of advance warning, several groups planned to observe the event. On the day of the occultation, a group of Cornell University astronomers, including James L. Elliot, Edward Dunham, and Doug Mink, was flying over the Indian Ocean in a converted Lockheed C-141A jet cargo transport that was fitted with a 90-centimeter telescope. The aircraft was owned and operated by NASA, being known as the "Kuiper Airborne Observatory (KAO)", after Gerard Kuiper.

The occultation turned out to be much more interesting than expected. The light from the star dimmed five times before the occultation began. After the occultation ended, the light from the star dimmed another five times, in the exact reverse order of the original pattern. Astronomers quickly came to the conclusion that Uranus has a system of five rings.

Further observations of stellar occultations over the next few years revealed a total of nine rings, all of which lie within a single planetary radius of the top of the Uranian atmosphere. Since there were multiple discoverers, the rings were given inconsistent names, with the outermost being named the "Epsilon" ring, followed by "Delta", "Eta", "Gamma", "Beta", "Alpha", "4", "5", and the lowest ring, "6". The rings were dark and narrow. At the time, the only ring system known was that of Saturn's, and the rings of Uranus were nothing like Saturn's. The rings, indeed all of Uranus and its moons, demanded a closer inspection.

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[16.3] VOYAGER 2 AT URANUS

* After flying by Jupiter and Saturn, the NASA Voyager 2 probe performed a flyby of Uranus, with closest approach on 24 January 1986. Planning for the encounter had been meticulous, since there was only one chance to get it right and Voyager 2 was showing its age. To complicate matters, the spacecraft's flyby would occur when the one of the planet's poles was aligned with the Sun, meaning that the probe would fly through the plane of the orbits of the planet's moons, not along it, minimizing the time available for observations.

One of the problems affecting Voyager 2 was the inevitable and steady degradation of the electrical output of its atomic radioisotope thermoelectric generators, and power management was a major headache. Subsystems had to be turned on and off in precisely the right sequence to ensure the most effective observations. A second problem was that the low light levels at the distance of Uranus meant long camera exposures. That not only reduced the number of images that could be obtained, but it required that the spacecraft be held very steady during the exposures, lest the images be blurred.

Mission planners used the spacecraft's thruster system to keep the probe on target, and even had to be careful in using the onboard magnetic tape data storage system, which caused a slight vibration every time the tape was started or stopped. A related problem was that the cameras had to be "panned", or turned slightly during an exposure, to stay pointed on target. This was also accomplished by careful control of the thruster system.

A third problem was that the data rate of communications fell as the distance of the probe from Earth increased and the received signal grew weaker. This problem was addressed by improving the efficiency of the spacecraft's data error correction scheme, reducing the amount of overhead information that it had to transmit just to ensure that the data bits were sent correctly; and by implementing a data compression scheme known as "delta modulation", where only changes in image data values were transmitted. The Earth-based ground stations were also improved for the Uranus flyby, and were used together as "arrays" in which their antenna areas were effectively "summed", providing a much more sensitive receiver.

* The efforts of the mission team proved highly successful, and the Voyager 2 flyby returned unprecedented new information about Uranus, its moons, and its rings. The probe sent back almost 6,000 images. In contrast to the gaudy multicolored bands of Jupiter and Saturn, Uranus simply looked like a featureless, icy cold, blue-green ball. Some faint weather patterns were later extracted from the Voyager 2 imagery by the clever method of creating a "graphical model" of how Uranus would look if it had no inherent variations in brightness at all, and then subtracting the "model" image data from the actual images.

The spacecraft's observations confirmed in more detail what had been generally believed about the composition of the planet. Uranus was made up of about 85% hydrogen and 12% helium by mass, similar to the general composition of the Sun, Jupiter, and Saturn. The blue-green appearance of Uranus was due to trace amounts of methane (CH4), with a faint haze of "smog" due to the photochemical conversion of methane into heavier hydrocarbons, particularly ethane (C2H6) and acetylene (C2H2).

It was once believed that Uranus had an internal structure similar to that of Jupiter and Saturn, consisting of a rocky core surrounded by a dense mantle of liquid helium, covered in turn by a gaseous atmosphere of mostly hydrogen and helium gas. However, Uranus does not bulge as much at its equator as would expected for such a composition, and such a rapid rate of spin; its equatorial diameter is only 2.4% bigger than its polar diameter. The planet is now thought to have a rocky core about the size of the Earth, with a mixed, superdense atmosphere that extends from the boundary of space down to the rocky core.

Since the Sun was above one of the poles of Uranus when the flyby occurred, mission scientists were expecting that there would be substantial temperature variation between the poles. That didn't prove to be the case. The maximum temperature variation was only two degrees Kelvin, with an average temperature of 52 degrees Kelvin. The feeble sunlight does not provide that much heating, and winds distribute heat quickly. Incidentally, Uranus emits 15% more energy than it obtains from the Sun, due to the breakdown of radioactive minerals in the planet's rocky core. Some astronomers thought that the orientation of Neptune during the flyby would result in winds moving from the north pole to the south pole, but the winds blew from east to west, in jet streams moving at 300 kilometers per hour, with the high speed due to the planet's rapid rotation rate.

Although Uranus was expected to have a magnetic field, Voyager 2's magnetic data still surprised astronomers. On the Earth, the magnetic and rotational poles are offset by about 11 degrees, and roughly the same is true for Jupiter and Saturn. However, on Uranus the magnetic and rotational poles are offset by 60 degrees. Furthermore, the axis of the magnetic field misses the center of Uranus by 7,000 kilometers.

Researchers suspected that Uranus was undergoing a magnetic pole reversal, which happens on Earth every now and then, but similar observations later made by Voyager 2 at Neptune in 1989 dampened enthusiasm for that theory. Refined theories suggest that the magnetic field is not produced in the planetary core, instead being generated at some "transition zone" deep in the planet's mantle, where ammonia and water are under enough pressure to make them conductive and capable of generating a magnetic field.

Observations of the magnetic field gave the first accurate measurement of the length of the planet's "day", 17.24 hours, though in fact, with the Sun over the north pole, one half of the planet was sunlit all "day" long, while the other half was in night.

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[16.4] THE URANIAN MOONS

* Voyager 2 provided the first real details on the five "classical" moons of Uranus, and also discovered ten new ones. In addition, the probe discovered two new rings, to add to the nine that had been discovered almost a decade earlier. The total mass of the entire moon-ring system of Uranus is small, with everything fitting into the volume of the Earth's Moon with space to spare.

moons of Uranus

The five classical moons were expected to be dull iceballs, but they turned out to be surprisingly diverse and interesting:

* The ten new moons discovered by Voyager 2 all have orbits below those of the five classical moons. The outermost and largest was given the provisional designation of "1985U1" and later given the formal name "Puck", and is about 170 kilometers in diameter. It is a lumpy sphere and has some large craters.

The other nine new moons are similar to Puck, with diameters ranging from 110 to 25 kilometers. They were given the names:

   Cordelia
   Ophelia
   Bianca
   Cressida
   Desdemona
   Juliet
   Portia
   Rosalind
   Belinda

-- in accordance with the tradition of naming the moons of Uranus from the works of William Shakespeare and Alexander Pope. All ten are much darker than the classical moons, described as "charcoal black". Their dark color and small size accounted for the fact that they were not observed by Earth-based telescopes. The black color is likely due to what the American astronomer Carl Sagan called "star tar", methane that has been converted into heavier hydrocarbons by Uranus' radiation belts or sunlight, or simply a residue of carbon compounds left over from the early days of the Solar System.

* From 1997 to 2001, six irregular moons of Uranus were discovered, verified, and given the names:

   Caliban
   Stephano
   Trinculo
   Sycorax
   Prospero
   Setebos

As with the irregular moons of Jupiter and Saturn, all six have eccentric, high inclination (in fact all retrograde) orbits well outside of the system of regular moons. However, they are not all just moonlets, little pieces of sky junk: Sycorax has an estimated diameter of 190 kilometers, while Stephano has an estimated diameter of 98 kilometers, and Prospero and Setebos both have diameters of about 30 kilometers.

Three more irregular moons of Uranus were discovered in 2001, with another three moons discovered in 2003, with only one being irregular, the other two being part of the inner regular moon system. One moon spotted in 1986 was also rediscovered, bring the increment of new moons to seven. The two regular moons were named "Mab" and "Cupid", while the irregulars were named:

   Trinculo
   Francisco
   Margaret
   Ferdinand
   Perdita
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[16.5] THE URANIAN RING SYSTEM

* Other than discovering two new rings, "1986U1R" and "1986U2R", Voyager 2 didn't provide much more information on the Uranian ring system, simply because there's so little to it. All eleven rings are thin and very tenuous. The outermost and most substantial of the set, the Epsilon ring, varies in width from 20 to 100 kilometers, while the lowest, 1986U2R, is diffuse and up to 2,500 kilometers wide. The others range from 1 to 12 kilometers wide. They are very dark in color, resembling the ten small moons in this respect, and in fact Voyager 2 had to use stellar occultations to help in mapping them.

The particles in the rings range from about the size of a softball to the size of a car, but the rings are relatively free of small particles, which suggests a rapid rate of depletion; collisions between large particles should create many small particles, but if so such small particles are being removed quickly. Voyager 2 detected an extended upper atmosphere of hydrogen that is slowly depleting the rings, causing their particles to fall down to the planet. The rings are believed to have arisen from the breakup of a small moon or moons due to cometary impact.

Interestingly, Voyager 2 mission scientists were surprised at finding ten new small moons of Uranus, not because there were so many, but because there were so few. This was because the nine known rings of the planet were so narrow. Narrow rings were puzzling because simple random motion should spread them out over time. Observations of Saturn's rings by the two Voyager probes had suggested to planetary astronomers that a narrow ring was created by a pair of small "shepherd moons" that straddled it, with the moon in orbit below the ring gravitationally kicking particles back up into the ring and the moon in orbit above the ring kicking particles back down. However, this pattern was not observed with the narrow rings of Uranus, and in fact Voyager 2 didn't observe it in 1989 for the narrow rings of Neptune, either. It is possible that there were shepherd moons that were simply too tiny to be observed by the probe, but planetary astronomers are still scratching their heads over the matter.

Voyager 2 also detected about a hundred very faint "dust bands" around the planet. They were not in the same range of orbits as the rings and were composed of particles a fraction of a millimeter in diameter.

* Having observed Uranus, Voyager 2 flew on towards Neptune. The spacecraft's look at Uranus was a short one, but provided far more information about the planet and its environment than any accumulated to that time. No new missions to Uranus are in planning at this time. It was extremely fortunate that NASA was able to take advantage of the rare "Grand Tour" trajectory to allow even this inspection of the planet, since otherwise it would be unlikely that we would have sent a mission to it at all.

For the time being, the planet is being studied by telescopes. Two new rings were discovered by the NASA Hubble Space Telescope in 2005. They were well outside the bounds of the previously-known rings, with the outermost new ring twice the diameter of the Epsilon ring. The outer ring is centered on the new moonlet Mab and has a blue color, like that of Saturn's E Ring. The blue color of Saturn's E Ring is apparently due to materials escaping from Enceladus, so it's not too remarkable to think that the blue ring of Uranus is due to materials escaping from Mab.

The big Keck telescopes in Hawaii have performed observations that would have been impossible two decades ago. Particularly interesting observations of the rings, suggesting that they are in a state of rapid change, were performed by the Kecks in 2007, when the rings were edge-on to the Earth's line of sight.

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[16.6] URANUS STATISTICS & COMMENTS

* Statistics for Uranus:

  __________________________________________________________________________

  mean distance from Sun              19.18 AU (2,869.6 x 10^6 kilometers)
  orbital period (sidereal)           84.01 years
  orbital eccentricity                0.046
  orbital inclination                 0.8 degrees
  equatorial diameter                 51,200 km / 4.01 Earth
  mass (relative to Earth)            14.500
  mean density (relative to water)    1.30
  gravity (relative to Earth)         0.91
  escape speed                        21.3 kilometers per second
  rotation period                     17.24 hours
  oblateness                          1/45
  inclination of equator              97.9 degrees (retrograde)
  albedo                              0.51
  surface temperature                 52 degrees Kelvin (cloud tops)
  atmosphere (major constituents )    H, He, CH4, clouds of CH4
  atmospheric pressure at surface     not applicable
  number of moons > 10 km in size     18, plus a handful of known moonlets
  __________________________________________________________________________

* Moons of Uranus, from outermost to innermost, with pronunciations in parenthesis.

Uranus scale diagram

Radii are given from the center of Uranus. The abbreviation "RU" stands for the radius of Uranus, while "RM" stands for the radius of the orbit of the Earth's Moon, and "M" stands for the diameter or mass of the Earth's Moon. Densities are given relative to water, which is equivalent to grams per cubic centimeter. Orbital and rotation periods are given in days and fractions of days, with days-hours-minutes added for periods under two days.

  __________________________________________________________________________

  CLASSICAL MOONS
  __________________________________________________________________________

  OBERON ("OH-buh-ron") / Uranus IV:

  mean orbital radius                 583,000 km / 22.77 RU / 1.52 RM
  orbital period (sidereal)           13.46 days
  orbital eccentricity                < 0.01
  orbital inclination                 0.1 degrees
  equatorial diameter                 1,550 km / 0.45 M
  mass                                0.040 M
  mean density                        1.64
  albedo                              0.24
  year of discovery                   1787 (Herschel)
  __________________________________________________________________________

  TITANIA ("ty-TAY-nee-uh") / Uranus III:

  mean orbital radius                 436,000 km / 17.03 RU / 1.13 RM
  orbital period (sidereal)           8.71 days 
  orbital eccentricity                0.002
  orbital inclination                 0.1 degrees
  equatorial diameter                 1,610 km / 0.46 M
  mass                                0.047 M
  mean density                        1.52
  albedo                              0.28
  year of discovery                   1787 (Herschel)
  __________________________________________________________________________

  UMBRIEL ("UM-bree-el") / Uranus II:

  mean orbital radius                 266,000 km / 10.39 RU / 0.69 RM
  orbital period (sidereal)           4.14 days
  orbital eccentricity                0.005
  orbital inclination                 0.36 degrees
  equatorial diameter                 1,190 km / 0.34 M
  mass                                0.017 M
  mean density                        1.52
  albedo                              0.19
  year of discovery                   1851 (Lassell)
  __________________________________________________________________________

  ARIEL ("AIR-ee-el") / URANUS I:

  mean orbital radius                 190,900 km / 7.46 RU / 0.50 RM
  orbital period (sidereal)           2.52 days
  orbital eccentricity                0.003
  orbital inclination                 0.31 degrees
  equatorial diameter                 1,160 km / 0.33 M
  mass                                0.018 M
  mean density                        1.56
  albedo                              0.40
  year of discovery                   1851 (Lassell)
  __________________________________________________________________________

  MIRANDA ("mi-RAN-duh") / Uranus V:

  mean orbital radius                 129,000 km / 5.04 RU / 0.34 RM
  orbital period (sidereal)           1.41 days / 1 day 9 hours 55 minutes
  orbital eccentricity                0.027
  orbital inclination                 4.2 degrees
  equatorial diameter                 485 km / 0.14 M
  mass                                0.001 M
  mean density                        1.15
  albedo                              0.34
  year of discovery                   1948 (Kuiper)
  __________________________________________________________________________

  VOYAGER 2 MOONS
  __________________________________________________________________________

  PUCK / Uranus XV:

  mean orbital radius                 86,000 km / 3.36 RU / 0.22 RM
  orbital period (sidereal)           0.76 days / 18 hours 17 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 0.31 degrees
  equatorial diameter                 170 km / 0.05 M
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1985 (Voyager 2)
  __________________________________________________________________________

  BELINDA ("buh-LIN-duh") / Uranus XIV:

  mean orbital radius                 75,300 km / 2.94 RU / 0.20 RM
  orbital period (sidereal)           0.62 days / 14 hours 58 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 < 0.1 degrees
  equatorial diameter                 65 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  ROSALIND ("RAHZ-uh-lind") / Uranus XIII:

  mean orbital radius                 70,000 km / 2.73 RU / 0.18 RM
  orbital period (sidereal)           0.56 days / 13 hours 24 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 < 0.1 degrees
  equatorial diameter                 55 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  PORTIA ("POR-shuh") / Uranus XII:

  mean orbital radius                 66,100 km / 2.58 RU / 0.18 RM
  orbital period (sidereal)           0.51 days / 12 hours 19 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 < 0.1 degrees
  equatorial diameter                 110 km / 0.03 M
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)

  __________________________________________________________________________

  JULIET / Uranus XI:

  mean orbital radius                 64,400 km / 2.52 RU / 0.17 RM
  orbital period (sidereal)           0.49 days / 11 hours 50 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 < 0.1 degrees
  equatorial diameter                 40 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  DESDEMONA ("des-duh-MOH-nuh") / Uranus X:

  mean orbital radius                 62,700 km / 2.45 RU / 0.16 RM
  orbital period (sidereal)           0.47 days / 11 hours 22 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 0.16 degrees
  equatorial diameter                 55 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  CRESSIDA ("KRES-sih-duh") / Uranus IX:

  mean orbital radius                 61,800 km / 2.41 RU / 0.16 RM
  orbital period (sidereal)           0.46 days / 11 hours 8 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 0.28 degrees
  equatorial diameter                 65 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  BIANCA ("bee-AHN-kah") / Uranus VIII:

  mean orbital radius                 59,200 km / 2.31 RU / 0.15 RM
  orbital period (sidereal)           0.43 days / 10 hours 26 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 0.156 degrees
  equatorial diameter                 45 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  OPHELIA ("oh-FEE-lee-uh") / Uranus VII -- outer Epsilon Ring shepherd:

  mean orbital radius                 53,800 km / 2.10 RU / 0.14 RM
  orbital period (sidereal)           0.33 days / 9 hours 2 minutes
  orbital eccentricity                0.01
  orbital inclination                 < 0.1 degrees
  equatorial diameter                 30 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  CORDELIA ("kor-DEE-lee-uh") / Uranus VI -- inner Epsilon Ring shepherd:

  mean orbital radius                 49,700 km / 1.94 RU / 0.13 RM
  orbital period (sidereal)           0.33 days / 8 hours 2 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 0.14 degrees
  equatorial diameter                 25 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   1986 (Voyager 2)
  __________________________________________________________________________

  MAB / Uranus XXVI 

  mean orbital radius                 97,734 km / 3.82 RU / 0.26 RM
  orbital period (sidereal)           0.923 days / 22 hours 9 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 0.14 degrees
  equatorial diameter                 25 km
  mass & density                      UNKNOWN
  albedo                              0.1
  year of discovery                   2003
  __________________________________________________________________________

  CUPID ("KYU-pid") / Uranus XXVII 

  mean orbital radius                 74,392 km / 2.9 RU / 0.19 RM
  orbital period (sidereal)           0.618 days / 14 hours 50 minutes
  orbital eccentricity                < 0.01
  orbital inclination                 < 0.01
  equatorial diameter                 18 km
  mass & density                      UNKNOWN
  albedo                              0.07
  year of discovery                   2003

  __________________________________________________________________________

* Irregular moons of Uranus:

  __________________________________________

                                preliminary
  moon                          designation
  __________________________________________

  CALIBAN       Uranus XVI       S/1997 U1 
  SYCORAX       Uranus XVII      S/1997 U2 
  PROSPERO      Uranus XVIII     S/1999 U3 
  SETEBOS       Uranus XIX       S/1999 U1 
  STEPHANO      Uranus XX        S/1999 U2 

  TRINCULO      Uranus XXI       S/2001 U1 
  FRANCISCO     Uranus XXII      S/2001 U3 

  MARGARET      Uranus XXIII     S/2003 U3 
  FERDINAND     Uranus XXIV      S/2001 U2 
  PERDITA       Uranus XXV       S/1986 U10
  __________________________________________

* Rings of Uranus, from outermost to innermost, with radii given from the center of the planet and widths in kilometers:

  __________________________________________________________________________

  EPSILON             51,140 km / 2.00 RU / 0.13 RM               20:100 km
  1986U1R             50,020 km / 1.95 RU / 0.13 RM                  1:2 km
  DELTA               48,290 km / 1.89 RU / 0.13 RM                  3:9 km
  GAMMA               47,630 km / 1.86 RU / 0.12 RM                  1:4 km
  ETA                 47,190 km / 1.84 RU / 0.12 RM                  1:2 km
  BETA                45,670 km / 1.78 RU / 0.12 RM                 7:12 km
  ALPHA               44,720 km / 1.75 RU / 0.12 RM                 7:12 km
  4                   42,580 km / 1.66 RU / 0.11 RM                  2:3 km
  5                   42,230 km / 1.65 RU / 0.11 RM                  2:3 km
  6                   41,840 km / 1.63 RU / 0.11 RM                  1:3 km
  1986U2R             38,000 km / 1.48 RU / 0.10 RM                2,500 km
  __________________________________________________________________________

* Incidentally, saying the name of the planet "Uranus" tends to provoke snickers among English speakers, because its popular pronunciation is "your anus". Classicists insist that the proper pronunciation is "YER-un-us", and that's the pronunciation used on TV and radio. That may be just an attempt to dodge the bullet, the popular pronunciation remains dominant, and the toilet jokes still circulate.

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