Space Today Online - Solar System - Planet Uranus - Moons
Margaret
2003
Click to enlarge Voyager 2 photos in 1986 of the Uranus moons Umbriel (left) and Ariel (right).
The planet Uranus has at least 27 moons that we know of.
Before the planet was visited by the spacecraft Voyager 2 on January 24, 1986, astronomers knew of only five. Voyager 2 discovered ten small moons. Today, astronomers suggest there may be more tiny satellites among the planet's rings.
The moons of Uranus are eye-openers. They have statuesque mountains towering more than ten miles high. Incredibly deep valleys. Vast plains, some with a mysterious dark surface.
Moon diameters range from a bit fatter than 25 miles up to about 1,100 miles.They circle Uranus in orbits ranging from every eight hours to every 14 days. Some of the moons are very tiny.
Click to enlarge Voyager 2 image of the planet Uranus
Voyager 2 found moons. The interplanetary probe Voyager 2 flew within 50,600 miles of Uranus' cloud tops on January 24, 1986, discovering a magnetic field and much ultraviolet light, known as dayglow.
Voyager 2 found 10 previously-unknown moons, the largest only 90 miles in diameter, bringing the total to 15. Astronomers previously had seen five large ice-and-rock moons. The innermost, Miranda, was found by Voyager to be one of the strangest bodies in the Solar System, with fault canyons up to 12 miles deep, terraced layers and mixed young and old surfaces.
Titania was marked by huge faults and deep canyons. Ariel had the brightest and youngest surface of Uranian moons, with many deep valleys and broad ice flows. Dark-surfaced Umbriel and Oberon looked older.
The moons Umbriel and Ariel each are about 75O miles in diameter. In NASA's 1986 pictures at right, Voyager 2 was about 346,000 miles from Umbriel. The Ariel picture is a mosaic of four images collected when Voyager 2 was about 8O,OOO miles from the moon
Voyager found nine rings around Uranus, but they were  quite different from Jupiter and Saturn rings. Uranus' rings may be young remnants of a shattered moon.
NASA image of the planet Uranus
Moons discovered in the 1990s. Until 1997, Uranus was the only one of the giant gas planets in our Solar System without any known tiny moonlets.
In 1997, J.J. Kavelaars of the Dominion Astrophysical Observatory in British Columbia, and a team of astronomers found the 16th and 17th satellites of Uranus. Those moons are named Caliban and Sycorax.
In 1999, Erich Karkoschka of the University of Arizona spotted an 18th Uranian satellite in a Voyager 2 photograph snapped in 1986.
In 1999, Kavelaars and a team of astronomers discovered the 19th and 20th moonlets orbiting Uranus. Designated 1999 U1 and 1999 U2, they are dark rocks less than 12 miles in diameter.
All newly-discovered Uranian moons are, or will be, named after Shakespearean characters.
New moonlet, in white circle, orbiting Uranus, bottom left
Source: International Astronomical Union
22 moons in 2002. Uranus has more than 20 satellites. The total number known in 2002 was 22, although astronomers were uncertain because newly-discovered moonlets were not always confirmed immediately by other astronomers.
Uranus was known to have ten so-called "regular" natural satellites that follow orderly paths around the planet all in about the same plane. Also known were six so-called "irregular" moons, including a newly-discovered tiny moonlet known as S/2001.
The irregulars were seen to be traveling in odd orbits around Uranus and may be remnants of a massive ancient collision between a comet and some other large object that had been orbiting Uranus.
S/2001 was the smallest Uranian satellite found so far, estimated to be 9 to 12 miles in diameter.
It was discovered in 2001 by Matthew Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, J.J. Kavelaars of the Dominion Astrophysical Observatory in British Columbia, and Dan Milisavljevic of McMaster University in Ontario.
The first sighting was made with the Canada-France-Hawaii Telescope and the Cerro-Tololo Inter-American Observatory in Chile.
»
October 24, 2001, was the 150th anniversary of William Lassell's 1851 discovery of Uranus'
Click to enlarge the NASA diagram depicting rings (red lines) and moons (blue dashed lines)  orbiting Uranus. The new rings are labeled R1 and U2. The newly-discovered moons are Mab and Cupid.
Credit: NASA, ESA, and M. Showalter of the SETI Institute.
Hubble sees moons and rings in 2005. The distant planet continues to surprise astronomers. Using NASA's Hubble Space Telescope in 2005, they uncovered a pair of giant rings girdling the planet.
The largest ring is twice the diameter of the planet's previously known ring system. The first Uranus ring discoveries were made in the 1970s.
The astronomers looking at Uranus with the Hubble Space Telescope saw two small moons – Mab and Cupid – discovered in 2003.
One of the moons shares an orbit with the outermost newly-found ring. That moon probably is the source of the fresh dust that replenishes the ring with matter knocked off the moon by meteoroid impacts. Without the ongoing replenishment, the ring would disappear as its dust spiraled slowly down onto Uranus.
The Hubble discoveries suggest Uranus has a young, dynamic system of rings and moons. Unlike the famous rings around Saturn, the rings around Uranus are dim and dark because they are mostly dust rather than ice.
Due to the extreme tilt of Uranus's axis, the rings appear to be nearly perpendicular relative to rings around other gas giant planets like Saturn.
Hubble Space Telescope image of Uranus ring
More moons will be found. British astronomers say they believe Uranus may have many more natural satellites than have been seen.
For instance, the moons Cordelia and Ophelia are shepherds, keeping the eleven large rings around Uranus from spreading into space. But, astronomers at London University say the two shepherd moons are not enough to explain the thin sharp edges on the rings. They figure there could be as many as 25 more small moonlets out there with their gravity pulling the rings into shape. Such small shepherd moons might range from one to 12 miles in diameter.
Ring gaps in some Voyager 2 photos, combined with a mysterious wave in one ring, may indicate two of the unseen moons could be 11 miles in diameter. They could be orbiting at 28,200 and 28,800 miles from the center of the planet.
Who names moons?
The International Astronomical Union (IAU), a worldwide association of 6,000 professional astronomers headquartered in Paris, has been responsible for naming planetary bodies since IAU was formed in 1919.
Harold Masursky, a planetary geologist with the U.S. Geological Survey in Flagstaff, Arizona, heads the IAU planetary system nomenclature working group.
Astronomers say, even though the Uranus moons were discovered by the United States, the natural satellites are international. Naming features or moons is not the property of one country. Going along with tradition receives international acceptance.
The IAU did name craters on Earth's Moon for the seven crew members -- Francis Scobee, Michael J. Smith, Judith Resnik, Ronald McNair, Ellison Onizuka, Gregory Jarvis and teacher-astronaut Sharon Christa McAuliffe -- killed in the 1986 shuttle Challenger launch explosion. The craters, 20 to 60 miles wide, are within a vast crater named Apollo on the far side of the Moon.
Also, seven asteroids were named in 1986 by the IAU for the Challenger astronauts, two months after the shuttle exploded.
The IAU previously had named craters on Earth's Moon for nine American astronauts and nine Soviet cosmonauts -- most living, some dead -- including Roger Chaffee, Gus Grissom and Ed White, killed when a launch pad fire swept the Apollo 1 spacecraft in 1967.
Shakespeare. Nine of the new Uranus moons found by Voyager in 1986 were named for Shakespearean characters: Bianca, Cordelia, Cressida, Desdemona, Juliet, Ophelia, Portia, Puck and Rosalind. The tenth was named for Belinda whose lock of hair was stolen in The Rape of the Lock.
Three of five previously-known Uranus moons – Oberon, Titania and Miranda – have Shakespearean names. Umbriel is from Pope's poem. Ariel is a name in both The Rape of the Lock and Shakespeare's The Tempest.
Moons of other planets mostly have been named for characters from Roman mythology. Use of literary names for moons of Uranus was started by English brewer and astronomer, William Lassell. He discovered Ariel, Oberon, Titania and Umbriel in 1851.
Voyager discovered the moons shortly before the January 28, 1986, shuttle Challenger disaster. Two U.S. congressmen had proposed naming the 10 moons for dead Challenger and Apollo 1 crew members. They prompted thousands of children, citizens and officials to write letters and sign petitions favoring naming moons for astronauts.
Lots and lots of moons. There are at least 156 moon in the Solar System. With a total of 63, Jupiter currently has the most known natural satellites of any planet in the Solar System. Saturn is second with 47. Uranus has 27. Neptune has 13. Pluto has 3. Mars has 2. Earth has 1.
The moon totals for Jupiter, Saturn, Uranus, Neptune and Pluto can be expected to increase as more powerful telescopes become available in the future.
More about the Solar System . . .
Star: