The planet Uranus has 27 known moons, most of which weren't discovered until the space age. They range from Titania, 981 miles (1,579 kilometers) in diameter, to tiny Cupid, only 11 miles (18 km) in diameter. All satellites of Uranus are named for characters in William Shakespeare plays or characters from Alexander Pope's "Rape of the Lock," according to International Astronomical Union guidelines.
Astronomers knew of five moons before the Voyager 2 spacecraft launched in 1977. The probe found an additional 10 when it swung by the system in 1986.
"When [Voyager 2] flew past in 1986, it was winter and dark on the whole northern hemispheres of all the moons, so we could only see a portion of their southern hemispheres," Jeff Moore, a planetary scientist at NASA Ames Research Center in California, told Space.com. "The encounter was also a bit like flying through a bull's-eye. Rather than one moon at a time, [Voyager 2] encountered the whole system at once."
No spacecraft has gone to Uranus since then, but astronomers have found new moons with the aid of generally improving telescope technology and techniques. The latest discoveries — Mab, Cupid and Margaret — were confirmed in 2003.
The moons of Uranus may have formed from the collision that knocked the planet over on its side.
"Material from the two [colliding] bodies is ejected in a debris disk, and finally satellites are formed from the debris disk," researcher Yuya Ishizawa, of Japan's Kyoto University, told Space.com. "It is possible to explain the axial tilt and the formation of the regular satellites of Uranus simultaneously."
Early moon discoveries
The discovery of Uranus and its first two moons came from the same astronomer: William Herschel. The English skygazer found Oberon and Titania in 1787, just six years after discovering the planet itself.
Close-up pictures two centuries later showed that Oberon is about half ice and half rock, with craters peppering its surface. Titania has fault lines running across its surface, hinting at past or present seismic activity.
English astronomer William Lassell spotted Umbriel, the darkest of Uranus' big moons, in 1851. In Voyager 2's quick fly-by, scientists were unable to figure out how the surface got so dark, or why there is a bright ring on Umbriel's surface that is 90 miles (140 kilometers) in diameter.
Ariel, another find from Lassell in 1851, has the youngest and brightest known surface among the moons. This could be due to meteorite hits, as well as possible geologic activity — among its features are grabens, or valleys surrounded by faults. Voyager 2 detected a silicate rock and water ice composition on this moon, with hints of carbon dioxide.
The last find before the space age was perhaps the most bizarre moon of all — Miranda. Found by Gerard P. Kuiper at the McDonald Observatory in 1948, the moon appears to be a jumble of disconnected features. It's unclear what made the mashed-up surface. Perhaps a huge meteorite blew the moon apart and it reassembled, or smaller meteorites melted the surface and caused slush to flow and freeze again.
Voyager 2's early discoveries
When Voyager 2 skimmed closest to Uranus' system in January 1986, its view turned up 10 new satellites within a month. While most of NASA's pictures of these moons show them as distant dots, if not for the spacecraft the moons probably would have gone unseen for decades if not longer.
Ophelia and Cordelia are sometimes called shepherd moons. They are close to Uranus' Epsilon ring, with their gravitational influence essentially keeping the ring from falling apart. No one knows how big Cordelia is, or what its composition might be. Ophelia is believed to be 12 miles (20 kilometers) in diameter.
Voyager 2 captured a picture of Puck that showed a mottled, dark surface. Astronomers believe it is made of carbonaceous material, and assume that much of the rest of the moons are of the same material. A 2003 study of the moon suggested water ice is on its surface.
Most of the other moons were so small and far away from the spacecraft that little is known about their composition and in some cases, their sizes. Belinda and Cressidahave low reflectivity, indicating they are probably carbonaceous moons as Puck is.
The other moons found right away were Portia, Rosalind, Juliet, Desdemona and Bianca. A 2001 study using the Hubble Space Telescope suggested these satellites (called the "Portia group" in this study) all have similar compositions and orbits.
Discoveries after Voyager
Since 1986, astronomers have turned up 12 new Uranus moons using the Voyager 2 data, as well as with newer observations with telescopes on or orbiting Earth.
Perdita has the most tangled history. The University of Arizona's Erich Karkoschka found the moon in 1999 using public Voyager 2 images and comparing them to some taken by the Hubble Space Telescope. At first, the IAU designated it a moon, but then rescinded that when other astronomers couldn't find it. Perdita was made official again in 2003 when Hubble spotted it again.
Caliban, found by a team at the Palomar Observatory in 1997, moves in an inclined opposite orbit to the rotation of Uranus. "These characteristics suggest that Caliban was an independent body that was captured by Uranus' gravity," NASA stated. The same team also found Sycorax, which appears red in telescopes and also has a retrograde orbit.
Three more discoveries happened in 1999 using the Canada-France-Hawaii Telescope at the Mauna Kea Observatory: Setebos, Prospero and Stephano. In 2001, a team at the Cerro Tololo Inter-American Observatory in Chile found Trinculo, Francisco and Ferdinand.
With no spacecraft visiting Uranus in the near future and its sheer distance from the Earth, the potential for observations of its moons is limited. That said, there have been several scientific examinations of the system in recent years, as well as proposals to send spacecraft there in the distant future. Some astronomers envision using small cubesats to reduce the cost of getting a spacecraft out that far, while still returning valuable science. A team also submitted a Uranus Pathfinder orbiter mission concept to the European Space Agency in 2010, and again in 2015, but neither concept was accepted for funding.
With the realization from Cassini observations that so many of Saturn's moons may have subsurface oceans, some astronomers have suggested that Uranus' moons could be host to oceans as well. The possibilities for life may be more limited because the moon is that much more distant from the sun, but it is hard to say for sure without doing detailed examination of the moons.
Some observations of the moons have been performed from Earth. In 2013, a group probed Miranda for evidence of volatiles such as ammonia hydrate, and could not exclude the possibility from their observations.
A 2015 Icarus publication based on Uranus system observations suggested that several of Uranus' moons have a mix of water and carbon dioxide ice on their surfaces.
Separately, a 2014 model of Miranda in the journal Geology suggested that its strange surface could at least in part have come by tidal heating, which created slow convection on the icy surface.
"Miranda has a really bizarre, deformed surface," Noah Hammond, a planetary scientist at Brown University in Rhode Island, told Space.com. "It's a really beautiful and exotic moon."
The thin rings that surround Uranus hint that the planet could have even more moons in orbit.
"At the edges of the rings … it's almost like the amount of stuff is going up and down in a periodic fashion that looks kind of like a wave, with crests and troughs," then-graduate student Robert Chancia, of the University of Idaho, told Space.com. "It seems consistent with something disturbing the rings there," he added.
"Based on the amplitude of this wave pattern and that distance from the ring … and our attempts to find the moon in images, it basically points toward if they exist, they're pretty tiny," Chancia said. He estimated that the moons, if they exist, are likely smaller than 3 miles (5 kilometers) in radius.
Additional reporting by Nola Taylor Redd, Space.com contributor.