Uranus' Moons

Uranus, the seventh planet of the Solar System, has 27 known moons, all of which are named after characters from the works of William Shakespeare and Alexander Pope. William Herschel discovered the first two moons, Titania and Oberon, in 1787, and the remaining spherical moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda). The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced ground-based telescopes.

Uranian moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner moons are small dark bodies that share common properties and origins with the planet's rings. The five major moons are massive enough to have achieved hydrostatic equilibrium, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, and about 20 times less massive than Earth's Moon. Uranus's irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at great distances from the planet.

Oberon and Titania (king and queen of the fairies) are the largest Uranian moons, and were first to be discovered - by William Herschel in 1787. William Lassell, who had been first to see a moon orbiting Neptune, discovered the next two, Ariel and Umbriel. Nearly a century passed before Gerard Kuiper found Miranda in 1948. And that was it until a NASA robot made it to distant Uranus.

Astronomers using the Hubble Space Telescope and improved ground-based telescopes have raised the total to 27 known moons. Spotting the post-Voyager moons is an impressive feat. They're tiny - as little as 12-16 km (8-10 miles) across, and blacker than asphalt. And of course, they're nearly 3 billion miles away.

All of Uranus's inner moons (those observed by Voyager 2) appear to be roughly half water ice and half rock. The composition of the moons outside the orbit of Oberon remains unknown, but they are likely captured asteroids.