UMBRIEL

one of the major satellites of the planet Uranus. It orbits the planet at a distance of 266,300 km (165,110 miles). Umbriel has an orbital period of 4.144 days. Its discovery is usually attributed to William Lassell in 1851, but William Herschel, who discovered Uranus and two other satellites, probably detected it more than 60 years earlier, in 1787, or possibly in 1798. Umbriel's diameter is estimated at 1,110 km (690 miles), and its mass is about 1.2 10-5 that of Uranus. Astronomers believe that the satellite is composed largely of water ice intermixed with small amounts of frozen methane and rocky material. Umbriel is distinct from the other major satellites of Uranus in that it shows no evidence of tectonic activity. However, like the Moon's, its surface is covered with impact craters, most measuring from about 100 to 200 km (60 to 125 miles) across.