Uranus The seventh planet in the solar system, discovered in 1781 by Sir William Herschel, although he described it as a comet. It was named Uranus by J. E. Bode. Its equatorial radius is 25 559 km and polar radius 24973 km; volume 6833 km³; mass 86.83 × 10²⁴ kg; mean density 1318 kg/m³; visual albedo 0.51; black-body temperature 35.9 K. The inclination of the equator to the plane of the ecliptic is 97.86°, so the planet is lying on its side (a fact discovered in 1846 by J. Galle). At its closest approach, Uranus is 2581.9 × 10⁶ km from Earth and at its furthest 3157.3 × 10⁶ km. Uranus has an atmosphere, with a surface atmospheric pressure well in excess of 100 bar. The atmosphere is composed of molecular hydrogen (89%) and helium (11%), with aerosols of methane, ammonia ice, water ice, ammonia hydrosulphide, and possibly methane ice (similar to that of Neptune). Wind speeds at the surface are 0-200 m/s and the average surface temperature is about 58 K. Two new satellites, so far unnamed, were discovered in 1997 (and designated S/1997U1 and S/1997U2), moving in eccentric orbits at a mean distance of 5.8 × 10⁶ km from the planet (227 Uranian radii), and are estimated to have diameters of 60 km and 80 km, bringing the total number of known satellites to 17 (see URANIAN SATELLITES) and it is likely that more remain to be discovered. Except for Titan, the uranian satellites are denser than those of Jupiter. Oberon and Titania, the two largest, were discovered in 1787 by Sir William Herschel. Umbriel and Ariel were discovered in 1851 by William Lassell. Miranda was discovered in 1948 by Gerard Kuiper. Ariel, Oberon, and Titania are probably made of water ice, other ices, and silicates. They are believed to be too cold to have a molten core, but on some there are signs of geological activity. The remaining 10 satellites were revealed in images transmitted to Earth from Voyager 2.