T he only spacecraft to encounter Uranus is Voyager 2, which collected data on the planet, its rings, and its satellites over a four-month period starting in late 1985. Voyager 2 passed within 107,000 km (66,500 miles) of the center of Uranus, which was about about 81,450 km (50,625 miles) above its cloud tops, in 1986.

M easurements by Voyager 2 verified that hydrogen is the main constituent of the visible atmosphere. Helium is the other major constituent, and makes up about 15% of the atmosphere, which is a much larger percentage than that of Jupiter and Saturn. Together hydrogen and helium make up more than 99 percent of Uranus's atmosphere, which extends to a depth of about 8,000 km (5,000 miles). A layer of methane ice clouds was also detected by Voyager 2.

P reviously little was known about Uranus's five major satellites: Miranda, Umbriel, Titania, Ariel, and Oberon. Ten small satellites were discovered in Voyager 2 photographs, all of them orbiting Uranus well inside the orbit of Miranda. One strange feature of the five major satellites, and probably the ten smaller ones as well, is that they appear to be in synchronous rotation.That means that the same side of the satellite faces the planet at all times. Oberon and Titania are remarkably similar in size, density color, and reflectivity. While Umbriel and Ariel also have a lot in common. Miranda is the innermost and smallest of the major moons. It has a very bizarre appearance that may be a frozen record of the late stages of its development.