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The exploration of Uranus has, to date, been solely through telescopes and NASA's ''Voyager 2'' spacecraft, which made its closest approach to Uranus on January 24, 1986. ''Voyager 2'' discovered 10 moons, studied the planet's cold atmosphere, and examined its ring system, discovering two new rings. It also imaged Uranus' five large moons, revealing that their surfaces are covered with impact craters and canyons. A number of dedicated exploratory missions to Uranus have been proposed, but none have been approved.〔〔 ==''Voyager 2''== ''Voyager 2'' made its closest approach to Uranus on January 24, 1986, coming within of the planet's cloud tops. This was the probe's first solo planetary flyby, since ''Voyager 1'' ended its tour of the outer planets at Saturn's moon Titan. Uranus is the third-largest planet in the Solar System. It orbits the Sun at a distance of about 2.8 billion kilometers (1.7 billion miles) and completes one orbit every 84 years. The length of a day on Uranus as measured by ''Voyager 2'' is 17 hours and 14 minutes. Uranus is distinguished by the fact that it is tipped on its side. Its unusual position is thought to be the result of a collision with a planet-sized body early in the Solar System's history. Given its odd orientation, with its polar regions exposed to sunlight or darkness for long periods, scientists were not sure what to expect at Uranus. The presence of a magnetic field at Uranus was not known until ''Voyager's'' arrival. The intensity of the field is roughly comparable to that of Earth's, though it varies much more from point to point because of its large offset from the center of Uranus. The peculiar orientation of the magnetic field suggests that the field is generated at an intermediate depth in the interior where the pressure is high enough for water to become electrically conductive. ''Voyager 2'' found that one of the most striking influences of the sideways position of the planet is its effect on the tail of the magnetic field, which is itself tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet. Radiation belts at Uranus were found to be of an intensity similar to those at Saturn. The intensity of radiation within the belts is such that irradiation would quickly darken (within 100,000 years) any methane trapped in the icy surfaces of the inner moons and ring particles. This may have contributed to the darkened surfaces of the moons and ring particles, which are almost uniformly gray in color. A high layer of haze was detected around the sunlit pole, which also was found to radiate large amounts of ultraviolet light, a phenomenon dubbed "electroglow". The average temperature of the atmosphere of the planet is about 59 kelvins. Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the same temperature at the cloud tops. ''Voyager 2'' found 10 new moons, bringing the total number to 15 at the time. Most of the new moons are small, with the largest measuring about in diameter. The moon Miranda, innermost of the five large moons, was revealed to be one of the strangest bodies yet seen in the Solar System. Detailed images from ''Voyager's'' flyby of the moon showed huge oval structures termed ''coronae'' flanked by faults as deep as , terraced layers, and a mixture of old and young surfaces. One theory holds that Miranda may be a reaggregation of material from an earlier time when the moon was fractured by a violent impact. The five large moons appear to be ice-rock conglomerates like the satellites of Saturn. Titania is marked by huge fault systems and canyons indicating some degree of geologic, probably tectonic, activity in its history. Ariel has the brightest and possibly youngest surface of all the Uranian moons and also appears to have undergone geologic activity that led to many fault valleys and what seem to be extensive flows of icy material. Little geologic activity has occurred on Umbriel or Oberon, judging by their old and dark surfaces. All nine previously known rings were studied by the spacecraft and showed the Uranian rings to be distinctly different from those at Jupiter and Saturn. The ring system may be relatively young and did not form at the same time as Uranus. Particles that make up the rings may be remnants of a moon that was broken by a high-velocity impact or torn up by gravitational effects. ''Voyager 2'' also discovered two new rings. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Exploration of Uranus」の詳細全文を読む スポンサード リンク
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