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The rings of Neptune consist primarily of five principal rings and were first discovered (as "arcs") in 1984 in Chile by Patrice Bouchet, Reinhold Häfner and Jean Manfroid at La Silla Observatory (ESO) during an observing program proposed by André Brahic and Bruno Sicardy from Paris Observatory, and at Cerro Tololo Interamerican Observatory by F. Vilas and L.-R. Elicer for a program led by William Hubbard.〔〔 They were eventually imaged in 1989 by the ''Voyager 2'' spacecraft.〔 At their densest, they are comparable to the less dense portions of Saturn's main rings such as the C ring and the Cassini Division, but much of Neptune's ring system is quite tenuous, faint and dusty, more closely resembling the rings of Jupiter. Neptune's rings are named after astronomers who contributed important work on the planet:〔 Galle, Le Verrier, Lassell, Arago, and Adams.〔Listed in increasing distance from the planet〕〔 Neptune also has a faint unnamed ring coincident with the orbit of the moon Galatea. Three other moons orbit between the rings: Naiad, Thalassa and Despina.〔 The rings of Neptune are made of extremely dark material, likely organic compounds processed by radiation, similar to that found in the rings of Uranus.〔 The proportion of dust in the rings (between 20% and 70%) is high,〔 while their optical depth is low to moderate, at less than 0.1.〔 Uniquely, the Adams ring includes five distinct arcs, named Fraternité, Égalité 1 and 2, Liberté, and Courage. The arcs occupy a narrow range of orbital longitudes and are remarkably stable, having changed only slightly since their initial detection in 1980.〔 How the arcs are stabilized is still under debate. However, their stability is probably related to the resonant interaction between the Adams ring and its inner shepherd moon, Galatea.〔 == Discovery and observations == The first mention of rings around Neptune dates back to 1846 when William Lassell, the discoverer of Neptune's largest moon Triton, thought he had seen a ring around the planet.〔 However, his claim was never confirmed and it is likely that it was an observational artifact. The first reliable detection of a ring was made in 1968 by stellar occultation, although that result would go unnoticed until 1977 when the rings of Uranus were discovered.〔 Soon after the Uranus discovery, a team from Villanova University led by Harold J. Reitsema began searching for rings around Neptune. On 24 May 1981, they detected a dip in a star's brightness during one occultation; however, the manner in which the star dimmed did not suggest a ring. Later, after the Voyager fly-by, it was found that the occultation was due to the small Neptunian moon Larissa, a highly unusual event.〔 In the 1980s, significant occultations were much rarer for Neptune than for Uranus, which lay near the Milky Way at the time and was thus moving against a denser field of stars. Neptune's next occultation, on 12 September 1983, resulted in a possible detection of a ring.〔 However, ground-based results were inconclusive. Over the next six years, approximately 50 other occultations were observed with only about one-third of them yielding positive results.〔 Something (probably incomplete arcs) definitely existed around Neptune, but the features of the ring system remained a mystery.〔 The ''Voyager 2'' spacecraft made the definitive discovery of the Neptunian rings during its fly-by of Neptune in 1989, passing by as close as above the planet's atmosphere on 25 August. It confirmed that occasional occultation events observed before were indeed caused by the arcs within the Adams ring (see below).〔 After the Voyager fly-by the previous terrestrial occultation observations were reanalyzed yielding features of the ring's arcs as they were in 1980s, which matched those found by Voyager almost perfectly.〔 Since Voyager's fly-by, the brightest rings (Adams and Le Verrier) have been imaged with the Hubble Space Telescope and Earth-based telescopes, owing to advances in resolution and light-gathering power.〔 They are visible, slightly above background noise levels, at methane-absorbed wavelengths in which the glare from Neptune is significantly reduced. The fainter rings are still far below the visibility threshold.〔 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「Rings of Neptune」の詳細全文を読む スポンサード リンク
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