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| density = ≈ 1.3 g/cm³ (estimate)〔 | surface_grav = m/s2〔Surface gravity derived from the mass ''m'', the gravitational constant ''G'' and the radius ''r'': ''Gm/r''2.〕 | escape_velocity = km/s〔Escape velocity derived from the mass ''m'', the gravitational constant ''G'' and the radius ''r'': .〕 | rotation = synchronous〔 | axial_tilt = zero〔 | albedo = 0.096〔〔 | magnitude = 19.7〔 | single_temperature = ≈ 51 K mean (estimate) | note = no }} Proteus (;〔In US dictionary transcription, .〕 Greek: ''Πρωτεύς''), also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by ''Voyager 2'' spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus orbits Neptune in a nearly equatorial orbit at the distance of about 4.75 equatorial radii of the planet. Despite being a predominantly icy body more than 400 km in diameter, Proteus's shape deviates significantly from an ellipsoid. It is shaped more like an irregular polyhedron with several slightly concave facets and relief as high as 20 km. Its surface is dark, neutral in color and heavily cratered. Proteus's largest crater is Pharos, which is more than 230 km in diameter. There are also a number of scarps, grooves, and valleys related to large craters. Proteus is probably not an original body that formed with Neptune; it may have accreted later from the debris created when the largest Neptunian satellite Triton was captured. == Discovery and orbit == Proteus was discovered from the images taken by ''Voyager 2'' space probe two months before its Neptune flyby in August 1989. It received the temporary designation S/1989 N 1.〔 Stephen P. Synnott and Bradford A. Smith announced its discovery on July 7, 1989, speaking only of "17 frames taken over 21 days", which gives a discovery date of sometime before June 16.〔 On 16 September 1991 S/1989 N 1 was named after Proteus, the shape-changing sea god of Greek mythology.〔 Proteus orbits Neptune at the distance approximately equal to 4.75 equatorial radii of the planet. Its orbit has a small eccentricity and is inclined by about 0.5° to the planet's equator.〔 Proteus is the largest of the regular prograde satellites of Neptune. It rotates synchronously with the orbital motion, which means that one face always points to the planet.〔 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「Proteus (moon)」の詳細全文を読む スポンサード リンク
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