Hyperion 土卫七(中英)

Hyperion is the sixteenth of Saturn's known satellites:
　　orbit:    1,481,100 km from Saturn
　　diameter: 286 km (410 x 260 x 220)
　　mass:    1.77e19 kg
　　In Greek mythology Hyperion was a Titan, the son of Gaea and Uranus and the father of Helios.
　　Discovered by Bond and Lassell in 1848.
　　Hyperion is the largest highly irregular (non-spherical) body in the solar system. Proteus is quite a bit larger but is almost spherical. It seems likely that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past.
　　Like most of Saturn's moons, Hyperion's low density indicates that it is composed of water ice with only a small amount of rock.
　　But unlike most of Saturn's moons, Hyperion has a low albedo (.2 - .3) indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus.
　　The Voyager images and subsequent ground based photometry indicate that Hyperion's rotation is chaotic, i.e. its axis of rotation wobbles so much that its orientation in space is completely unpredictable. There is only one other known body in the solar system (asteroid 4179 Toutatis) that rotates chaotically but simulations seem to indicate that other irregular satellites may have done so in the past. Hyperion is unique in that it is very irregularly shaped, has a highly eccentric orbit, and is near another large moon (Titan). These factors combine to restrict the set of conditions under which stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make chaotic rotation more likely.
　　Hyperion's odd rotation probably accounts for the fact that Hyperion's surface is more or less uniform, in contrast to many of Saturn's other moons which have distinctly different leading and trailing hemispheres.
　　Hyperion 是土星已知卫星中距土星第十六近的一颗：
　　公转轨道：距土星 1,481,100 千米
　　卫星直径：286 千米 (410 x 260 x 220)
　　质量：1.77e19 千克
　　在希腊神话中，Hyperion是一个巨人，该亚与乌拉诺斯的儿子，Helios的父亲。
　　它由波德和Lassell于1848年发现。
　　土卫七是太阳系中最大的一颗高度不规则（非球形）天体。海卫八稍大一些，但差不多是个球体。看起来土卫七像是一颗大星体的碎片，大星体因在远古时期的碰撞而碎裂。
　　就像土星的其他卫星，土卫七的低密度表明它由少量的岩石混合着冰水组成。
　　不像土星的其他卫星的是，土卫七的反照率较小(.2 - .3)，表明它至少覆盖着一层薄薄的暗色物质。这可能是来自远于土卫八的更暗的土卫九上的物质。
　　旅行者号带来的图片与基于地的光度测定表明土卫七的自转混乱，比如它的自转轴不停摇晃，在空间的方向无法确定。土卫七是太阳系已知星体中唯一一颗自转混乱的星体，不过其他星体可能在远古时期也这样。土卫七在其他不规则外形的星体中是独一无二的，公转偏心率较大，与另一颗大卫星土卫六很近。这些因素集合在一起，限制了它稳定自转的条件。土卫六与七的公转共振比为3:4，使它自转混乱更有可能。
　　土卫七的奇怪自转可能起因于外形的不规则，与那些具有明显正半球与后半球的土星卫星形成对比。