海卫八

普罗透斯
	航海家2号于1989年拍摄的海卫八
发现
发现者	航海家2号 ; 斯蒂芬·辛诺特（日语：Stephen P. Synnott）
发现日期	1989年6月16日
编号
命名依据	Πρωτεύς Prōteys
其它名称	S/1989 N 1
形容词	普罗透斯 (/ˈproʊtiːən/ or /proʊˈtiːən/)
轨道参数
	历元 18 August 1989
近心点	117584±10 km
远心点	117709±10 km
半长轴	117647±1 km (4.75 RN)
离心率	0.00053±0.00009
轨道周期	1.12231477±0.00000002 d
平均轨道速度	7.623 km/s
轨道倾角	0.524° 相对海王星赤道) ; 0.026°±0.007° (到拉普拉斯变换)
隶属天体	海王星
物理特征
大小	424 × 390 × 396 km
平均半径	210±7 km
表面积	554,200 km2
体积	(3.4±0.4)×107 km3
质量	4.4×1019 kg (7.3672×10−6 Earths)
平均密度	≈ 1.3 g/cm3 (estimate)
表面重力	0.07 m/s2
逃逸速度	0.17 km/s
自转周期	潮汐锁定
转轴倾角	zero
反照率	0.096
温度	平均 ≈ 51 K （估计）
视星等	19.7

海卫八（S/1989 N 1，Proteus）是环绕海王星运行的第二大的卫星。它的大小已经接近不规则星体的极限，也是海王星的最大内卫星。其英文名字“Proteus”（普罗透斯）源于希腊神话中会变形的海神。

有一颗小行星叫作小行星9313.

Quick Facts 发现, 发现者 ...

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发现

海卫八是由旅行者2号在1989年飞经海王星所摄取的图像而发现的。它当时被暂命名为S/1989 N 1。和于1989年7月7日的IAUC 4806会议上宣布了海卫八的发现。根据公布说明的“21天中获取的17幅图像”来推算，海卫八的发现日期是6月16日略前。海卫八在1991年9月16日被正式命名。^[9]

表面特征

海卫八的直径超过400千米，比海卫二大。但是因为海卫八非常靠近行星，容易被其反射太阳光掩盖，所以它未能被地基的望远镜所发现。

海卫八是已知太阳系内最暗的天体之一，它像煤烟那么黑。和土卫九相似，它们都只反射被照射的太阳光的6%。海卫八的陨石坑众多，陨石坑也没有被地质活动改变的迹象；目前只有一个最大的撞击坑被命名，该撞击坑内还包含着其他较小的撞击坑。海卫八不规则的形状让科学家们相信它已经到了类似密度天体在尚未被自身引力拉至球状的最大极限。作为对比，土卫一拥有更小的质量，但形状远较海卫八规则。

撞击坑

海卫八上的撞击坑，以埃及港口的地名命名。

More information 名称, 直径（公里） ...

名称	直径（公里）	名字来源
法罗斯撞击坑	230-260	法罗斯岛，埃及港口亚历山大附近的离岛。

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图片

哈伯太空望远镜拍摄的海王星、海卫八和海卫一图像
航海者2号拍摄的海卫八彩色图像
航海者2号拍摄的海卫八图像与法罗斯撞击坑
航海者2号拍摄的海卫八图像，经过数位处理以显示表面特征。

参考资料

注解

[a]
In the earlier papers slightly different dimensions were reported. Thomas and Veverka in 1991 reported 440×416×404 km.^[3]^[4] Croft in 1992 reported 430×424×410 km.^[5] The difference is caused by the use of different sets of images and by the fact that the shape of Proteus is not described well by a triaxial ellipsoid.^[3]
[b]
The mass was calculated by multiplying the volume from Stooke (1994)^[3] by the assumed density of 1,300 kg/m³. If one uses slightly larger dimensions from the earlier papers the mass will increase to 5×10¹⁹ kg.^[6]
[c]
Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r².
[d]
Escape velocity derived from the mass m, the gravitational constant G and the radius r:2Gm/r

来源

[1]
Protean. 牛津英语词典 (第三版). 牛津大学出版社. 2005-09 （英语）.
[2]
Jacobson, R. A.; Owen, W. M., Jr. The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations. Astronomical Journal. 2004, 128 (3): 1412–1417. Bibcode:2004AJ....128.1412J. doi:10.1086/423037 .
[3]
Stooke, Philip J. The surfaces of Larissa and Proteus. Earth, Moon, and Planets. 1994, 65 (1): 31–54. Bibcode:1994EM&P...65...31S. S2CID 121825800. doi:10.1007/BF00572198.
[4]
Williams, Dr. David R. Neptunian Satellite Fact Sheet. NASA (National Space Science Data Center). 2008-01-22 [2008-12-12]. （原始内容存档于2008-02-01）.
[5]
Croft, S. Proteus: Geology, shape, and catastrophic destruction. Icarus. 1992, 99 (2): 402–408. Bibcode:1992Icar...99..402C. doi:10.1016/0019-1035(92)90156-2.
[6]
Planetary Satellite Physical Parameters. JPL (Solar System Dynamics). 2010-10-18 [2011-10-11]. （原始内容存档于2013-11-01）.
[7]
Proteus By The Numbers. solarsystem.nasa.gov/. [September 4, 2020]. （原始内容存档于2022-03-16）.
[8]
Karkoschka, Erich. Sizes, shapes, and albedos of the inner satellites of Neptune. Icarus. 2003, 162 (2): 400–407. Bibcode:2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.
[9]
IAU Circular No. 5347. September 16, 1991 [2007-04-10]. （原始内容存档于2019-09-11）.

外部链接

Proteus In Depth at NASA's Solar System Exploration site
Proteus page （页面存档备份，存于互联网档案馆） at The Nine Planets
Proteus, A Moon Of Neptune （页面存档备份，存于互联网档案馆） on Views of the Solar System
Ted Stryk's Proteus Page
Neptune's Known Satellites (by Scott S. Sheppard)

[earlier_papers-6] [a]
In the earlier papers slightly different dimensions were reported. Thomas and Veverka in 1991 reported 440×416×404 km.^[3]^[4] Croft in 1992 reported 430×424×410 km.^[5] The difference is caused by the use of different sets of images and by the fact that the shape of Proteus is not described well by a triaxial ellipsoid.^[3]

[mass_calc-9] [b]
The mass was calculated by multiplying the volume from Stooke (1994)^[3] by the assumed density of 1,300 kg/m³. If one uses slightly larger dimensions from the earlier papers the mass will increase to 5×10¹⁹ kg.^[6]

[10] [c]
Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r².

[11] [d]
Escape velocity derived from the mass m, the gravitational constant G and the radius r:2Gm/r

[1] [1]
Protean. 牛津英语词典 (第三版). 牛津大学出版社. 2005-09 （英语）.

[Jacobson2004-2] [2]
Jacobson, R. A.; Owen, W. M., Jr. The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations. Astronomical Journal. 2004, 128 (3): 1412–1417. Bibcode:2004AJ....128.1412J. doi:10.1086/423037 .

[Stooke1994-3] [3]
Stooke, Philip J. The surfaces of Larissa and Proteus. Earth, Moon, and Planets. 1994, 65 (1): 31–54. Bibcode:1994EM&P...65...31S. S2CID 121825800. doi:10.1007/BF00572198.

[Williams2008-nssdc-4] [4]
Williams, Dr. David R. Neptunian Satellite Fact Sheet. NASA (National Space Science Data Center). 2008-01-22 [2008-12-12]. （原始内容存档于2008-02-01）.

[Croft1992-5] [5]
Croft, S. Proteus: Geology, shape, and catastrophic destruction. Icarus. 1992, 99 (2): 402–408. Bibcode:1992Icar...99..402C. doi:10.1016/0019-1035(92)90156-2.

[jplssd-7] [6]
Planetary Satellite Physical Parameters. JPL (Solar System Dynamics). 2010-10-18 [2011-10-11]. （原始内容存档于2013-11-01）.

[NASA_BTN-8] [7]
Proteus By The Numbers. solarsystem.nasa.gov/. [September 4, 2020]. （原始内容存档于2022-03-16）.

[Karkoschka2003-12] [8]
Karkoschka, Erich. Sizes, shapes, and albedos of the inner satellites of Neptune. Icarus. 2003, 162 (2): 400–407. Bibcode:2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.

[13] [9]
IAU Circular No. 5347. September 16, 1991 [2007-04-10]. （原始内容存档于2019-09-11）.

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