User:Franklsf95/Sandbox/小行星951
 图片来自NASA，颜色经过处理 | |
| 发现 | |
|---|---|
| 發現者 | G. N. Neujmin |
| 發現日期 | 1916年6月30日 |
| 編號 | |
| 其它名稱 | SIGMA 45; A913 YA; 1955 MG1 |
| 小行星分類 | 小行星主带（花神星族） |
| 軌道參數 | |
| 曆元 2006年3月6日（JD 2453800.5） | |
| 遠日點 | 388.102 Gm (2.594 AU) |
| 近日點 | 272.985 Gm (1.825 AU) |
| 半長軸 | 330.544 Gm (2.210 AU) |
| 離心率 | 0.174 |
| 軌道週期 | 1199.647 d (3.28 a) |
| 平均軌道速度 | 19.88 km/s |
| 平近點角 | 53.057° |
| 軌道傾角 | 4.102° |
| 升交點黃經 | 253.218° |
| 近日點參數 | 129.532° |
| 物理特徵 | |
| 大小 | 18.2×10.5×8.9 km [1] |
| 質量 | 2–3×1016 kg (estimate) |
| 平均密度 | ~2.7 g/cm³ (estimate) [2] |
| 表面重力 | ~0.002 m/s² (estimate) |
| ~0.006 km/s (estimate) | |
| 自轉週期 | 0.293 d (7.042 h) [3] |
| 反照率 | 0.22 [4] |
| 溫度 | ~181 K max: 281 K (+8°C) |
| 光譜類型 | S |
| 絕對星等(H) | 11.46 |
小行星951（951 Gaspra，发音为/ˈɡæsprə/）是一颗非常接近小行星带内层边缘的S-型小行星。它的名字是发现者根据克里米亚半岛上的一个旅游胜地所起[5]。小行星951是第一颗人类飞行器所探访过的小行星。1991年10月29日，伽利略号在前往木星的途中掠过了该颗小行星。
特征
除了大量的小型环形山，小行星951上还存在着6个以上的大面积平原地区和凹面地区。其中的一片平原（Dunne Regio）的范围达到5×7km，海拔起伏不超过200m。目前无法确定这些是撞击的结果还是它在与其母星分离时的琢面。不平衡的重力作用会很容易地使撞击坑呈现出平坦的模样，lopsided shapes, making this determination difficult. The flat facets and concavities give Gaspra a very angular appearance.
Gaspra appears to be fairly olivine-rich among the S-type asteroids (the surface appears to contain olivine and pyroxene in the proportions 4:1 to 7:1 [6]). There are no prominent albedo or color patterns, although a subtle color variation is seen across the surface (see image above).
Gaspra's surface lacks unambiguous craters of a size comparable to its radius, like those seen e.g. on 253 Mathilde. A probable reason is that the collision that produced the Flora family and Gaspra was relatively recent on an astronomical timescale, so that Gaspra has not yet had the opportunity to acquire many large craters since. Analysis of cratering rates suggests the age of the surface is between about 20 to 300 million years [7].
Grooves about 100-300m wide, up to 2.5 km long, and tens of meters deep are seen on Gaspra's surface, which may be related to Gaspra's formation along with the rest of the Flora family in an asteroid collision. Their presence also suggest that it is a single coherent body, rather than a rubble pile. The grooves were likely created by impacts that shattered the underlying rock. A system of much more prominent grooves is seen on the Martian moon Phobos. The pitted appearance of some grooves may suggest that the surface is covered by a regolith [7].
The extensiveness of regolith on Gaspra and its presence overall is a matter of debate, and not fully understood. Visually, the somewhat subdued and mantled appearance suggests a substantial regolith. Also, correlations are seen between the subtle color variations and local topography, and it has been suggested that this is caused by the slow migration of regolith to lower areas. It is, however, difficult to explain the origin of a putative regolith. Firstly, Gaspra's escape velocity is very small, so small that it is difficult to understand how it could keep a significant portion of fragments ejected by impacts from escaping. This may be alleviated if Gaspra is a porous body or started with a large regolith, but one has to explain how the original regolith appeared. A possible resolution of the issue may be that Gaspra obtained a regolith during the Flora-family forming impact that also created Gaspra itself. Secondly, it has been estimated that the matter ejected by all the craters would be only enough to cover Gaspra with 10 m of regolith. However, some craters are much deeper than this without showing any structural difference on their walls [8].
Gaspra's pole has been determined to point in the direction of RA 0h40m±10m, Declination 27±2° [1]. This is equivalent to ecliptic coordinates (β, λ) = (21°, 20°) and gives an axial tilt of 72°.
The Galileo flyby was too distant for a body of Gaspra's small size to noticeably affect Galileo's trajectory, so no information on Gaspra's mass was obtained. (Galileo also visited 243 Ida where it discovered a moon, allowing a mass estimate there.)
Gaspra's surface area has been calculated at about 525 km² [1], which, for comparison, is about half the land area of Hong Kong.
Exploration
Gaspra was discovered by Russian astronomer G. N. Neujmin in 1916. Neujmin named it after Gaspra, a Black Sea retreat that was visited by his contemporaries, such as Gorky and Tolstoy.
Galileo flew by Gaspra on 29 October, 1991, passing within 1,600 kilometers at a relative speed of about 8 kilometers per second (18,000 mph). 57 images were returned to Earth, the closest taken from a distance of 5300 km. The best images have a resolution of about 54 meters/pixel. The area around the southern pole was not seen during the flyby, but the remaining 80% of the asteroid was imaged [7].
Because Gaspra's position was only known to within about 200 km before the encounter, and the camera's field of view was only about 5° across, Galileo would not know where to point to capture images of the asteroid once it was closer than 70,000 km. This would render the encounter not very interesting scientifically. To overcome this problem, a pioneering optical navigation campaign was implemented by the Galileo spacecraft team to reduce the uncertainty of Gaspra's position using images captured during the approach to Gaspra. This was spectacularly successful and allowed the spacecraft to obtain images from as close as 5300 km. At this closest range, the pointing was still not known quite accurately enough, but the camera actually took a 51 image mosaic so as to capture Gaspra on at least one image [7]. Similar optical navigation techniques have been used on all spacecraft flybys of asteroids since.
参考文献
- ^ 1.0 1.1 1.2 （英文）P. C. Thomas, J. Veverka, D. Simonelli, P. Helfenstein, B. Carcich, M. J. S. Belton, M. E. Davies, C. Chapman. The Shape of Gaspra. Icarus. 1994, 107 (1): 23–36. doi:10.1006/icar.1994.1004.
- ^ Krasinsky, G. A.; Pitjeva, E. V.; Vasilyev, M. V.; Yagudina, E. I. Hidden Mass in the Asteroid Belt. Icarus. 2002, 158 (1): 98–105. doi:10.1006/icar.2002.6837. 已忽略未知参数
|month=（建议使用|date=） (帮助); - ^ （英文）PDS lightcurve data
- ^ （英文）Supplemental IRAS Minor Planet Survey
- ^ （英文）Gaspra Rotation Sequence. Observatorio ARVAL. [2009-8-9].
- ^ J.C. Granahan F.P. Fanale, & M.S. Robinson. A Galileo Multi Spectral Instrument Analysis of 951 Gaspra. Abstracts of the 25th Lunar and Planetary Science Conference, held in Houston, TX, 14-18 March 1994. 1994: 453.
- ^ 7.0 7.1 7.2 7.3 Veverka, J.; Belton, M.; Klaasen, K.; Chapman, C. Galileo's Encounter with 951 Gaspra: Overview. Icarus. 1994, 107 (1): 2–17. doi:10.1006/icar.1994.1002.
- ^ M.J.S. Belton; et al. Galileo Encounter with 951 Gaspra: First Pictures of an Asteroid (abstract). Science. 1992, 257: 1647. doi:10.1126/science.257.5077.1647.
外部链接
- short intro: 2 photos
- JPL images
| 前一小行星： 950 Ahrensa |
小行星列表 | 後一小行星： 952 Caia |