使用者:Franklsf95/Sandbox/小行星951

**951 Gaspra**
	圖片來自NASA，顏色經過處理
發現
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	小行星主帶（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
質量	2–3×1016 kg (estimate)
平均密度	~2.7 g/cm³ (estimate)
表面重力	~0.002 m/s² (estimate)
逃逸速度	~0.006 km/s (estimate)
自轉週期	0.293 d (7.042 h)
反照率	0.22
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，發音為/ˈɡæsprə/）是一顆非常接近小行星帶內層邊緣的S-型小行星。它的名字是發現者根據克里米亞半島上的一個旅遊勝地所起^[5]。小行星951是第一顆人類飛行器所探訪過的小行星。1991年 10月29日，伽利略號在前往木星的途中掠過了該顆小行星。

特徵[編輯]

除了大量的小型環形山，小行星951上還存在着6個以上的大面積平原地區和凹面地區。其中的一片平原（Dunne Regio）的範圍達到5×7km，海拔起伏不超過200m。目前無法確定這些是撞擊的結果還是它在與其母星分離時的琢面。不平衡的重力作用會很容易地使撞擊坑呈現出平坦的模樣，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[編輯]

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.

參考文獻[編輯]

^ ^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. 已忽略未知參數|month=（建議使用|date=） (幫助);
^ （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.

外部連結[編輯]

Franklsf95/Sandbox/小行星951 at the JPL Small-Body Database
- Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters

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

[Thomas94-1] 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-2] 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. 已忽略未知參數|month=（建議使用|date=） (幫助);

[PDS_lc-3] （英文）PDS lightcurve data

[IRAS-4] （英文）Supplemental IRAS Minor Planet Survey

[5] （英文）Gaspra Rotation Sequence. Observatorio ARVAL. [2009-8-9].

[Granahan94-6] 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.

[Veverka94-7] 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.

[Belton92-8] 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.

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