Pluto資料！！十萬火急！

冥王星是太陽系天體。它於1930年1月被發現，並以羅馬神話中的冥王普魯托（Pluto）命名，中文意譯為冥王星。發現後長期以來一直與地球、土星等並列為九大行星。但是在2006年8月24日於布拉格舉行的第26屆國際天文聯會中通過第五號號決議，將冥王星劃為矮行星（dwarf planet）在行星之列中除名，並賦與成為小行星134340號。


發現

藝術家想像的冥王星和其「衛星」查龍。1930年被發現後冥王星一直被當作行星，至2006年才被歸類為矮行星。1894年，美國亞利桑那州的天文學家帕西瓦爾·羅威爾建造了以其名字命名的羅威爾天文臺。他試圖在那處搜尋一顆可能存在的新行星，一顆攝動海王星公轉的天體，稱之為「行星X」。羅威爾計算出了那顆行星的所在位置，然而在他有生之年卻未能找到這顆行星。

1916年羅威爾去世後，天文學家克萊德·湯博繼續在羅威爾天文臺進行搜尋，把在同一天空、不同時間拍攝的照片底片，在背後燈光的照射下輪流先後顯示，就會看到所有的恆星都沒有變動，只有被拍攝到的行星會有位置變化，這樣就能發現行星和小行星。

1930年1月18日與23日，湯博在雙子座拍攝兩張照片，在這兩張照片上發現一個移動的小點，從而發現冥王星。他在同年3月13日公開這項發現。

之前多次搜索冥王星失敗的原因是它比人們預計的要暗弱得多。在1919年，天文學家休姆遜曾以攝影方法紀錄到冥王星，但其中一張照片中冥王星的像在污點上，在另一張相片中冥王星則靠在明亮的恆星附近，結果沒有被發現。


[編輯] 漫長的公轉
冥王星是離太陽的平均距離約為59億公里，是地球與太陽平均距離的40倍。而且，它環繞太陽運行的速度只有地球的六分之一，因而要花上248個地球年才能圍繞行太陽一圈。冥王星於1989年9月5日通過近日點（下次為2237年9月16日）、並將於2114年2月19日過遠日點（上次為1866年6月6日）。

冥王星的軌道是一個非常扁的橢圓，在遠日點約有74億公里；近日點也有44億公里。另外，軌道偏心率較大，使冥王星有時比海王星離太陽還要略近一些（例如在1989年～1999年2月9日），但冥王星不會因軌道與海王星相交而與之碰撞（近日點時冥王星在海王星軌道以北公轉，兩顆行星之間的距離在3.78億公里以上）。


[編輯] 小小世界
1988年6月9日，冥王星剛好運行到一顆恆星的前面，根據恆星被遮掩的時間，天文學家們測定冥王星直徑約2344千米，比月球還要小，其質量也只有月球的五分之一。所以冥王星是個小小的世界。

冥王星離太陽極其遙遠，因而在冥王星上看到的太陽，也只是一顆普通的恆星而已。即使是最靠近太陽的時候，它所獲得的太陽光也只有地球的九百分之一，所以冥王星十分寒冷（從－212℃到－234℃）。

行星地位的爭論
冥王星由於尺度小（比其他八大行星小得多）、軌道扁長，許多人對它能不能算一顆真正的行星表示質疑：

其它的一些天體，例如小行星2060（喀戎）的軌道與冥王星十分相似。
太陽系中的一些行星還有著7個比冥王星更大的衛星。
在海王星外有一沿軌道運行的天體帶——柯伊伯帶。許多天文學家認為，冥王星就是這一軌道帶上最大的天體之一，並相信海王星是最後一顆「真正的」行星。
有說冥王星擁有衛星—冥衛一（查龍），因此它該作行星論，但天文學家及後相繼發現小行星243（愛達）等部份小行星同樣皆有衛星，所以擁有衛星被認為不再是判定行星的標準。

1998年曾有建議把冥王星剔除太陽系行星之列，但當年國際天文聯盟（IAU）否決。2006年8月24日下午，在第26屆國際天文聯會通過第決議，由天文學家以投票正式將冥王星劃為矮行星，自行星之列中除名。

2006年9月7日，國際小行星中心把已知或即將成為矮行星的天體賦與編號，冥王星現編號為小行星134340號。

衛星
冥王星現有三顆已知的天然衛星。

冥衛一名為Charon（開隆），於1978年被發現。據天文學家計算所得，它與冥王星形成了一個雙行星系統：它們的質心在冥王星表面以外，因此冥衛一並非圍繞冥王星旋轉。

2005年5月哈伯太空望遠鏡發現S/2005 P1及S/2005 P2兩顆冥王星的新衛星，並於翌年6月底的國際天文學合會會議上命名為Nix（尼克斯）與Hydra（許德拉）。
詳情 : http://zh.wikipedia.org/w/index.php?title=%E5%86%A5%E7%8E%8B%E6%98%9F&variant=zh-tw
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Pluto (IPA: /ˈpluːtəʊ/), designated (134340) Pluto in the Minor Planet Center catalogue, is the second-largest known dwarf planet in the solar system and the tenth largest body orbiting the Sun. It orbits between 29 and 49 AU from the Sun, and was the first Kuiper Belt object to be discovered. Approximately one-fifth the mass of the Earth's Moon, Pluto is primarily composed of rock and ice. It has an eccentric orbit that is highly inclined with respect to the planets and takes it closer to the Sun than Neptune during a portion of its orbit. Pluto and its largest satellite, Charon, could be considered a binary system because they are closer in size than any of the other known celestial pair combinations in the solar system, and because the barycentre of their orbits does not lie within either body. However, the International Astronomical Union (IAU) has yet to formalize a definition for binary dwarf planets, so Charon is currently regarded as a moon of Pluto. Two smaller moons, Nix and Hydra, were discovered in 2005. Pluto is smaller than several of the natural satellites or moons in our solar system (see the list of solar system objects by radius).

From its discovery by Clyde Tombaugh in 1930, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st century, many similar objects were discovered in the outer solar system, most notably the trans-Neptunian object Eris which is slightly larger than Pluto. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which was then reclassified under the new category of dwarf planet along with Eris and Ceres.[1] Pluto is also classified as the prototype of a family of trans-Neptunian objects.[2][3] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[4][5]

詳情 : http://en.wikipedia.org/wiki/Pluto

Pluto was discovered in 1930
Pluto's rotation period is 6.387 days, the same as its satellite Charon. Although it is common for a satellite to travel in a synchronous orbit with its planet, Pluto is the only planet to rotate synchronously with the orbit of its satellite. Thus being tidally locked, Pluto and Charon continuously face each other as they travel through space.
Unlike most planets, but similar to Uranus, Pluto rotates with its poles almost in its orbital plane. Pluto's rotational axis is tipped 122 degrees. When Pluto was first discovered, its relatively bright south polar region was the view seen from the Earth. Pluto appeared to grow dim as our viewpoint gradually shifted from nearly pole-on in 1954 to nearly equator-on in 1973. Pluto's equator is now the view seen from Earth.
During the period from 1985 through 1990, Earth was aligned with the orbit of Charon around Pluto such that an eclipse could be observed every Pluto day. This provided opportunity to collect significant data which led to albedo maps defining surface reflectivity, and to the first accurate determination of the sizes of Pluto and Charon, including all the numbers that could be calculated therefrom.
The first eclipses (mutual events) began blocking the north polar region. Later eclipses blocked the equatorial region, and final eclipses blocked Pluto's south polar region. By carefully measuring the brightness over time, it was possible to determine surface features. It was found that Pluto has a highly reflective south polar cap, a dimmer north polar cap, and both bright and dark features in the equatorial region. Pluto's geometric albedo is 0.49 to 0.66, which is much brighter than Charon. Charon's albedo ranges from 0.36 to 0.39.
The eclipses lasted as much as four hours and by carefully timing their beginning and ending, measurements for their diameters were taken. The diameters can also be measured directly to within about 1 percent by more recent images provided by the Hubble Space Telescope. These images resolve the objects to clearly show two separate disks. The improved optics allow us to measure Pluto's diameter as 2,274 kilometers (1413 miles) and Charon's diameter as 1,172 kilometers (728 miles), just over half the size of Pluto. Their average separation is 19,640 km (12,200 miles). That's roughly eight Pluto diameters.
Average separation and orbital period are used to calculate Pluto and Charon's masses. Pluto's mass is about 6.4 x 10-9 solar masses. This is close to 7 (was 12 x's) times the mass of Charon and approximately 0.0021 Earth mass, or a fifth of our moon.
Pluto's average density lies between 1.8 and 2.1 grams per cubic centimeter. It is concluded that Pluto is 50% to 75% rock mixed with ices. Charon's density is 1.2 to 1.3 g/cm3, indicating it contains little rock. The differences in density tell us that Pluto and Charon formed independently, although Charon's numbers derived from HST data are still being challenged by ground based observations. Pluto and Charon's origin remains in the realm of theory.
Pluto's icy surface is 98% nitrogen (N2). Methane (CH4) and traces of carbon monoxide (CO) are also present. The solid methane indicates that Pluto is colder than 70 Kelvin. Pluto's temperature varies widely during the course of its orbit since Pluto can be as close to the sun as 30 AU and as far away as 50 AU. There is a thin atmosphere that freezes and falls to the surface as the planet moves away from the Sun. NASA plans to launch a spacecraft, the Pluto Express, in 2001 that will allow scientists to study the planet before its atmosphere freezes. The atmospheric pressure deduced for Pluto's surface is 1/100,000 that of Earth's surface pressure.