萬物簡史 第27期:走進太陽系(5)

　　As for Pluto itself, nobody is quite sure how big it is,or what it is made of, what kind of atmosphere ithas, or even what it really is. A lot of astronomersbelieve it isn't a planet at all, but merely thelargest object so far found in a zone of galactic debrisknown as the Kuiper belt.

　　至于冥王星本身，誰也不大清楚它有多大，是什么組成的，有什么樣的大氣，甚至它到底是個什么東西。許多天文學(xué)家認為，它其實算不上是顆行星，而只是我們在銀河的廢墟帶(稱之為凱珀帶)發(fā)現(xiàn)的最大的物體。

　　The Kuiper belt was actually theorized by an astronomer named F. C. Leonard in 1930, butthe name honors Gerard Kuiper, a Dutch native working in America, who expanded the idea.The Kuiper belt is the source of what are known as short-period comets—those that come pastpretty regularly—of which the most famous is Halley's comet. The more reclusive long-periodcomets (among them the recent visitors Hale-Bopp and Hyakutake) come from the much moredistant Oort cloud, about which more presently.

　　凱珀帶理論實際上是1930年由一位名叫F.G.倫納德的天文學(xué)家提出來的，他用這個名字來紀念一位在美國工作的荷蘭人杰勒德·凱珀。凱珀發(fā)展了這個理論。凱珀帶是所謂短命彗星的源泉--那種經(jīng)常一閃而過的星星--其中最著名的就是哈雷彗星。比較長命的彗星(其中有最近光顧的海爾-博普彗星和百武彗星)產(chǎn)生于遙遠得多的奧爾特云，我們過一會兒就會談到這個問題。

　　It is certainly true that Pluto doesn't act much like the other planets. Not only is it runty andobscure, but it is so variable in its motions that no one can tell you exactly where Pluto will bea century hence. Whereas the other planets orbit on more or less the same plane, Pluto'sorbital path is tipped (as it were) out of alignment at an angle of seventeen degrees, like thebrim of a hat tilted rakishly on someone's head.

　　冥王星的表現(xiàn)與別的行星很不一樣，這種看法肯定沒錯兒。它不但又小又模糊，而且它的運行方式變化不定，一個世紀以后誰也說不準冥王星到底會在哪里。別的行星多少在同一平面上轉(zhuǎn)動，而冥王星的運行軌道(似乎)是傾斜的，不和別的行星處于同一平面，而是形成一個17度的角，猶如有人頭上瀟灑地歪戴著帽子。

　　Its orbit is so irregular that for substantial periods on each of its lonely circuits around theSun it is closer to us than Neptune is. For most of the 1980s and 1990s, Neptune was in factthe solar system's most far-flung planet. Only on February 11, 1999, did Pluto return to theoutside lane, there to remain for the next 228 years.

　　它的軌道很不規(guī)則，在它寂寞地繞太陽轉(zhuǎn)動的過程中，每一圈都在相當(dāng)長的時間里比海王星距離我們更近。事實上，在20世紀80年代和90年代的大部分時間里，海王星是太陽系里離我們最遠的行星。只是到了1999年2月11日，冥王星才回到外側(cè)的軌道，此后它將在那里停留228年的時間。