宇宙時空 彗星和小行星有何區別?

彗星和小行星有何區別?

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【新三才編譯首發】彗星和小行星有很多共同之處:兩者都繞著太陽運轉,都比行星小得多,都是大約50億年前太陽系形成過程中的殘留物。

那麼,彗星和小行星之間有什麼區別呢?

彗星是什麼?

彗星被比喻作骯髒的雪球,這是一個貼切的描述。它們由大量的冰、灰塵、岩石鬆散地聚集著。彗星的核心通常只有幾英里寬,主要由水冰和冰凍的二氧化碳、一氧化碳、甲烷、氨所組成。

大多數彗星繞行太陽的軌道遠遠超過海王星,而且繞行一週需要好幾百年。有時彗星的軌道會使它接近太陽,此時,來自太陽的熱量會使彗星的核心從冰體(稱為「彗髮」,coma)形成可見的蒸汽雲,其其延伸長度可達數千萬英哩。

在某些情況下,彗星的尾巴很長,以至於肉眼就能看見。

地球的軌道週期性地穿過彗星的尾部。當尾部的碎片掉入地球的大氣層時,它會燃燒起來,形成短暫而明亮的光紋,稱之為流星或流星雨。每年會有幾次的流星雨,包括每年8月出現在英仙座的流星雨,它與斯威夫特-塔特爾彗星(Comet Swift-Tuttle)有關。

夜空中的哈雷彗星

小行星(Asteroid)是什麼?

「小行星」主要由岩石和金屬所組成,使它成為可在太空中開採的有價礦源。

小行星有時被稱為「太空岩石」(space rocks),大小從直徑幾英尺到山脈或大陸一般大。已知最大的小行星穀神星(Ceres)體積約為月球的四分之一,被認為是一顆「矮行星」(dwarf planet)。

在火星和木星軌道之間的小行星帶中約190萬顆大型小行星圍繞太陽運行,而穀神星是其中之一。一些小行星進入太陽系內部,它們很少與我們的行星相撞。

日本的探測器Hayabusa 2拍攝下Ryugu小行星的圖像

彗星和小行星來自何方?

小行星和彗星都是在約50億年前太陽系星體旋轉時所遺留的碎片。岩石和金屬小行星的形成更像似太陽,而冰彗星的形成就比較不像。

彗星由於具有引人注、易於辨識的尾巴,自古以來就為人所知。彗星實際上來自古希臘語中的「長髮」。

小行星很難被看見,它是到19世紀才被發現,也多虧當時的望遠鏡已經足夠精密,才得以分辨出它們的存在。在望遠鏡中,一顆小行星看起來像一顆昏暗的恆星,只是一個光點。

歷史上的彗星和小行星

彗星經常出現在人類的歷史中,在古代,它們有時被視為災難或征服的預兆。

已知的第一個對彗星的描繪出現在11世紀的貝葉掛毯(Bayeux Tapestry)中,描繪1066年的諾曼征服英格蘭(Norman Conquest)。這顆彗星在決戰前幾個月被看到,後來被解讀為一種預兆。

1705年,英國天文學家哈雷(Edmond Halley)確定1758年將出現一顆彗星,它在之前至少已經出現3次:1531年、1607年、1682年,每次相隔約75年。後來的研究證實,現在已知的哈雷彗星就是當時在諾曼征服英格蘭之前看到的同一顆彗星。

哈雷彗星最後一次出現在地球上空是1986年,下一次出現將在2061年。

世界各地的隕石坑提供了許多小行星在久遠的過去曾經多次撞擊地球的證據。最大的隕石坑,近200英里,是南非的弗雷德福特隕石坑(the Vredefort Crater),它的形成是在大約20億年前。

相比下,大約在6,600萬年前撞擊今日墨西哥尤卡坦半島(the Yucatan Peninsula)的小行星就下微不足道,其直徑不到10英里。但它被認為因此導致氣候巨變和恐龍滅絕。。

小行星和彗星也可能播種和毀滅生命:科學家們提出,地球上的生命可能是由彗星或小行星攜帶的微生物、來自其他行星甚至是遙遠恆星系統,通過一種稱為「胚種論」(Panspermia)的機制所播下的。

(作者:Tom Metcalf)

(編譯:汪水)

(文章來源:新三才編譯首發)

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