本篇文章共分 7 段，從彗星的結構變化原因開始，逐步說明彗星尾巴的類型與形成原因，彗星大小，以及彗星生成與消滅等與彗星相關的科學問題。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Structure and Composition of Comets 彗星的結構和組成

1. 距離太陽遠近影響彗星結構

   Astronomers now have a fairly good idea of what a comet really is. When it is far from the Sun, it is a very small object only a few kilometers across. It consists mainly of ices (water, methane, ammonia) with bits of dust embedded in it—a kind of dirty ice ball. As it approaches the Sun, radiation from the Sun vaporizes the icy matter and releases some of the dust. This forms a gigantic halo around the ice ball. This halo—called the coma—extends out tens of thousands of kilometers from the icy core, which is the nucleus of the comet. Sunlight reflected off the dust particles makes the coma visible to observers on Earth. Ultraviolet radiation from the Sun breaks down the vapor molecules into their constituents. These components can be excited by absorbing radiation from the Sun. In returning to lower-energy states, the excited atoms and ions emit light, contributing to the luminosity of the coma.

   天文學家現在對彗星到底是什麼有了一個相當完整的概念。當它遠離太陽時，它是一個非常小的物體，只有幾公里寬。它主要由冰（水、甲烷、氨）和嵌入其中的灰塵碎片組成—一種骯髒的冰球。當它接近太陽時，來自太陽的輻射使冰雪物質汽化，並釋放出一些塵埃。這在冰球周圍形成了一個巨大的光環。這個光環被稱為彗星，從作為彗星核心的冰核延伸出去數萬公里。從塵埃粒子上反射的陽光使地球上的觀察者看到了彗星。來自太陽的紫外線輻射將蒸氣分子分解成它們的基本成分。這些成分可以通過吸收太陽的輻射而被激發。在返回到低能量狀態時，被激發的原子和離子會發光，成為彗星亮度的來源。

2. 塵埃彗尾

   When the comet gets even closer to the Sun, one of its most spectacular parts begins to form—the tail. Actually, there are two kinds of tails: the dust tail and the ion tail. The dust tail is produced by the light from the Sun reflecting off the dust particles in the coma. A photon carries momentum. In bouncing off a dust particle, it imparts a tiny, but perceptible, momentum change to the dust particle, driving it away from the coma. As the comet sweeps along its orbit, it leaves a curving trail of dust behind in its path. This visible dust tail can extend for tens or hundreds of millions of kilometers out from the nucleus. The dust tail is characterized by its gently curving shape and its yellowish color.

   當彗星更加接近太陽時，它最壯觀的部分之一開始形成—彗尾。實際上，有兩種彗尾，即塵埃彗尾和離子彗尾。塵埃彗尾是由太陽的光反射離開彗星中的塵埃顆粒時產生的。一個光子帶有動量。在從塵埃粒子上反彈時，它給塵埃粒子帶來一個微小的、但可感知的動量變化，促使它離開彗星。當彗星沿其軌道掠過時，它在其路徑上留下了一條彎曲的塵埃彗尾。這個可見的塵埃彗尾可以從彗星核中延伸出數千萬或數億公里。塵埃彗尾的特點是它緩緩彎曲的形狀和淡黃色。

3. 離子彗尾

   A different mechanism is responsible for the ion tail. Near the Sun, ultraviolet radiation from the Sun (solar wind) ionizes and excites the atoms in the coma. As the solar wind sweeps through the coma, the high-velocity charged particles of the solar wind interact with the electrically charged excited ions in the coma, driving them away from the head of the comet. In returning to lower-energy states, these excited ions emit photons and form a luminous, bluish-colored tail extending out from the comet directly away from the Sun. Since both kinds of tails are produced by radiation streaming out from the Sun, they extend out from the coma in the general direction away from the Sun. A comet may exhibit several tails of each kind.

   另一個不同的機制是產生了離子彗尾。在太陽附近，來自太陽的紫外線輻射（太陽風）使彗星中的原子電離並被激發。當太陽風掃過彗星時，太陽風的高速帶電粒子與彗星中帶電的激發離子相互作用，驅使它們離開彗星的頭部。在返回到低能量狀態時，這些受激離子會發出光子，並形成一個發光的、帶藍色的彗尾，從彗星直接延伸到遠離太陽的地方。由於這兩種彗尾都是由來自太陽的輻射產生的，它們從彗星上延伸出來，朝著遠離太陽的大致方向。一顆彗星可能表現出幾種不同類型的彗尾。

4. 彗星的大小範圍

   Although the nucleus is of the order of a few kilometers in size, the diameter of the coma may be tens or hundreds of thousands of kilometers, the tails typically extend out tens or hundreds of millions of kilometers away from the coma.

   儘管彗星核的大小為幾公里，但彗星的直徑可能是幾萬或幾十萬公里，彗尾通常從彗星延伸出幾千萬或幾億公里遠。

5. 與地球相交會

   A comet leaves a trail of matter behind it as it moves through the inner solar system. Some of this debris may get strewn across Earth’s orbit around the Sun. When Earth passes through this part of its annual path, it sweeps through the dust trail. The particles enter Earth’s atmosphere at high velocity. The air friction can cause one of these bits of matter to produce a brief streak of light as it burns up in the atmosphere.

   彗星在穿過內太陽系時，會在身後留下一串物質。其中一些碎片可能會散落在地球圍繞太陽的軌道上。當地球經過其常年路徑的此一部分時，它就會掃過塵埃的痕跡。這些顆粒以高速進入地球的大氣層。空氣摩擦會導致這些物質的某個碎片在大氣層中燃燒時產生短暫的光亮線條。

6. 彗星的生命

   Since a comet loses matter on each pass by the Sun, eventually it will be depleted to the point where it is no longer visible. Comets that approach the Sun have finite lifetimes. Given the typical sizes of comets and the typical rates at which they lose matter, astronomers have concluded that the lifetimes of comets with orbits that bring them near enough to the Sun to be seen from Earth are very much shorter than the age of the solar system. Where do the new comets come from to replace the old ones that dissipate and vanish from view?

   由於彗星在每次經過太陽時都會損失物質，最終它將被耗盡，以至於不可復見。接近太陽的彗星的壽命是有限的。考慮到彗星的一般大小和它們失去物質的一般速度，天文學家們得出結論指出，擁有特定軌道使它們足夠接近太陽以致能從地球上看得到的彗星的壽命比太陽系的年齡短很多。新的彗星從哪裡來，以取代那些消散和消失在視野中的舊彗星？

7. 彗星的生成

   Dutch astronomer Jan Oort proposed that a giant cloud of matter left over from the formation of the solar system surrounds the Sun and extends out to about 50,000 astronomical units. This cloud contains large chunks of matter like the nuclei of comets. The gravitational influence of a passing star can be sufficient to perturb the orbit of one of these chunks to send it toward the inner solar system and bring it near the Sun.

   荷蘭天文學家楊．奧爾特提出，太陽系形成時留下的巨大物質雲環繞著太陽，並延伸到大約 5 萬個天文單位。這片雲包含像彗星核般的大塊物質。一顆經過的恆星的引力影響足以擾亂這些大塊物質中某個的軌道，將其送向太陽系內部，並使其靠近太陽。