Imagine a baseball zipping1） along at 30，000 miles per hour. That's how big and fast many meteors are. And though some are bigger than baseballs， most are more like grains of sand.

Entry into the Atmosphere

When they plow through the atmosphere， meteors are heated to more than 3000 degrees Fahrenheit， and they glow. Meteors are not heated by friction2）， as is commonly thought. A phenomenon called ram pressure3） is at work. A meteor compresses4） air in front of it. The air heats up， in turn heating the meteor.

The intense heat vaporizes5） most meteors， creating what we call shooting stars. （Most become visible at around 60 miles up.） Some large meteors splatter6）， causing a brighter flash called a fireball， and an explosion， which can often be heard up to 30 miles away. When meteors hit the ground， they're called meteorites. Some meteors are bits broken off asteroids， others—mere cosmic dust—are cast off by comets.

想象一下一個棒球以3萬英里（約4.8萬公里）的時速飛馳的場景。很多流星體就是這個大小和速度。盡管有些流星體會比棒球大點，但大多數(shù)流星體都更像沙粒那么大。

進(jìn)入大氣層

流星體劃過地球大氣層的時候，溫度能升高到 3000多華氏度（約1649攝氏度），并會燃燒發(fā)光。它們并不像通常認(rèn)為的那樣是因摩擦生熱，而是一種叫做動壓的現(xiàn)象在起作用。流星體會擠壓其前方的空氣，使得空氣升溫，空氣反過來又讓流星體變熱。

熾熱的溫度直接汽化了大多數(shù)流星體，形成我們所說的流星（大多在離地約60英里[約96公里]的高空時開始被肉眼所見）。一些大的流星體會崩裂飛濺，形成一道我們稱之為火球的更亮光芒一閃即逝，并伴有爆炸，常常在遠(yuǎn)達(dá)30英里（約48公里）的地方都能聽到聲響。流星撞到地面之后叫做隕石。一些流星體是小行星的碎屑，其他的只是宇宙塵埃，是從彗星上掉下來的。

◎ By Robert Roy Britt

譯 / Evan Chao

流星通常被賦予了浪漫的元素，不論是看到流星許愿，還是陪著喜歡的人一起看流星雨，都聽起來十分美好。然而事實上，流星這種美麗的天象能否帶來好運我們不得而知，但它有時候會帶來災(zāi)難卻是確鑿的事實。想要了解其背后的原因，就要從解釋這種“天外來客”是如何形成的開始啦。

1. zip [z?p] vi. 快速移動；飛速行動

2. friction [?fr?k?n] n. 摩擦；摩擦力

3. ram pressure： 動壓，與靜壓相對，指的是物體在流體中運動時，在正對流體運動的方向的表面，流體完全受阻，此處的流體速度為零，其動能轉(zhuǎn)變?yōu)閴毫δ?。流星體在地球大氣中高速運動時，由于流星體表面的大氣受阻而被迅速壓縮到極致，此過程中形成的動壓造成了大氣溫度升高，從而也使流星體溫度升高。ram [r?m] n. 撞擊

4. compress [k?m?pres] vt. 壓緊；壓縮

5. vaporize [?ve?p?ra?z] vt. 使汽化；使蒸發(fā)

6. splatter [?spl?t?（r）] vi. 濺潑；灑落

7. oblique [??bli?k] adj. （線或角）斜的，傾斜的

8. withstand [w???st?nd] vt. 經(jīng)受??；承受；頂住

9. ensuing [?n?sju???] adj. 接著發(fā)生的；隨之產(chǎn)生的

10. extraterrestrial [?ekstr?t??restri?l] adj. 地外的

11. rebound [?r??ba?nd] n. 反彈

12. rim [r?m] n. （圓形物體的）邊框，邊緣

13. terrace [?ter?s] n. 梯田；階地

14. basin [?be?sn] n. 盆地；洼地

Meteor Breakup

Whether an object breaks apart depends on its composition， speed and angle of entry. A faster meteor at an oblique7） angle suffers greater stress. Meteors composed of ironwithstand8） the stress better than those made of stone. Even an iron meteor will usually break up as the atmosphere becomes denser.endprint

A meteor sometimes explodes above the surface， causing widespread damage from the blast and ensuing9） fire. This happened in 1908 over Siberia.

Impact with Earth

Extraterrestrial10） objects that hit the ground， their speed roughly half what it was upon entry， blast out craters 12 to 20 times their size. Craters on Earth form much as they would on the moon or any rocky planet. Smaller objects create simple， bowl-shaped craters. Larger impacts cause a rebound11） that creates a central peak； slipping along the rim12） forms terraces13）. The largest impacts form basins14） in which multiple rebounds form several inner peaks.

Meteorite Events

In 1807， a fireball exploded over Connecticut， and several meteorites rained down. By then the first handful of asteroids had been discovered， and a new theory emerged suggesting meteorites were broken bits off asteroids or other planets. （A theory that still holds.）

One of the most significant meteorite events in recent history destroyed hundreds of square miles of forest in Siberia on June 30， 1908. Across hundreds of miles， witnesses of the Tunguska event saw a ball of fire streak15） through the sky， suggesting the meteor entered the atmosphere at an oblique angle. It exploded， sending out hot winds and loud noises and shook the ground enough to break windows in nearby villages. Small particles blown into the atmosphere lit the night sky for several days. No meteorite was ever found， and for years many scientists thought the devastation was caused by a comet. Now， the prevailing theory holds that a meteor exploded just above the surface.

The largest meteorite recovered in the United States fell in a wheat field in southern Nebraska in 1948. Witnesses saw a giant fireball in the afternoon that some said was brighter than the sun. The meteorite was found buried 10 feet deep in the ground. It weighed 2，360 pounds.

The most famous meteorite crater in the United States is Barringer Crater. It's in Arizona， and it's huge. The rim rises 150 feet from the surrounding plain， and the hole is 600 feet deep and nearly a mile wide. It was the first crater that was proved to be caused by a meteorite impact， which occurred between 20，000 and 50，000 years ago.

Meteor Showers

When a comet nears the sun， a trail of dust and other debris16） burns off and remains in solar orbit. As Earth orbits the sun， it passes through this debris field spread across its path. Small bits burn up in the atmosphere， creating meteors. Meteors come from other sources， too， but comet debris streams are the source of sometimes dramatic meteor showers.endprint

流星體解體

天體是否會解體取決于它的構(gòu)成、速度和進(jìn)入地球大氣層的角度。流星體的速度越快，角度越小，受到的壓力就越大。由鐵構(gòu)成的流星體比由巖石構(gòu)成的流星體更能承受壓力。但即便是由鐵構(gòu)成的流星體通常也會隨著大氣層密度的增加而解體。

流星體有時還未落地就會發(fā)生爆炸，爆炸帶來的沖擊以及繼而引起的大火會造成大范圍的破壞。1908年西伯利亞上空就發(fā)生過這樣的事情。

撞擊地球

外來天體撞到地面的速度大約是其進(jìn)入地球大氣層時速度的一半，撞地后炸出自身體積12～20倍大的隕石坑。地球上隕石坑的形成和月球或其他任何巖石星球上隕石坑的形成沒有什么兩樣。較小的流星體會撞出簡單的碗狀隕石坑，而較大的撞擊則會引起反彈，形成一個中心錐，并且沿著坑緣滑下的地方還會形成階梯狀斷層。最大的撞擊可以形成盆地，且由于多次反彈，盆地中還會形成多個內(nèi)部錐形峰。

流星撞擊事件

1807年，一個火球在美國康涅狄格州上空爆炸，好幾顆隕石落了下來。當(dāng)時，第一批小行星已被發(fā)現(xiàn)，由此出現(xiàn)了一個新的理論，該理論認(rèn)為隕石是小行星或其他行星的碎屑（這一理論現(xiàn)在仍然成立）。

近代歷史上最大的流星撞擊事件之一發(fā)生在1908年6月30日，摧毀了西伯利亞數(shù)百平方英里的森林。發(fā)生在西伯利亞通古斯地區(qū)的這起事件在方圓數(shù)百英里都有目擊者，他們看到天空飛過一個火球，這表明流星體是斜著進(jìn)入地球大氣層的?；鹎虮▏姵鰺犸L(fēng)，發(fā)出巨響，引起地面劇烈震動，足以將附近村莊的窗戶都震碎。被吹到大氣層中的碎粒連續(xù)數(shù)日讓夜空亮如白晝。人們沒有發(fā)現(xiàn)任何隕石，多年來許多科學(xué)家都認(rèn)為這次破壞是由彗星引起的。而目前盛行的理論認(rèn)為，當(dāng)時是一顆流星在地表上方爆炸了。

在美國發(fā)現(xiàn)的最大的隕石是1948年落在內(nèi)布拉斯加州南部一片麥田的那顆。目擊者在當(dāng)天下午看到了一個巨大的火球，有人說這個火球比太陽還亮。隕石被發(fā)現(xiàn)的時候，深埋在地下10英尺（約3.05米）的地方。這顆隕石重2360磅（約1.07噸）。

美國最著名的隕石坑是巴林杰隕石坑。該隕石坑位于亞利桑那州，巨大無比?？泳壐叱鲋車皆?50英尺（約46米），坑深600英尺（約183米），直徑近一英里（約1.6公里）。這是首個被證實是流星撞擊形成的坑，撞擊時間發(fā)生在2～5萬年之前。

流星雨

當(dāng)彗星靠近太陽的時候，尾部的塵?；蚱渌樾紩紵撀?，遺留在太陽軌道內(nèi)。地球繞著太陽運轉(zhuǎn)時會經(jīng)過散布在地球公轉(zhuǎn)軌道上的這個碎屑區(qū)。許多細(xì)小的碎屑進(jìn)入地球大氣層開始燃燒，形成流星。雖然流星還可能來自其他地方，但不時出現(xiàn)的那些壯觀的流星雨都來自彗星的碎屑流。

Discovery

探索之旅

When to watch 流星雨的觀看時間

The part of Earth where dawn is breaking is always at the leading edge17） of our planet's plunge18） along its orbital path around the sun. This part of the planet tends to "catch" oncoming meteors left by a comet， whereas the other side of Earth， where it is dusk or late evening， outruns the debris. For that reason， the hours between midnight and dawn are typically the best time to watch a meteor shower.

地球迎來黎明的區(qū)域始終處于地球沿著公轉(zhuǎn)軌道行進(jìn)的前端。這一區(qū)域往往會“捕捉”到彗星留下的、迎頭撞來的流星體，而地球處于黃昏或深夜的另一半?yún)^(qū)域則避開了彗星留下的這些碎屑。因此，午夜到黎明這段時間通常是觀看流星雨的最佳時間。

15. streak [stri?k] vi. 飛奔；疾馳

16. debris [?debri?] n. 殘骸

17. edge [ed?] n. 邊緣

18. plunge [pl?nd?] n.【地】傾入endprint