A meteor explosion over the Bering Sea late last year unleashed 10 times as much energy as the atomic bomb that destroyed Hiroshima, scientists have revealed. The fireball tore across the sky off Russia’s Kamchatka Peninsula on Dec. 18 and released energy equivalent to 173 kilotonnes of TNT. It was the largest air blast since another meteor hurtled into the atmosphere over Chelyabinsk, in Russia’s south-west, six years ago, and the second largest in the past 30 years.
Unlike the Chelyabinsk meteor, which was captured on CCTV, mobile phones and car dashboard cameras, the December arrival from outer space went largely unnoticed at the time because it exploded in such a remote location. NASA received information about the blast from the US air force after military satellites detected visible and infrared light from the fireball.
Lindley Johnson, planetary defense officer at NASA, told BBC News that blasts of this size were expected only two or three times a century. The space agency’s analysis shows that the meteor, probably a few meters wide, barreled into Earth’s atmosphere at a speed of 32km per second and exploded at an altitude of around 26km. The blast released about 40 percent of the energy of the meteor explosion over Chelyabinsk, according to Kelly Fast, NASA’s near-Earth objects observations program manager, who spoke at the 50th Lunar and Planetary Science conference near Houston.
Photo: AFP
照片：法新社
Since the event came to light, meteor researchers have been asking airlines for any sightings of the fireball, which came in close to routes used by commercial carriers flying between North America and Asia. Peter Brown, a meteor specialist at Western University in Canada, spotted the blast independently in measurements made by global monitoring stations. The explosion left its mark in data recorded by a network of sensors that detect infrasound, which has a frequency too low for the human ear to pick up. The network was set up to detect covert nuclear bomb tests.
The Bering Sea event is another reminder that, despite efforts to identify and track space rocks that could pose a threat to Earth, sizable meteors can still arrive without warning. NASA is working to identify 90 percent of near-Earth asteroids larger than 140m by 2020, but the task could take another 30 years to complete.
The 20m-wide meteor that detonated over Chelyabinsk lit up the morning sky on Feb. 15, 2013. At its most intense, the fireball burned 30 times brighter than the sun. The flash quickly gave way to a shockwave that knocked people off their feet and shattered windows in thousands of apartments. No one was killed but more than 1,200 people were injured, many by flying glass. Some sustained retinal burns from watching the spectacle.
In 1908, the most powerful meteor blast in modern times shook the ground in Russia. The rock exploded over Tunguska, a sparsely populated region in Siberia, and flattened an estimated 80 million trees over an area of 2,000 square kilometers.
(The Guardian)
科學家透露，白令海上空去年底發生一起流星爆炸，釋放出相當於摧毀廣島原子彈的十倍能量。這顆火球在去年十二月十八日劃過俄羅斯的堪察加半島上空，釋放出的能量相當於十七萬三千噸的黃色炸藥。這起事件是繼六年前另一顆流星猛烈衝進俄國西南部車里雅賓斯克上空的大氣層以來，最大的一場空中爆炸，同時也是過去三十年來第二大的爆炸事件。
不同於車里雅賓斯克那顆流星，被閉路電視、手機以及行車記錄器廣泛拍攝到，這位外太空訪客去年十二月抵達時幾乎無人察覺，原因在於爆炸地點實在是人跡罕至。後來因為軍用衛星在十二月偵測到這顆火球發出可見光和紅外線，美國國家航空暨太空總署（NASA）才從美國空軍方面獲得爆炸的相關資訊。
美國太空總署的行星防禦官員林德里‧約翰遜向英國廣播公司新聞網表示，這個規模的爆炸每一百年預期只會發生兩到三次。根據太空總署分析顯示，該顆流星約數公尺寬，以每秒約三十二公里的速度衝進地球的大氣層，並在大約二十六公里的高空中爆炸。美國太空總署的近地天體觀測計畫主管凱莉‧法斯特，近日於休士頓附近召開的第五十屆月球與行星科學研討會中表示，這次爆炸釋放的能量約為車里雅賓斯克上空流星爆炸的百分之四十。
自從這起事件曝光後，流星學者紛紛向航空公司詢問是否有任何這顆火球的目擊記錄，畢竟火球相當接近商用客機往來於北美洲和亞洲使用的飛行路徑。加拿大西安大略大學的流星專家彼得‧布朗，在全球多個監測中心測量到的數值中，獨立發現這起爆炸。它留下的痕跡，可見於一組感應器網路記錄到的數據，這些感應器可偵測頻率太低而無法被人耳接收到的超低頻音，而該網路是被設置用來偵測暗中進行的核彈試爆。
這起白令海爆炸事件再度提醒我們，儘管人類致力於找出並追蹤可能對地球造成威脅的太空巨石，大尺寸的流星仍然會毫無預警地來到地球。美國太空總署目前正努力在二○二○年之前，對大於一百四十公尺的近地小行星完成百分之九十的辨識，但這項任務可能需要再花三十年才能全部完成。
二○一三年二月十五日，二十公尺寬的流星在車里雅賓斯克上空爆炸，點亮早晨的天空。在燃燒最劇烈的時候，整顆火球的亮度比太陽高出三十倍。眩目的閃光很快就被衝擊波所取代，直接震倒民眾，並震碎數千棟公寓窗戶。雖然無人身亡，這起事件仍造成超過一千兩百名民眾受傷，許多人遭到飛散的玻璃割傷。有些民眾則因為觀看這場奇觀過久而導致視網膜灼傷。
現代歷史上威力最強大的流星爆炸在一九○八年撼動俄國土地。那顆石頭在通古斯加──西伯利亞一處人煙稀少的區域──上空爆炸，夷平兩千平方公里土地上的八千萬棵樹。
（台北時報章厚明譯）
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