什麽才是真正“人類命運共同體”的悄然進行時

一顆命名為“2019 OK”的小行星，以不足地球月球距離五分之一的距離，在八月二日從地球“擦身而過”。它幾乎是在最後時刻被發現的。而發現這顆小行星的是三位巴西的天文科學家。因為發現的很倉促，著實讓天文學家們驚出一身冷汗。但是經過NASA（美國國家航空航天局），ESA（歐洲航天局）還有巴西、澳大利亞等 ISON（the International Scientific Optical Network ）的天文學家們通力合作，在極短時間內將小行星的運行軌道計算出來，預測無危害通過。下一次再會是2024年，但距離會很遠。

NASA，ESA 和 ISON 等組織對與地球規定有重合的小行星們的觀測，已經有好多年了。這些活動，除了預報危害外，還可以（據提案）對具有嚴重威脅的小行星發射火箭衝撞，迫使其改變軌道。數年前，NASA曾發射一枚飛船著陸在類似的一顆（足球場大小的）小行星上，對起經常探測。所以用火箭碰撞改變小行星軌道的提案也是有依可據的。

這才是“人類命運共同體”的一個實例。

下麵內容來自中文媒體的報道: 

威力可摧毀兩個紐約！一顆小行星差點撞上地球(圖) (http://www.hi.chinanews.com/hnnew/2019-08-02/497086.html)（部分內容見下）：

　　中新網8月2日電 (李弘宇)近日，在很多人都不知情的情況下，一顆名為“2019 OK”的小行星以大約8.7萬公裏每小時的速度，在不足五分之一的地月距離處掠過地球，令天文學家們著實“捏了把汗”。

　　雖然美國國家航空航天局(NASA)一直密切跟蹤大型小行星，以識別那些可能對地球構成威脅的小行星，但這顆威力足以夷平一整座城市的小行星，卻直到其接近地球的前一天，才被天文學家們發現。在此之前，人類對它一無所知。

　　天文學家先前已經預測到，近期會有幾顆小行星通過地球。可為何這顆小行星，直到最後一刻才被發現？

　　與地球擦肩而過

　　為何前一天才被發現？

　　澳大利亞蒙納士大學天文學副教授邁克·布朗稱，“小行星‘2019 OK’不是我們追蹤的星體，不知道它到底從哪裏冒出來”，“它可能是這麽多年來，如此接近地球的行星之中，體積最大的一顆。”

　　外媒報道稱，這顆小行星直徑大約在“57米至130米之間”，而現在服役的最大客機(空中客車A380-800)也才長約73米。在浩瀚宇宙中發現這顆小行星，就如在太空中找到一架大型客機，難度可想而知。

　　布朗表示，“2019 OK”過去1個月裏，非常靠近太陽，夜晚時，與太陽一同落到地平線下，因此觀測不到。天文學家們隻有黃昏與黎明的時候，在太陽剛剛落入地平線以下，小行星還在地平線以上時，才可能察覺其微弱亮度。

　　據報道，“2019 OK”從地球身旁掠過的3天前，亮度比平時還要黯淡1000倍，所以天文學家與觀測站的天文望遠鏡都沒能及時發現它。

 

圖為NASA發布的拍攝於2013年2月15日阿根廷境內的小行星掠過地球的圖像。

　　若撞地球後果多嚴重？

　　或是廣島原子彈爆炸能量30倍

　　如果地球被“2019 OK”撞上，將產生什麽後果？1908年，一顆直徑比“2019 OK”還要小的小行星，在西伯利亞通古斯河附近發生爆炸，將幾乎是兩個紐約市大小的森林夷為平地。

　　澳大利亞斯威本科技大學天文學家艾倫·達菲稱，“若撞擊地球，其威力或超過廣島原子彈爆炸能量的30倍”，它是一個“城市毀滅者”。布朗表示，它就像是個“非常巨型的核武器”，威力足以摧毀一座城市，不容小覷。

　　美國全天自動超新星勘測項目(ASAS-SN)團隊在社交媒體上發文稱，雖然這顆小行星是安全的，但“一顆直徑為100米的小行星若與地球相撞，會造成相當大的影響：製造直徑約1公裏的隕石坑，威力或相當於幾百萬噸的TNT炸藥當量。”

 

ASAS-SN在社交媒體上發文稱，直徑為100米的小行星若與地球碰撞，會造成相當大的影響：製造直徑約1公裏的隕石坑，威力或相當於幾百萬噸的TNT炸藥當量。”(圖片來源：ASAS-SN社交媒體截圖)

　　據報道，天文學家們過去曾發現數以千計的近地小行星，但很少有像“2019 OK”那樣接近地球。達菲表示，“我當時驚呆了，真實地受到驚嚇……小行星的距離近到讓人很不安。”

　　有研究稱，直徑超過1公裏的近地小行星，都能夠產生毀滅性的破壞力，比地球上現存核武庫裏所有軍火爆炸的威力，還要高百倍甚至千倍。

　　6500萬年前，一顆直徑逾10公裏的小行星撞上地球，在墨西哥尤卡坦半島造成巨大的“希克蘇魯伯隕石坑”，被認為是導致全球氣候劇變、恐龍滅絕的直接原因。

　　2013年2月15日，一顆直徑約20米的小行星在穿越大氣層時摩擦燃燒，在俄羅斯車裏雅賓克斯地區上空爆炸，致使近3000座建築受損，1200人受傷。

=====================

來自英文媒體的報道：

Big Asteroid Buzzed Past Earth Last Week: 2019 OK

 

A 330-foot- (100 m) wide asteroid designated 2019 OK passed just 43,500 miles (70,000 km) from Earth on July 25, 2019. It was discovered by the Brazilian SONEAR survey just days ago, and its presence was announced mere hours before it zoomed past our planet. 2019 OK is not a threat to Earth right now. However, this and other near-Earth asteroids do pose a genuine risk. The Tunguska explosion in 1908 and the Chelyabinsk meteor in 2013 were equivalent to large nuclear explosions, and under the wrong circumstances a meteor impact could devastate a city.

An illustration of an asteroid in space. Image credit: NASA / JPL / Caltech.

Searching for danger

Astronomers are well aware of the risks posed by asteroids hitting Earth. Meteor craters can be found around the globe, and some relatively fresh examples include Wolfe Creek in northern Australia and the imaginatively named Meteor Crater in Arizona. A huge asteroid impact 65 million years ago near Chicxulub in modern-day Mexico initiated the fall of the dinosaurs.

Consequently, astronomers across the globe have devoted considerable effort to determining the level of threat posed by near-Earth asteroids, and to identifying individual asteroids that could pose a significant threat. Asteroid surveys include Pan-STARRS, ATLAS, SONEAR, and the Catalina Sky Survey.

Asteroids are typically so far from Earth that they resemble stars, rather than the craggy rocks they are. However, because asteroids travel around the Solar System, they move relative to the distant stars. Thus astronomers can discover asteroids by taking sequences of images and looking for objects that move from image to image.

Using this approach to survey large swathes of sky, astronomers have discovered thousands of near-Earth asteroids, including more than 2,000 during 2017 alone.

And yet, some asteroids still manage to sneak up on us. Why?

Astronomers are good at discovering asteroids that are visible at night, but less good at spotting asteroids during the daytime. Asteroids also are fainter the further they get from Earth.

The 2013 Chelyabinsk meteor snuck up on us from the direction of the Sun. 2019 OK was visible at night, and at closest approach it would have been visible with a pair of binoculars as point of light drifting slowly across the sky. But three days before that it was 1,000 times fainter, and thus harder to spot.

2019 OK was finally tracked down by the SONEAR survey on Wednesday, and soon after that it was independently detected by the ASAS-SN telescope network. Both of these surveys use relatively small telescopes with sensitive cameras to search large areas of sky, rather than using large telescopes to study small patches of sky.

Trees flattened by the intense shock wave created in the atmosphere as the space rock exploded above Tunguska on June 30, 1908. The photograph was taken by the Soviet Academy of Science 1929 expedition led by Leonid Kulik. 500,000 acres, the size of a large metropolitan city, were flattened. Flattening trees requires an immense shock wave.

Close calls

Before its discovery as a near-Earth asteroid, 2019 OK was imaged by other telescopes, but its significance wasn’t recognised. But these earlier images did help astronomers nail down the asteroid’s orbit.

2019 OK has a very elliptical orbit, taking it from the asteroid belt beyond Mars to within the orbits of both Earth and Venus. As each orbit takes 2.7 years, it isn’t always going to pass as close to Earth as it did this time. It will make close approaches in the future, but hopefully not quite this close.

Other near-Earth asteroids are also on track to make close approaches to our planet. The 1,312-foot- (400 m) wide Apophis will pass roughly 18,700 miles (30,000 km) from Earth on Friday April 13, 2029, which will only come as bad news if you’re particularly superstitious.

Both 2019 OK and Apophis are far larger than the Chelyabinsk meteor, which was just 66 feet (20 m) across. The risk of them hitting Earth may be small, but they would be devastating if they did.

Avoiding Armageddon

If we find an asteroid on an actual collision course with Earth, is there anything we can do? With just a day or week’s notice we would be in real trouble, but with more notice there are options.

We are already sending spacecraft to near-Earth asteroids, with NASA’s OSIRIS-REx currently visiting Bennu and Japan’s Hayabusa2 currently visiting Ryugu.

However, these are missions of discovery rather than destruction. Indeed, destroying a near-Earth asteroid may be counterproductive, potentially creating multiple destructive asteroids.

So how can we stop catastrophe? The solution may be to give dangerous asteroids a gentle nudge rather than a vicious kick. If an asteroid’s velocity can be changed by just 0.6 mph (1 kmh), over years that adds up to thousands of miles’ difference in position. Given that the pale blue dot of Earth is just 8,000 miles (12,750 km) across, a small nudge to a big rock may be enough to avoid annihilation.

_____

J. Bulger et al. 2019. 2019 OK. Minor Planet Electronic Circular # 2019-O56

Author: Michael J.I. Brown, associate professor in astronomy at Monash University.

This article was originally published on The Conversation.