On April 13th 2015 the Minor Planet Centre （MPC）， an office with a staff of six which looks after such matters for the International Astronomical Union， recorded hundreds of newly discovered asteroids—a typical daily haul. The one to which it assigned the name 2015 PDC， though， stood out.

美國小行星觀測(cè)中心（MPC）是為國際天文學(xué)聯(lián)合會(huì)負(fù)責(zé)小行星事務(wù)的機(jī)構(gòu)，有6名工作人員。在2015年4月13日這一天，它記錄了幾百顆新發(fā)現(xiàn)的小行星——一般每天都發(fā)現(xiàn)這么多。但被它定名為2015PDC的小行星引人注目。

The calculations for 2015 PDC showed that on September 3rd 2022 the Earth would cross the corridor where the asteroid might be. The two might collide.

對(duì)2015PDC的計(jì)算結(jié)果顯示，2022年9月3日，地球?qū)⒋┰竭@顆小行星可能會(huì)經(jīng)過的路線。兩者有可能相撞。

That was not， in itself， too worrying. Newly discovered asteroids have had measurable possibilities of hitting the Earth before; further observations， and thus a more precise understanding of the rocks’ orbits， have always ruled those impacts out. The one exception to that rule so far， 2008 TC3， was an asteroid only 4 metres （13 feet） across; when it exploded in the upper atmosphere in 2008， just days after its discovery， it mattered to no one except the meteorite hunters who rushed to Sudan to find the remains that fell to Earth.

這件事本身不是太令人擔(dān)心。先前也有新發(fā)現(xiàn)的小行星具有撞擊地球的明顯可能性，但進(jìn)一步的觀測(cè)并由此對(duì)那些小行星軌道的更精確了解一般都排除了撞擊的可能性。迄今這一規(guī)律的唯一例外是2008TC3，一顆直徑只有4米的小行星。它在2008年被發(fā)現(xiàn)后沒幾天就在高層大氣爆炸了。沒人關(guān)心此事，除了搜尋隕石的人。他們趕往蘇丹，找尋落到地球上的殘骸。

As 2015 PDC’s orbit became better known the corridor in which it might be on that fateful day in 2022 shortened. But it still contained the Earth. By June the probability of an impact had risen to 1%， making it the most threatening asteroid astronomers had ever seen. By September governments in America， Europe， Russia and China had started work on space missions aimed at changing the asteroid’s orbit by ramming into it. Even at a speed of more than 10km （6 miles） a second， hitting a billion-tonne asteroid with a few tonnes of spacecraft will make only a minute difference to the asteroid’s orbit.

隨著2015PDC的軌道變得更清楚，它在2022年那個(gè)重要的日子可能經(jīng)過的路線縮短了。但地球仍在那條路線上。到6月，相撞概率升高到1％，使其成為天文學(xué)家見過的最具威脅性的小行星。到9月，美國、歐盟、俄羅斯和中國政府開始研究通過沖撞改變這顆小行星軌道的太空任務(wù)。即便以每秒超過10千米的速度，用幾噸重的飛船撞擊一顆10億噸重的小行星也只會(huì)對(duì)小行星的軌道作出微小的改變。

But a minute difference， made early enough， can provide the margin between a near miss and a hit that is palpable on a planetary scale. And 2015 PDC looked like providing such a hit. Early estimates put its diameter between 140 and 400 metres. Even if it were at the small end of that range， though， when it hit the Earth at 11km a second it would release as much energy as hundreds of large nuclear warheads set off simultaneously. At the large end the hundreds would become thousands.

但如果發(fā)生得夠早，微小的改變也能在行星尺度上造成明顯差距——在擦肩而過與相撞之間。而2015PDC看起來會(huì)造成這樣的撞擊。據(jù)初步估計(jì)，其直徑在140米至400米之間。即使其直徑在這個(gè)范圍的低端，當(dāng)它以每秒11千米的速度撞擊地球時(shí)，所釋放的能量也相當(dāng)于幾百枚大型核彈頭同時(shí)爆炸釋放的能量。如果其直徑在這個(gè)范圍的高端，幾百枚將變成幾千枚。

The remaining uncertainty was about where， exactly， it would hit. The risk corridor was wrapped around the Earth on a great-circle arc that began in the tropical Pacific and ran north-west over the Philippines and across the South China Sea before passing over a swathe of Asia beginning in Vietnam and ending in Iran. The track passed over a surprisingly large fraction of the world’s population and three megacities—Dhaka， New Delhi and Tehran. You would have been hard put to have imagined a more threatening rock.

仍然不確定的是撞擊的確切位置。那條危險(xiǎn)路線像一個(gè)大圓弧環(huán)繞地球，從熱帶太平洋開始，向西北經(jīng)過菲律賓，越過南中國海，然后跨越亞洲起于越南、止于伊朗的一長條地帶。所占比例驚人的世界人口和三座大都市——達(dá)卡、新德里和德黑蘭處在這條軌跡上。你很難想象還有比這更具威脅性的小行星了。

As with all missions to other parts of the solar system， interceptors aimed at 2015 PDC could be launched only at a specific time defined by the asteroid’s orbit. They would have to take off in late August 2019 in order to reach the rock in early March 2020， 900 days before the impact. The largest of the interceptors originally proposed fell by the wayside because the rocket could not be readied in time. Still， the six remaining spacecraft—three American attempts and one each from China， Russia and Europe—were to offer more than enough kinetic energy to change the asteroid’s orbit by the two centimetres a second required to make it miss the Earth in 2022.

就像前往太陽系其他區(qū)域的所有任務(wù)一樣，以2015PDC為目標(biāo)的攔截器只能在由這顆小行星軌道限定的一個(gè)特定時(shí)間發(fā)射。它們必須在2019年8月底發(fā)射升空，以便在2020年3月初、即距撞擊還有900天的時(shí)候飛抵小行星。最初擬定的最大一枚攔截器半途而廢了，因?yàn)榛鸺裏o法及時(shí)準(zhǔn)備好。不過，剩下的6艘飛船——美國3艘，中、俄和歐盟各1艘——將提供綽綽有余的動(dòng)能，以每秒2厘米的速度改變小行星的軌道，而這正是讓小行星在2022年避免撞上地球所需要的速度。

What was not much discussed—other than by Bhavya Lal of the Science and Technology Policy Institute in Washington， DC， who was representing India in the role-playing—was whether the deflection the spacefaring powers were undertaking was a good idea. By August 2019 the possible impact sites had been narrowed down to an arc stretching from the Philippines to Vietnam across the South China Sea. An impact in the middle of the sea would have threatened 80m people living on its coasts， almost half of them in China， with tsunami-like waves. The damage could be enormous—but unlike the hazards posed by most natural disasters its timing would be known， to within an hour， years in advance， and its effects could be modelled. Breakwaters could be built in front of the larger cities， evacuation plans perfected， nuclear plants at risk from the waves shut down， populations resettled either for a few crucial weeks or for good.

除在角色扮演中代表印度的美國科技政策研究所的巴夫亞·拉爾，其他人沒有過多討論的一件事是，航天強(qiáng)國要采取的改變小行星軌道的做法是否是個(gè)好主意。到2019年8月，可能的碰撞地點(diǎn)已經(jīng)縮小到從菲律賓到越南、橫跨南中國海的一條圓弧。在海洋上空的碰撞將造成海嘯般的巨浪，威脅到8000萬沿海居民，其中近一半在中國。破壞將是巨大的。但與大多數(shù)自然災(zāi)害構(gòu)成的險(xiǎn)情不同，此次險(xiǎn)情的發(fā)生時(shí)間會(huì)提前幾年就為人所知，精確度在1個(gè)小時(shí)以內(nèi)，而且其影響是可以模擬的。我們可以在較大城市沿岸修建防波堤，完善疏散計(jì)劃，關(guān)閉受大浪威脅的核電站，在關(guān)鍵的幾個(gè)星期里或永久性地把居民安置到別處。（王海昉譯自英國《經(jīng)濟(jì)學(xué)人》周刊8月1日一期文章）