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如何阻止'末日'小行星撞擊地球

What we would actually do to stop a 'doomsday’ asteroid
如何阻止'末日'小行星撞擊地球

Imagine the day observatories confirm that an asteroid is on a collision course with Earth; space-faring nations agree we need to stop it. What happens next depends on how much time the asteroid watchers say we have. None of these options are easy, and at least one of them would require the use of nuclear weapons.

如果天文觀測臺證實一顆小行星即將與地球相撞,航天大國都表示應該采取行動。接下來發生的事情將取決于天文觀測所測算的我們有多少時間來阻止相撞。這些選擇并非易事,且其中至少有一種方案需要用到核武器。

Major asteroid strikes are rare. The last one that could have caused massive loss of life was the Tunguska event of 1908, in which what was believed to be a meteorite exploded some 10 kilometres above a remote area of Siberia.

大規模行星撞擊地球事件是罕見的。上一次造成大規模傷亡的事件是在1908年的通古斯(Tunguska)大爆炸,科學家們認為在這次事件中,一顆隕石在西伯利亞(Siberia)的一個偏遠地區十公里左右的上空發生爆炸。
That type of impact occurs once every few centuries. Siberia, however, is remote; even today the population is small and scattered across a huge area. Had the same object arrived four of five hours later, the major city of St Petersburg would have been hit with the equivalent of a megaton-scale nuclear explosion.

此類撞擊事件大概幾百年發生一次。西伯利亞是一片人跡罕至的地方,即使今天仍然地區廣闊,人口稀少。如有相同大小物體將在4-5小時后撞上地球,其影響力對于圣彼得堡這樣的大城市相當于受到一個萬噸級的核爆炸當量的沖擊。

We have seen a smaller version of this nightmare scenario more recently. In 2013, the Chelyabinsk Meteor, which disintegrated at an altitude of 30 kilometres, shattered windows and injured 1,400 people in the western Russian city. It delivered an explosion equal to about 500 kilotons – about 30 or so Hiroshima bombs – though it exploded high enough to not cause too much structural damage. That kind of impact is much more common, occurring about three times per year. Most occur over the ocean or in remote areas, so we usually don't notice. But the question isn't if an impact will happen, but when.

最近的另一次小規模撞擊事件使我們再一次感受到了類似這種噩夢般的場景。車里雅賓斯克小行星撞擊事件(Chelyabinsk Meteor)發生在2013年俄羅斯西部的一個城市。隕石在30公里高度發生爆炸,建筑物玻璃破碎,導致1400多人受傷。爆炸相當于500000噸–約30個左右的廣島原子彈的威力,但由于爆炸點較高,因此并未摧毀太多建筑。此類規模的爆炸每年發生三次左右,較為常見。而其中大多數發生在海洋或偏遠地區,所以通常我們不會注意到。但問題不在于撞擊是否會發生,而是什么時候發生。

Governments are taking this seriously; taking the first tentative steps to preventing a dangerous impact. In January, Nasa formed the Planetary Defense Coordination Office to act as a clearing house for asteroid observations and work with other space agencies to discuss how large space rocks on a collision course with Earth might be dealt with.

各國政府都在嚴肅對待這個問題,并采取了試探性措施防止危險撞擊事件。在今年一月,美國航空航天局(Nasa)設立了行星防御協調辦公室(Planetary Defense Coordination Office),用來作為行星觀測和與各國其他航天機構協調交流的機構,討論如何處理大型隕石與地球的碰撞所產生的問題。

Right now, the PDCO spends much of its effort in detection, coordinating various observation programs, says Lindley Johnson, Nasa's planetary defense officer. That's because you can't deal with the space rocks until you know where they are. "We try to detect anything that might be a threat years, if not decades, in advance," he says. Once a dangerous asteroid is identified, the actual plans for stopping one enter the picture.

美國宇航局的行星防御官(planetary defense officer)林德利.約翰遜(Lindley Johnson)說目前,行星防御協調辦公室(PDCO)的主要工作在于檢測、協調各種觀測項目。“因為你只有確定了他們的位置才能討論應對措施。”“這些年,將近幾十年以來我們一直在提前預測任何有可能會造成威脅的物體,”他說。一旦危險的小行星位置確定,應對計劃就會應然而生。

The simplest method is a kind of planetary billiards, using a space probe to send a heavy object (or the probe itself) smashing into it. The asteroid will then, hopefully, be pushed off course and miss the Earth.

最簡單可行的方法是一種類似行星臺球的方法,用空間探測器發送一個有分量的物體(或探針本身)砸向行星。然后,這顆小行星或將會被推離既定軌道,避免與地球發生碰撞。

A joint European Space Agency and Nasa mission will test such technology in the next few years, under the name Asteroid Impact and Deflection Assessment (Aida). The mission consists of two spacecraft, one called the Asteroid Impact Mission (Aim), which will launch in late 2020, and the second, the Double Asteroid Redirection Test (Dart), in 2021.

歐洲航天局(European Space Agency)和美國航空航天局聯合制定的小行星偏移&評估任務計劃(Aida)將在未來幾年驗證該方案的可行性。該項目包含兩個航天器,一個叫做小行星撞擊任務(AIM),將在2020年底發射,第二個是雙小行星改道測試任務(DART),預計于2021年發射升空。

In 2022 they'll arrive at a double asteroid called 65803 Didymos, which is accompanied by a companion called Didymoon. Didymos measures some 780 metres across, while Didymoon is about 170 metres. The smaller of the two orbits the larger one every 11.9 hours, and they are close, only 1,100 metres apart. The Aim craft will rendezvous with the asteroid and study its composition. Once Dart arrives it will crash into Didymoon, and Aim will see what the effect on the smaller rock's orbit is. The idea is to find out exactly how much one can move an asteroid without running the risk of sending it on a dangerous trajectory, a kind of baby step to actually redirecting it.

兩個航天器將于2022抵達目的地—代號為65803的迪代莫斯(Didymos)行星,該小行星另有一顆伴星-迪代莫恩(Didymoon)。迪代莫斯直徑780米,迪代莫恩直徑為170米。較小的行星環繞較大的行星一圈的時間為11.9小時,兩者之間只有1100米的間隔。AIM飛行器將瞄準小行星并研究其成分,DART飛行器抵達后將直接撞向迪代莫恩,之后AIM飛行器將對撞擊表現進行分析。執行此任務的目的在于研究到底人類能在何種程度上能夠左右行星的運行軌道,而不至于將其送上危險的軌道上。這是改變行星運行軌跡的第一步。

To put the potential of the mission in perspective, the famous Meteor Crater in the US state of Arizona was probably made by an object only one-third the size of Didymoon, and that's 170 metres in diameter. A Didymos-size rock hitting the Earth about 15.5 kilometres per second – about the minimum speed it would be travelling at – would release something like two megatons worth of energy; easily enough to destroy a city. At its maximum speed (about 34.6 kilometres per second) it would release four megatons of energy – the equivalent of four million tonnes of TNT.

為了更好的理解此次任務的潛力,不妨看一下美國亞利桑那(Arizona)州著名的隕石坑(Meteor Crater),而造成這個直徑為1.18公里隕石坑的隕石體積大概只有迪代莫恩三分之一大。一個類似迪代莫斯體積的隕石與地球發生撞擊的速度大概在每秒鐘15.5公里,這也是其飛行的最低速度,其爆炸當量相當于兩兆噸的能量,可以輕而易舉的夷平一座城市。在其飛行的最大速度(約34.6公里/秒)時能夠釋放約四兆噸能量–相當于四百萬噸TNT當量的沖擊力。

"We want to alter the orbit of the moon around the primary," says Patrick Michel, a senior researcher at France's National Centre for Scientific Research and one of the Aida team leaders, "because the orbital velocity of moon around primary is only 19cm per second." Even small change could be measured from Earth, he adds, changing Didymoon's orbital period by about four minutes.

“我們想改變初級月球運行的軌道,”法國國家科學研究中心(National Centre for Scientific Research)的高級研究員,AIDA的團隊倡導者之一,帕特里克米歇爾(Patrick Michel)說,“因為月亮在原軌道速度只有每秒19厘米。”即使是很小的變化也可以從地球測量中檢測到,他補充說,這樣的變化能夠改變迪代莫恩的軌道運行周期約四分鐘。

It's also important to see if the impactor works. "All the models [of impacts] that we do are based on understanding of collision physics that is only at lab scale – centimetre-ised targets," Michel says. Whether those models are true on real asteroids is still something of an open question.

撞擊器工作的有效性也是一個重要指標。”我們所做的有關[影響]的模型是基于實驗室規模-百分之一厘米大的目標-的碰撞物理學的理解,”米歇爾說。這些模型是否影響真實存在的小行星仍然是一個懸而未決的問題。

Johnson adds that the technology is the most mature – humans have demonstrated the ability to get to asteroids already, notably with the Dawn mission to Ceres and Esa's Rosetta mission to comet 67P/Churyumov-Gerasimenko.

約翰遜補充說,該技術是目前最為成熟的–人類已經證明能夠接觸到小行星的能力,特別是探索灶神星(Ceres)的黎明號使命和歐洲航天局探索彗星67P/Churyumov-Gerasimenko的羅塞塔任務。

Beyond the impact approach, there's using gravity – simply putting a relatively massive spacecraft in orbit around an asteroid and letting their mutual gravitational pull gently nudge it into a new path. The advantage of this is that it only requires that the spacecraft gets there. The orbit would be a "halo" – a roughly circular path centered on the point where the gravitational force on an object would be the same from the Sun as the asteroid. Nasa's Asteroid Redirect Mission might indirectly test the idea; part of the plan is to bring an asteroid back to near-Earth space.

除了使用撞擊器的方法,科學家們還利用重力–直接在行星運行軌道中放入一個大型的航空飛船,并利用兩個飛行物之間的相互引力將其推入一個新的運行軌道。這種方法的好處是飛船抵達行星附近就可以實行。軌道將成為一個“光環”,一個以某點為圓心的圓形的路徑,其對物體的引力相當太陽對于行星的引力作用。美國航空航天局的小行星改道任務可能會間接測試這個想法;該計劃的一部分是將一顆小行星引回近地空間。

直徑為250米的行星就足以造成阿里桑拿州隕石坑規模的破壞。
However, a key element in these methods is time; it takes a good four years to mount a space mission beyond Earth orbit and the spacecraft will take another year or two to get to the relevant asteroid. And if there's less time, we might be left to try something else.

然而,在這些方法中的一個關鍵因素是時間。需要整整四年才能實施一個脫離地球軌道的太空發射任務,此外航天器需要再花一到兩年時間才能抵達相關行星。如果時間不夠,我們恐怕不得不嘗試其他方法。

Qicheng Zhang, a physicist at the University of California, Santa Barbara, thinks lasers could be another answer. A laser won't blow the asteroid up like a Death Star. Instead it would vaporise a small part of the surface. Zhang, along with his colleagues working under experimental cosmologist Philip Lubin, presented a set of orbital simulations in a paper to the Astronomical Society of the Pacific.

位于圣塔巴巴拉(Santa Barbara)的加利福尼亞大學(University of California)的物理學家張啟程(Qicheng Zhang)認為激光可以提供另外一種途徑。激光不會像一顆死星(Death Star)那樣把小行星炸毀,相反,它會蒸發行星表面的一小部分物質。在實驗宇宙學家菲利普.魯賓(Philip Lubin)的帶領下,張和他的同事向太平洋天文學會(Astronomical Society of the Pacific)提交了一份關于模擬軌道的學術論文。

This might seem like it would be ineffective, but remember that if it’s done early enough even a tiny nudge can alter an orbiting body's course by many thousands of kilometres. Zhang says the advantage of a laser is that a large one can be built in Earth orbit, and won’t need to chase an asteroid down. A laser with a power on the order of one gigawatt, firing for a month, could move an 80-metre-wide (264ft) asteroid – like that which created the Tunguska event – by the equivalent of two Earth radii (around 8,000 miles, or 12,800 kilometres). That's just enough to avoid a collision.

這看起來似乎沒什么效果,但請不要忘記,如果時間充裕下手早,僅僅是輕輕的一推就能將行星推離軌道數千公里。張說使用激光的優點在于可以在地球軌道上建造一個大的固定激光設備,而不需要去追著行星走。一個一千兆瓦級功率的激光持續運行達一個月的時間就可以將一個80米寬的(264ft)小行星–相當于造成通古斯大爆炸的行星尺寸–推離出相當于兩個地球半徑的距離(約8000英里,或12800公里)。這樣就足以避免撞擊了。

Another version of this idea is to send a spacecraft equipped with a less-powerful laser, though in that case it would have to reach the asteroid and follow it relatively closely. Since the laser would be smaller – in the 20 kilowatt range – it would have to operate for years, though Zhang's simulations suggest that a satellite following an asteroid, reaching it 15 years in advance, could also push it the necessary distance.

這種想法的另一個做法是發送一個搭載了較低功率激光的飛船,在這種情況下,飛船必須到達小行星附近并保持近距離飛行。由于激光相對較弱--20千瓦左右--這將花費數年才能產生效果。不過張的模擬測試顯示,如果激光能夠提前十五年到達行星位置,也可以將其推出需要的距離。

Zhang says the good thing about his Earth-orbiting proposal is that following an asteroid or comet in orbit is not as easy as it looks, even though we've already done it. "Rosetta was originally intended to target a different comet (46P) until a launch delay forced them to switch targets due to 46P no longer being in a favourable position. If a comet decides to target Earth though, we don't have the luxury of switching it out for a better one." Asteroids aren't quite as difficult to track, but it will still take around three years to reach one, he says.

張認為他的環地球軌道建議的一個優點在于盡管人類已經成功完成在軌追蹤行星或彗星,但實際上任務本身很有難度。羅塞塔計劃(Rosetta)原本的目標是另一顆彗星(46P),但是由于發射時間的延遲導致不得不臨時更換目標,因為46P所處位置發生了變化,不便于追蹤。“如果一顆彗星的運行目標是地球的話,恐怕我們沒有那么多時間更換這個目標了”。小行星不是很難追蹤,但仍然需要三年左右的時間才能抵達,”他說。

Johnson, though, notes one of the big problems with using a laser of any sort is that nobody has launched a kilometre-sized object of any sort into orbit, let alone an array of lasers. "There's a lot that I wouldn't consider mature; converting solar power to laser power reliably enough to operate for that long is one."

而約翰遜則注意到了一個大問題,至今沒有人能夠發射一個任何一種尺寸為一公里長的物體進入太空,更不要提一系列激光了。“該方法有很多問題我認為并不成熟,例如如何將太陽能轉換為激光使其可以進行長時間有效運轉就是其中之一。”

Then there's the "nuclear option". If you've seen the movie Armageddon or Deep Impact it sounds straightforward enough, but it's actually a lot harder than it sounds. "You'd need to send up an entire infrastructure," says Massimiliano Vasile, at the University of Strathclyde. He proposed detonating a nuclear bomb at some distance from the target. As with the laser, the plan here is to vaporise some of the surface, which would generate thrust and alter the asteroid's orbit. "If you ablate [erode] you have the advantage of high energy efficiency," he says.

除此之外,還有一種選項是使用核能。如果你看過電影《世界末日》或《天地大沖撞》的話你會覺得這個選擇很簡單直接,但實際上比聽起來難多了。”你需要把整個基礎設施發射到太空,”斯凱萊德大學(University of Strathclyde)的馬西米利亞諾.瓦西里(Massimiliano Vasile)說。他提出了距目標一定距離引爆核炸彈的提議。如同激光方法的原理,該計劃將蒸發行星表面物質由此產生助推力,改變行星的運行軌道,”如果采用消融[侵蝕]的辦法,這樣的辦法能效比很高,”他說。

While lasers and nuclear bombs can work when an asteroid is closer, in those cases composition becomes a significant factor, since the vaporisation temperatures will differ from one asteroid to the next. Another issue is flying rubble. There are many asteroids that might be just that – agglomerated chunks of rock that are only loosely bound. An impactor might not be as effective on that kind of body. That's a big plus for the gravity tractor method, Johnson notes – the composition and cohesion don't matter so much.

激光與核彈在接近行星時可以發揮作用,而行星表面的物質成分將是一個主要因素,因為成分本身將決定蒸發溫度,不同行星各有不同。另一個問題是飛出來的碎石。有許多的小行星其實就是一堆松散石塊的集合體。一個撞擊器對于這樣的物體效果不大。重力牽引法則更為有效,約翰遜指出,成分與物質之間的粘合度并不重要。

Any of these methods, however, might run into a final obstacle: politics. The 1967 Outer Space Treaty bans the use of nuclear weapons and their testing in space, and deploying a gigawatt laser in orbit might make some people nervous.

然而,所有這些方法終將遇到一個共同的障礙:政治。1967年的《外層空間條約》禁止在外太空進行核武器試驗,在太空軌道上部署一個千兆瓦的激光設備會令人緊張。

Zhang noted that if the power in an orbiting laser is lowered to 0.7 gigawatts, that only pushes the asteroid about 0.3 Earth radii – about 1,911 kilometers. "Smaller asteroids that could wipe out a city are far more common than giant planet killers. Now consider such an asteroid on a trajectory targeting New York. Depending on the circumstances, attempting and partially failing to deflect the asteroid from Earth might shift the site of impact to London instead, for example. If that were a substantial risk, the Europeans might not be so willing to agree to letting the US to deflect the asteroid."

張指出,如果將在軌激光設備的功率降低到0.7千兆瓦,那也僅僅能夠將小行星推離約0.3個地球半徑,也就是約1911公里。“能夠將一座城市夷為平地的小行星比巨型的行星殺手更為常見?,F在想一下,這樣的一顆小行星正在向紐約城飛來,試圖或者未能完全改變小行星運行軌道有可能會將撞擊點轉移到倫敦,試想如果面臨這樣巨大的風險,歐洲人是不會同意由美國人來主導改變行星軌道的。”

Such obstacles might be less than anticipated. "There's an escape clause in these treaties," Johnson says, referring to the Outer Space and Comprehensive Test Ban Treaties. The OST doesn't prohibit the launching of ballistic missiles, for example, which travel through space and might be armed with nuclear weapons. And in light of the need for planetary defence, criticism over their use might be muted.

這種障礙或許比預期的要少。”在這些條約中有一個免責條款,“約翰遜指的是《外太空和綜合測試禁止條約》(Outer Space and Comprehensive Test Ban Treaties)。例如該條約并不禁止向太空發射攜帶核武器的彈道導彈。出于行星防御系統的需要,批評者或許會保持沉默。

Michel notes that unlike any other natural disaster, this is one we can avoid. "This is something for which the natural risk is very low, compared to tsunamis and such. But it's the only one for which we can do something."

米歇爾說與任何其他自然災害相比,這個災害我們是可以避免的。”發生這樣的自然災害的機率非常低,不像是海嘯什么的。但這是我們唯一可以提前預防的災害。”
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