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如何阻止'末日'小行星撞击地球

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

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.

如果天文观测台证实一颗小行星即将与地球相撞,航天大国都表示应该采取行动。接下来发生的事情将取决于天文观测所测算的我们有多少时间来阻止相撞。这些选择并非易事,且其中至少有一种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.

大规模行星撞击地球事件是罕见的。上一次造成大规模伤亡的事件是在1908年的通古斯(Tunguska)大爆炸,科学家们认为在这次事件中,一颗陨石在西伯利亚(Siberia)的一个偏远地区十公里左右的上空发生爆炸。
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.

此类撞击事件大概几百年发生一次。西伯利亚是一片人迹罕至的地方,即使今天仍然地区广阔,人口稀少。如有相同大小物体将在4-5小时后撞上地球,其影响力对于圣彼得堡这样的大城市相当于受到一个万吨级的核爆炸当量的冲击。

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.

最近的另一次小规模撞击事件使我们再一次感受到了类似这种噩梦般的场景。车里雅宾斯克小行星撞击事件(Chelyabinsk Meteor)发生在2013年俄罗斯西部的一个城市。陨石在30公里高度发生爆炸,建筑物玻璃破碎,导致1400多人受伤。爆炸相当于500000吨–约30个左右的广岛原子弹的威力,但由于爆炸点较高,因此并未摧毁太多建筑。此类规模的爆炸每年发生三次左右,较为常见。而其中大多数发生在海洋或偏远地区,所以通常我们不会注意到。但问题不在于撞击是否会发生,而是什么时候发生。

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.

各国政府都在严肃对待这个问题,并采取了试探性措施防止危险撞击事件。在今年一月,美国航空航天局(Nasa)设立了行星防御协调办公室(Planetary Defense Coordination Office),用来作为行星观测和与各国其他航天机构协调交流的机构,讨论如何处理大型陨石与地球的碰撞所产生的问题。

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.

美国宇航局的行星防御官(planetary defense officer)林德利.约翰逊(Lindley Johnson)说目前,行星防御协调办公室(PDCO)的主要工作在于检测、协调各种观测项目。“因为你只有确定了他们的位置才能讨论应对措施。”“这些年,将近几十年以来我们一直在提前预测任何有可能会造成威胁的物体,”他说。一旦危险的小行星位置确定,应对计划就会应然而生。

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.

最简单可行的方法是一种类似行星台球的方法,用空间探测器发送一个有分量的物体(或探针本身)砸向行星。然后,这颗小行星或将会被推离既定轨道,避免与地球发生碰撞。

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.

欧洲航天局(European Space Agency)和美国航空航天局联合制定的小行星偏移&评估任务计划(Aida)将在未来几年验证该方案的可行性。该项目包含两个航天器,一个叫做小行星撞击任务(AIM),将在2020年底发射,第二个是双小行星改道测试任务(DART),预计于2021年发射升空。

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.

两个航天器将于2022抵达目的地—代号为65803的迪代莫斯(Didymos)行星,该小行星另有一颗伴星-迪代莫恩(Didymoon)。迪代莫斯直径780米,迪代莫恩直径为170米。较小的行星环绕较大的行星一圈的时间为11.9小时,两者之间只有1100米的间隔。AIM飞行器将瞄准小行星并研究其成分,DART飞行器抵达后将直接撞向迪代莫恩,之后AIM飞行器将对撞击表现进行分析。执行此任务的目的在于研究到底人类能在何种程度上能够左右行星的运行轨道,而不至于将其送上危险的轨道上。这是改变行星运行轨迹的第一步。

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.

为了更好的理解此次任务的潜力,不妨看一下美国亚利桑那(Arizona)州著名的陨石坑(Meteor Crater),而造成这个直径为1.18公里陨石坑的陨石体积大概只有迪代莫恩三分之一大。一个类似迪代莫斯体积的陨石与地球发生撞击的速度大概在每秒钟15.5公里,这也是其飞行的最低速度,其爆炸当量相当于两兆吨的能量,可以轻而易举的夷平一座城市。在其飞行的最大速度(约34.6公里/秒)时能够释放约四兆吨能量–相当于四百万吨TNT当量的冲击力。

"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.

“我们想改变初级月球运行的轨道,”法国国家科学研究中心(National Centre for Scientific Research)的高级研究员,AIDA的团队倡导者之一,帕特里克米歇尔(Patrick Michel)说,“因为月亮在原轨道速度只有每秒19厘米。”即使是很小的变化也可以从地球测量中检测到,他补充说,这样的变化能够改变迪代莫恩的轨道运行周期约四分钟。

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.

撞击器工作的有效性也是一个重要指标。”我们所做的有关[影响]的模型是基于实验室规模-百分之一厘米大的目标-的碰撞物理学的理解,”米歇尔说。这些模型是否影响真实存在的小行星仍然是一个悬而未决的问题。

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.

约翰逊补充说,该技术是目前最为成熟的–人类已经证明能够接触到小行星的能力,特别是探索灶神星(Ceres)的黎明号使命和欧洲航天局探索彗星67P/Churyumov-Gerasimenko的罗塞塔任务。

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.

除了使用撞击器的方法,科学家们还利用重力–直接在行星运行轨道中放入一个大型的航空飞船,并利用两个飞行物之间的相互引力将其推入一个新的运行轨道。这种方法的好处是飞船抵达行星附近就可以实行。轨道将成为一个“光环”,一个以某点为圆心的圆形的路径,其对物体的引力相当太阳对于行星的引力作用。美国航空航天局的小行星改道任务可能会间接测试这个想法;该计划的一部分是将一颗小行星引回近地空间。

直径为250米的行星就足以造成阿里桑拿州陨石坑规模的破坏。
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,在这些方法中的一个关键因素是时间。需要整整四年才能实施一个脱离地球轨道的太空发射任务,此外航天器需要再花一到两年时间才能抵达相关行星。如果时间不够,我们恐怕不得不尝试其他方法。

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)的加利福尼亚大学(University of California)的物理学家张启程(Qicheng Zhang)认为激光可以提供另外一种途径。激光不会像一颗死星(Death Star)那样把小行星炸毁,相反,它会蒸发行星表面的一小部分物质。在实验宇宙学家菲利普.鲁宾(Philip Lubin)的带领下,张和他的同事向太平洋天文学会(Astronomical Society of the Pacific)提交了一份关于模拟轨道的学术论文。

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.

这看起来似乎没什么效果,但请不要忘记,如果时间充裕下手早,仅仅是轻轻的一推就能将行星推离轨道数千公里。张说使用激光的优点在于可以在地球轨道上建造一个大的固定激光设备,而不需要去追着行星走。一个一千兆瓦级功率的激光持续运行达一个月的时间就可以将一个80米宽的(264ft)小行星–相当于造成通古斯大爆炸的行星尺寸–推离出相当于两个地球半径的距离(约8000英里,或12800公里)。这样就足以避免撞击了。

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.

这种想法的另一个做法是发送一个搭载了较低功率激光的飞船,在这种情况下,飞船必须到达小行星附近并保持近距离飞行。由于激光相对较弱--20千瓦左右--这将花费数年才能产生效果。不过张的模拟测试显示,如果激光能够提前十五年到达行星位置,也可以将其推出需要的距离。

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.

张认为他的环地球轨道建议的一个优点在于尽管人类已经成功完成在轨追踪行星或彗星,但实际上任务本身很有难度。罗塞塔计划(Rosetta)原本的目标是另一颗彗星(46P),但是由于发射时间的延迟导致不得不临时更换目标,因为46P所处位置发生了变化,不便于追踪。“如果一颗彗星的运行目标是地球的话,恐怕我们没有那么多时间更换这个目标了”。小行星不是很难追踪,但仍然需要三年左右的时间才能抵达,”他说。

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."

而约翰逊则注意到了一个大问题,至今没有人能够发射一个任何一种尺寸为一公里长的物体进入太空,更不要提一系列激光了。“该方法有很多问题我认为并不成熟,例如如何将太阳能转换为激光使其可以进行长时间有效运转就是其中之一。”

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,还有一种选项是使用核能。如果你看过电影《世界末日》或《天地大冲撞》的话你会觉得这个选择很简单直接,但实际上比听起来难多了。”你需要把整个基础设施发射到太空,”斯凯莱德大学(University of Strathclyde)的马西米利亚诺.瓦西里(Massimiliano Vasile)说。他提出了距目标一定距离引爆核炸弹的提议。如同激光方法的原理,该计划将蒸发行星表面物质由此产生助推力,改变行星的运行轨道,”如果采用消融[侵蚀]的办法,这样的办法能效比很高,”他说。

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.

激光与核弹在接近行星时可以发挥作用,而行星表面的物质成分将是一个主要因素,因为成分本身将决定蒸发温度,不同行星各有不同。另一个问题是飞出来的碎石。有许多的小行星其实就是一堆松散石块的集合体。一个撞击器对于这样的物体效果不大。重力牵引法则更为有效,约翰逊指出,成分与物质之间的粘合度并不重要。

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,所有这些方法终将遇到一个共同的障碍:政治。1967年的《外层空间条约》禁止在外太空进行核武器试验,在太空轨道上部署一个千兆瓦的激光设备会令人紧张。

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."

张指出,如果将在轨激光设备的功率降低到0.7千兆瓦,那也仅仅能够将小行星推离约0.3个地球半径,也就是约1911公里。“能够将一座城市夷为平地的小行星比巨型的行星杀手更为常见。现在想一下,这样的一颗小行星正在向纽约城飞来,试图或者未能完全改变小行星运行轨道有可能会将撞击点转移到伦敦,试想如果面临这样巨大的风险,欧洲人是不会同意由美国人来主导改变行星轨道的。”

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.

这种障碍或许比预期的要少。”在这些条约中有一个免责条款,“约翰逊指的是《外太空和综合测试禁止条约》(Outer Space and Comprehensive Test Ban Treaties)。例如该条约并不禁止向太空发射携带核武器的弹道导弹。出于行星防御系统的需要,批评者或许会保持沉默。

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."

米歇尔说与任何其他自然灾害相比,这个灾害我们是可以避免的。”发生这样的自然灾害的机率非常低,不像是海啸什么的。但这是我们唯一可以提前预防的灾害。”
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