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  • 对生命的威胁

    For all of us, even the Earth, and even the Sun, the end is coming.  Here we describe what may be in the future for our Solar System and our Sun and what it will mean for life on Earth.
    ::对我们所有人都来说,甚至地球,甚至太阳,末日即将到来。在这里,我们描述了太阳系和太阳未来可能发生的事情,以及它对地球上的生命将意味着什么。

    Near-Earth Asteroids
    ::近地小行星

    Yes, there's an app for that! There is even a twitter that tracks and monitors the location and trajectories of   near-Earth asteroids   (NEAs), asteroids that cross or closely approach the Earth's orbit. Asteroids are rocky debris left over from the formation of the Solar System and are likely to be a component of extrasolar systems as well. Most orbit in a "belt" located between Mars and Jupiter where there are millions of asteroids varying in sizes from 1 km to hundreds of kilometers.
    ::是的,有一个应用程序! 甚至还有一个推特跟踪和监测近地小行星(近地小行星)的位置和轨迹,这些小行星跨越或接近地球轨道。小行星是太阳系形成时遗留下来的岩石碎片,很可能也是太阳系外太阳系的组成部分。大多数轨道位于火星和木星之间的“带状”中,那里有数百万个小行星,大小从1公里到数百公里不等。

    Potentially hazardous asteroids   (PHA) have been with us for millions of years, weaving in and out of Earth's orbital path. Classification as a PHA depends on the three things: 1) mass, 2) material strength, and 3) location. The mass determines the amount of force the asteroid will impart if it collides with the Earth. The material strength determines how well the asteroid will survive through the Earth's protective atmosphere, in which many meteoroids burn up. Location determines if the asteroid will collide with the Earth at all.  These three things together determine how likely it is for an asteroid to survive to hit the Earth and cause great damage.
    ::具有潜在危险的小行星(PHA)与我们在一起已有数以百万年的时间,在地球的轨道路径中编织和脱离地球轨道路径。作为PHA的分类取决于三件事sad1)质量,(2)物质强度和(3)位置。质量决定了小行星与地球相撞时的强度。物质强度决定了小行星在地球保护性大气中的生存能力,许多流星体在其中燃烧。位置决定小行星是否会与地球发生碰撞。这三件事一起决定小行星生存到地球并造成巨大破坏的可能性。

    Detection campaigns work to spot comets or PHAs against background stars. Farther away stars provide an inert frame of reference, allowing closer, moving objects to be distinguished. As of October 2016, approximately 15,000 near-Earth asteroids have been found, with an average of 30 new asteroids discovered each week. No, no, do not panic yet. Only a few are hazardous for Earth and they are all being kept close track of.
    ::在背景恒星中检测彗星或PHAs的探测工作。远处的恒星提供了一个惰性参照框架,允许更近距离、更移动的天体进行区分。截至2016年10月,已发现约15,000个近地小行星,平均每星期新发现30个小行星。不,不,不,还没有惊慌。只有少数恒星对地球有害,而且它们都在紧随其后。

     

     

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    Orbits of some of the asteroids in our Solar System are shown in blue. While there is quite a concerning number, it is clear that they are being watched and well tracked. How can we figure out the orbit of an asteroid even if we only see the asteroid move for a short amount of time? Similarly, how do you know where a ball is going to fall after it is thrown even before it lands?
    ::我们太阳系中有些小行星的轨道以蓝色显示。 虽然数字相当多, 但显然它们正在被监视和跟踪。 我们如何找到小行星的轨道呢? 即使我们只看到小行星在短短的时间内移动? 同样, 你如何知道一颗小行星在落地前就被扔到哪里会掉落呢?

    There are three classes of NEAs.   Apollo asteroids   have semi-major axes larger than 1 AU, the semi-major axis of the Earth. These make up about 62% of all NEAs, including the asteroids that made Earth-crossing orbits in December 2019.   Aten asteroids , making up just 6% of NEAs, have a semi-major axes of smaller than 1 AU.   Amor asteroids   (32%) merely approach Earth's orbit, but crosses orbits with asteroids that orbit between the Earth and Mars.
    ::阿波罗小行星的半主轴大于1AU,即地球的半主轴,占所有非主轴的大约62%,包括2019年12月成为地球跨轨道的小行星。小行星仅占非主轴的6%,小行星的半主轴小于1AU。阿摩尔小行星(32%)仅接近地球轨道,但穿越轨道的小行星在地球和火星之间运行。

     

     

    The three different types of near-earth asteroids. Why might there be so many apollo asteroids and relatively little aten asteroids?
    ::近土小行星的三种不同类型。为什么会有如此多的单极小行星和相对较少的原子小行星?

    Impact probes are used to better understand the strength and composition of asteroids. These probes are sent to crash into asteroids, causing material from the asteroid to vaporize around the probe  which can then   be analyzed. Probes can also   measure  the water content of asteroids, which gives a clue to the water abundance in the Solar System. It is theorized that much of the water on Earth was once delivered to Earth through asteroids.
    ::撞击探测器被用来更好地了解小行星的强度和组成情况,这些探测器被送入小行星,造成小行星材料在探测器周围蒸发,然后加以分析。探矿还可以测量小行星的水含量,这为太阳系的水量提供了线索。据推测,地球上的许多水曾经通过小行星输送到地球。

    In 1908, the Tunguska asteroid exploded over Siberia with a force comparable to 10-15 megatons of TNT. Thought to measure 120 m across, the explosion fortunately occurred over a largely uninhabited area. Some settlers reported seeing a flash and a large explosion as the asteroid burst in the air about the ground. Shock waves knocked down millions of trees in a 800 mile radius and registered a 5.0 on the Richter scale. Now, 100 years later, the trees are only just beginning to recover from the explosion.
    ::1908年,通古斯卡小行星在西伯利亚上空爆炸,其威力相当于10-15兆吨梯恩梯。 为测量120米宽,幸运的是,爆炸发生在一个基本上无人居住的地区。一些定居者报告说,当小行星在地面的空中爆炸时,看到闪光和大爆炸。冲击波在800英里半径范围内击落了数百万棵树木,记录了里氏5.0吨。现在,100年后,树木才刚刚开始从爆炸中恢复过来。

     

     

    The site of the Tunguska asteroid explosion.
    ::通古斯卡小行星爆炸现场。

    Russia was hit again in February 2013 when a 20 m wide meteor crashed down  with   the force of 500 kilotons of TNT.  Shock waves from the explosion shattered windows  in hundreds of buildings.  The explosion itself was  hot and bright enough to cause skin and retinal burns.
    ::2013年2月,俄罗斯再次被击中,当时20米宽的陨石被500千吨TNT的威力击落。 爆炸的冲击波粉碎了数百座建筑物的窗户。 爆炸本身的热度和亮度足以造成皮肤和视网膜灼伤。

    Asteroid impacts release substantial amounts of collision energy and can cause the type of devastation witnessed by all the now super-dead dinosaurs of the Cretaceous period. What then, are the real chances that the Earth, or any other habitable planet, is hit by an asteroid capable of that type of global destruction?
    ::小行星撞击释放了大量碰撞能量,并可能造成白鲸时代所有超死恐龙所目睹的那种破坏。 那么,地球或任何其他可居住行星被能够遭受这种全球毁灭的小行星撞击的真正可能性是什么?

    Every day, earth is bombarded by more than 100 tons of dust and sand-sized particles from space that burn up in the atmosphere. Roughly once per year, an automobile-sized asteroid will hit the Earth's atmosphere. It will create a glowing fireball in the atmosphere, but at this size, the asteroid will still burn up before reaching Earth's surface. From calculations, we know that roughly every 2,000 years, an asteroid larger than the size of a football field could hit the Earth and cause significant local damage. Only every million years or so will there be an object with large enough mass to threaten the Earth's entire civilization.
    ::每天,地球都会被100吨以上的尘埃和沙尘大小的微粒从太空中轰炸,这些尘埃在大气层中燃烧。每年约有一次,一个汽车大小的小行星将撞击地球大气层。它将在大气层中产生一个发光的火球,但以这种规模,小行星仍将在到达地球表面之前燃烧。根据计算,我们知道大约每2000年,一个大于足球场大小的小行星可能撞击地球并造成重大的当地破坏。只有每100万年左右,才会有一颗大到足以威胁地球整个文明的物体。

     

     

    Map of known meteor craters on Earth's continents.
    ::地球各大洲已知陨石坑地图

    Is there no hope? Yes, there is hope. There are a number of proposals on how to deal with approaching asteroids. We could always blow up the asteroid, but it will be hard to mitigate the debris from such an attack, and we currently only have the resources to dispel an asteroid of about 1 km in diameter.   Th e   Treaty on Principles Governing the Activities of States in the Exploration and Other Uses of Outer Space   disallows the use of nuclear weapons in space.
    ::没有希望吗?是的,有希望。有许多关于如何处理接近小行星的建议。我们总是可以炸毁小行星,但很难减轻这种攻击造成的碎片,目前我们只有资源驱散直径约1公里的小行星。《关于各国探索和其他使用外层空间活动所应遵守原则的条约》不允许在空间使用核武器。

    Other suggestions propose changing the orbit of approaching asteroids just enough so that they are no longer on a collision course with the Earth. This could be done using a gravity tractor or solar sails, or by attaching a ballast to the asteroid that will change its center of mass  just enough to change   its trajectory. It may also be possible to use concentrated lasers to vaporize part of an incoming asteroid. The vaporized areas of the rock will let off plumes of super-heated gases and debris that will have the same effect as a rocket and propel the asteroid in a different direction. While certainly threatening, asteroids pose only an occasional threat to life and perhaps not a threat at all to advanced enough civilizations.
    ::其他建议建议改变接近小行星的轨道,使其不再处于与地球相撞的轨道上。这可以通过重力牵引机或太阳帆,或者对小行星附加压载器,从而改变其质量中心,从而改变其轨道轨迹。还有可能使用集中激光来蒸发一部分即将进入的小行星。岩石的蒸发区将释放超热气体和碎片的羽流,这些热气体和碎片将产生与火箭同样的效果,并将小行星推进到不同的方向。虽然小行星当然具有威胁性,但只是偶尔对生命造成威胁,也许不会对足够先进的文明造成威胁。

       Domesticated Asteroids
    ::家化小行星

    While it seems   most people would agree  that the only good asteroid is a far away asteroid, others have proposed sending out a probe to capture an asteroid and bring it back. It could be parked near the moon and enable astronauts to regularly go and examine what is usually so inaccessible. In July 2014, NASA sent out a call of proposals for ideas on how to accomplish such a mission and received over 400 proposals from private companies to international organizations!

    Solar System Stability
    ::太阳能系统稳定

    Planets are kept in their orbits primarily through gravitational interactions between the planet and their host star. However, planets also feel a much smaller but non-negligible gravitational pull from the other planets in the system. Though the planets in our Solar System are relatively well separated, this is not always the case in other Solar Systems, and may not even always be the case for our own system. The closer two bodies are to each other, the stronger their gravitational pull will be on each other.
    ::行星留在轨道上,主要通过行星与其主星之间的引力相互作用。然而,行星也感到与系统中其他行星的引力拉动较小,但不可忽略。 尽管太阳系中的行星相对分离,但其他太阳系中的行星并不总是如此,甚至不一定总是如此。两个体彼此距离越近,它们的引力拉动就会越强。

    Close encounters between planets can affect their orbits' eccentricities, semi-major axes, and relative inclination. These effects are accentuated in the case of resonant systems.   Resonance   describes the situation in which two planets have periods that form an integer ratio. For example, if one planet has a period of two years and a second planet has a period of four years, these two planets are said to be in resonance. Planets in resonance will always approach each other at the same point in their orbit. This means their strongest gravitational interactions will always occur at the same point and become additive. Today, we can use computer simulations to determine what will happen to different systems of planets as they evolve through time that will expose these types of interactions.
    ::行星之间的近距离接触可能会影响它们的轨道偏心、半主轴和相对倾角。 这些效应在共振系统的情况下会更加突出。 共振描述两个行星的时段形成一个整数比率的情况。 例如,如果一个行星的时段为两年,而另一个行星的时段为四年,这两个行星据说是共振的。 共振中的行星总是在其轨道的同一点相互接近。 这意味着它们最强大的重力相互作用总是在同一点发生,并成为添加剂。 今天,我们可以使用计算机模拟来确定不同行星系统随着时间的演变会发生什么样的变化,从而暴露这些类型的相互作用。

     

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    Resonance among the moons of Jupiter. This animation shows the orbits of three of Jupiter's largest moons that happen to be in resonance. Whenever two moons reach their closest approach, they flash a corresponding color. Note that these flashes occur at the same time each time in the orbit. This translates to each moon feeling the strongest gravitational pull from another moon at the same place each orbit. The same happens with planets. What would happen to these orbits if the inner planet was the smallest planet? What if the outer planet was the smallest planet?
    ::木星卫星之间的共振。 这个动画显示了木星最大的三个卫星的轨道, 碰巧是共振的。 当两个月到达最接近时, 它们会闪烁一个相应的颜色。 注意这些闪光会同时发生, 每次在轨道上。 这让每个月感到每个轨道上从另一个月中 最强烈的引力拉动。 行星也会发生同样的情况。 如果内行星是最小的行星, 这些轨道会发生什么? 如果外行星是最小的行星呢?

    Planets may also move around during formation. This may push smaller, more-likely rocky planets into the host star, or send debris like asteroids and comets into the path of other planets. Such motion in our Solar System is thought to have caused the period of late heavy bombardment, which left its mark on our Moon.  This type of mechanism may have caused the strange position of hot Jupiters, which are commonly found and much closer in than where they could have formed.
    ::行星在形成过程中也可能四处移动。 这可能会将更小、更有可能的岩石行星推入主星,或者像小行星和彗星一样将碎片送入其他行星的轨道。 我们的太阳系中的这种运动被认为已经造成了后期猛烈轰炸的时期,在月球上留下了它的印记。 这种机制可能造成热木星的奇怪位置,这些位置通常被发现,而且远比它们可能形成的位置更近。

     

     

    A proposed model of planet migration in the early Solar System. The different a, b, c, and d lines represent different snapshots in time. The x-axis is distance from the Sun in AU. Smaller circles represent planetessimals before they have formed into full planets. The movement of Jupiter and Saturn in this model appears to have affected the formation of the close-in terrestrial planets, which are favorites for habitability. How does the current position of the asteroid belt figure into this model? What about the Kuiper belt?
    ::早期太阳系中拟议的行星迁移模型。 不同的 a、 b、 c 和 d 线代表不同时间的截图。 X 轴与 AU 中的太阳距离很远。 小圆圈代表了尚未形成完整的行星的行星。 这个模型中的木星和土星的移动似乎影响了近距离地球行星的形成, 这些行星是适合居住的最喜爱的。 小行星带目前的位置如何出现在这个模型中? Kuiper 带呢?

    In 2008, a paper by Batygin and Laughlin showed that on million-year time scales, planetary orbits may evolve in random, but ultimately stable ways. However, on longer timescales, orbits can rapidly and spontaneously evolve into chaotic states hard for even the best computer simulations to predict. Subtle interactions between the different planets and objects with mass in the solar system are capable of enacting significant change over the dynamical state of a system of planets. Close encounters will amplify these changes, rearrange orbits, and eject small bodies out of the Solar System. Resonant affects could even depopulate entire areas in a planetary system. The architecture and stability of a planetary system are important contributors to a planet's potential habitability.
    ::2008年,Batygin和Laughlin的一篇论文显示,在百万年的时间尺度上,行星轨道可能以随机的方式演变,但最终是稳定的方式演变。然而,在更长的时间尺度上,轨道可以迅速和自发地演变成混乱状态,即使是最好的计算机模拟也难以预测。不同行星和太阳系中质量物体之间的细微相互作用能够使行星系统的动态状态发生重大变化。近距离相遇将扩大这些变化,调整轨道,并将小体从太阳系中分离出来。共振效应甚至可以使行星系统中的整个地区均分解。行星系统的构造和稳定性是行星系统潜在可居住性的重要贡献者。

    Solar Evolution
    ::太阳能进化

    The Sun, the light of our lives, formed 4.6 billion years ago and will last in its current state for about 5 billion more years. Recall that the Sun is fueled by nuclear reactions in its core that fuse hydrogen atoms to form helium and energy. This makes it a main-sequence star. These reactions serve to generate the outward pressure that balances with the Sun's gravity which pulls inwards.
    ::太阳,我们生命的光芒,在46亿年前形成,并将在目前状态下再持续约50亿年。回顾太阳的核心核反应为太阳提供了燃料,核反应将氢原子结合成氦和能量。这使得太阳成为主序列恒星。这些反应有助于产生与太阳引入的引力平衡的外向压力。

    In 5 billion years, the sun will run out of the hydrogen it needs to fuel this outward push. It will instead begin to combine helium atoms to make carbon atoms, which will release far more energy. The Sun will begin to expand and blow up into a   red giant , whose radius will be approximately the orbit of the Earth.
    ::在50亿年的时间里,太阳将耗尽它所需的氢气,以刺激这种向外推力。相反,太阳将开始结合氦原子来制造碳原子,这将释放更多的能量。太阳将开始膨胀,并爆炸成红巨人,其半径将接近地球轨道。

    Ultimately, the Sun will also run out of helium to burn. It will eject its outer envelope of hot, charged plasma, which is misleadingly named a   planetary nebula . What remains is a   white dwarf , which still has the approximate mass of the once great Sun, but is about the size of Earth. The incredibly dense white dwarf is held up only by   electron pressure,   generated  from the inability of electrons to occupy too close a space with one another.
    ::最终,太阳也将用完燃烧。 它会喷出热电荷等离子体外包, 它被误导地命名为行星星云。 剩下的是白色矮星, 它仍然拥有曾经伟大的太阳的近似质量, 但它与地球的大小有关。 异常稠密的白矮星只能靠电子压力支撑, 电子无法相互占据太近的空间而产生。

     

     

    A time line of how the Sun will evolve. How would you expect the timeline to differ for larger or smaller stars?
    ::太阳如何演进的时间线。 您如何期待大恒星或小恒星的时间线会有所不同 ?

    Distant planets may continue to orbit the dead stars since the mass of the star remains nearly constant, but planets within the orbit of Earth are not similarly saved. As a red giant, the Sun will expand nearly to the Earth's orbit. Close-in planets, such as Mercury and Venus, will become engulfed within the Sun itself. If the Sun does not engulf the Earth, it will certainly irradiate the Earth with an excessive amount of high-energy radiation that no amount of sunscreen can withstand. The different mass distribution of the Sun throughout its different evolutionary stages may also play significant havoc with close-in orbits. The habitability of close-in planets is strongly upper limited by the time the host star remains on the main sequence, which is determined by the mass of the star.
    ::由于恒星质量几乎保持不变,远方行星可能继续绕着死星环绕着恒星运行,但地球轨道内的行星却没有类似的保存。作为红巨星,太阳将几乎扩展到地球轨道上。近距离行星,如水星和金星,将会被太阳本身吞没。如果太阳不吞没地球,它肯定会用过多的高能辐射对地球进行辐照,而太阳无法承受这种辐射。太阳在不同进化阶段的不同质量分布也可能对近距离轨道造成重大破坏。当主恒恒留在主序列上时,近距离行星的可居住性将受到极大的限制,而主序列是由恒星的质量决定的。

     

     

    A cartoon comparing the Sun as a main-sequence star and a red giant. The orbits of the planets, though not to scale with each other, are provided for reference. After the Sun transitions to a red giant, Mercury and Venus will become completely engulfed by the Sun. Why is the habitable zone so much further for the Sun as a red giant?
    ::将太阳比作一个主序列恒星和一个红巨星的漫画。 行星的轨道, 虽然不相互相向, 却被提供参考。 在太阳向红巨星过渡后, 水星和金星会被太阳完全吞没。 为什么太阳作为红巨星的可居住区会更远呢 ?

       Planet Eaters
    ::地球食人者

    White dwarfs no longer have nuclear fusion providing an outward force. This causes heavier elements to settle down to the core of a white dwarf due to the effects of gravity (like the way sediment will settle in a glass if it is kept still). Therefore, white dwarfs should have only the lightest element, hydrogen, on their surfaces.
    ::白矮星不再有核聚变提供外向力量。 这导致重元素沉积到白矮星核心,因为重力效应(比如沉积在玻璃中沉积的方式,如果沉积不动的话 ) 。 因此,白矮星表面应该只有最轻元素氢。

    In 1917, astronomer vanMaanen found a white dwarf that showed traces of oxygen, magnesium, silicon, and iron on its surface, elements that commonly indicate the presence of rocks. This may well have been the first discovered exoplanet - or rather, the remains of an exoplanet plastered onto the surface of its once hospitable host star.
    ::1917年,天文学家范马纳宁发现了一个白色矮星,其表面有氧、镁、硅和铁的痕迹,这些元素通常表明岩石的存在。 这很可能是第一个发现的外行星,或者说,一个外行星的残骸被粘贴在曾经好客的宿主恒星的表面。