【新闻周刊 20150611】我们可以逃离小行星撞毁地球的命运

满仓

【中文标题】我们可以逃离小行星撞毁地球的命运
【原文标题】We Can Save Ourselves From Earth-Killing Asteroids, But Someone Has to Pay
【登载媒体】新闻周刊
【原文作者】NINA BURLEIGH
【原文链接】http://www.newsweek.com/2015/06/19/we-can-save-ourselves-earth-destroying-asteroids-someone-has-pay-341823.html




今年春天，利比亚的暴力和混乱让数千名难民登上临时拼凑的船只前往意大利，数名难民溺死在水中。与此同时，科学家们齐聚罗马市郊外，讨论另外一个人类大灾难的应对方式。来自世界顶尖机构的天文学家和物理学家在商讨一个恐怖的预测：一个大约直径1300英尺的小行星——足够给地球带来巨大的灾难——正在向地球飞来，位于其着陆点的国家属于世界上最贫穷、最动荡的地区。决策者们争论的话题是应该将其摧毁，还是改变其运行轨道。如果让小行星偏离轨道，它有可能落在其它国家的领土，而这些国家不惜为此而开战。

别紧张，这仅仅是一个演习。如果真正的危机爆发，你就需要跟地球吻别了。

有两个美国人在全面关注这次为期5天的小行星战争模拟游戏，一位来自科学界，另一位来自军事领域。这些所谓行星领域的资深政治家的责任是提醒决策者，地球——以及地球上的所有生命——都曾遭受来自外太空巨型岩石撞击的影响。戴夫•莫里森和其他一些人曾经建议，与受小行星影响而灭绝的恐龙不同，我们或许可以保护自己。美国空军前中尉林迪•约翰逊后来被任命为宇航局“近地物体”项目的负责人，他在90年代建议空军要密切关注小行星体。让他们以及全世界所有密切关注行星防卫事务的人骄傲的是，目前最大的问题已经从如果太空中的一块岩石向我们冲来该怎么办——我们都知道巨大的影响不可避免，演变成我们该如何应对。

这个问题是4月中旬在罗马郊区弗拉斯卡蒂举行的一个会议的主要课题。欧洲航天局邀请了宇航员、物理学家、核工程师和数学家，他们在讨论太空岩石撞击地球，造成区域性灾难，甚至文明的终结这种极其渺茫的可能性。和前六次行星防御会议一样，这次会议的主题是分享小行星威胁的信息和拯救人类的方法。

今年会议重点关注的问题是，各个国家在面对威胁时，是否会联合起来。今天的科学家可以十分肯定地告诉我们，一个物体在，比如说，200年之后将会撞击地球，而且他们认为我们有足够的技术来避免灾难的发生。但是没人知道人类在全球大灾难面前，是否能够，或者愿意联合起来。当很多政客连人类行为对气候造成的影响都不愿承认时，我们可以指望他们相信小行星的威胁是真正存在的吗？


人类是地球上唯一可以发现小行星威胁，并且可以思考如何阻止行星杀手撞击地球的物种。

大盲点

作为卡尔•萨根的第一个博士学生，莫里森在1989年与克拉克•查普曼合著的一本书《宇宙大灾难》中最先向公众警告小行星的威胁。他说：“三十年前没有任何针对近地物体的研究，人们几乎一无所知，也没有什么可以研究的东西。”

从那以后，这个研究逐渐囊括了国家太空署、国会、联合国和有诸多数学家、物理学家、工程师、火箭科学家、甚至核武器设计者的实验室。在他们的努力下，超过15万颗小行星已经被史密森尼小行星中心登记注册。这些捍卫者估计，还有数万颗到数十万颗小行星无法被观测到，很多都位于我们的盲点——被太阳的光线所掩盖。大约有1.27万颗小行星被确认为“近地物体”，他们的轨道距太阳1.21亿英里之内。美国宇航局认为，大约1000颗近地物体属于具备终结地球文明的能量，它们的直径超过0.5英里。这些庞然大物都没有撞击地球的威胁，但是另外1600个近地物体或许与地球的运行轨道有交集，撞击将导致数百万人丧生。


一位艺术家创作的织女星小行星带。织女星是银河系中遥远的一颗星球，它是科学家在太阳系之外研究最多的一颗星星。

尤卡坦的大灭亡

人类在十七世纪发现了第一颗彗星，但类似彗星的物体在历史上屡见不鲜，圣经和其它古老的文献中都有相关的记载。第一颗小行星在十九世纪被发现，但是直到二十世纪初，我们才意识到有些小行星会穿越地球的轨道。科学家现在知道有数千个“地球穿越者”，而且我们极为幸运，因为木星和土星吸引了很多本来会把地球打得千疮百孔的小行星。

已故地质学家基恩•苏梅克是一个科学天才，他在19岁时毕业于加州理工学院。50年代，他在美国太空项目中检视月球的环形山，他确定这些环形山是撞击造成的。后来，他被任命为美国地质勘探太空科学中心在亚利桑那旗杆镇的负责人。他和他的团队开始观测小行星，研究陨石撞击的规律。他和另外一位科学家爱德华•巢一起发现了柯石英——一种在剧烈撞击之后出现的二氧化硅。但是他在行星防御领域最重要的发现是苏梅克－列维9号彗星，这颗彗星在1994年撞击木星。这是人类第一次预测并且真正观测到的外太空星体撞击现象。这个成就让科学家有信心认为，类似的计算也可以应用在地球身上。

当苏梅克整理他有关亚利桑那温斯洛附近的陨石坑中不寻常的硅沉淀物笔记时，地质学家沃尔特•阿尔瓦雷茨在分割白垩纪和第三级的地层中——也就是恐龙年代与我们目前的年代之间，发现了富含铱元素的泥土。铱在地球上非常稀少，但在陨石中非常普遍。地质学家很快在世界其它地方也发现了类似富含铱元素的地层。他们因此推测，大约在恐龙灭绝的同时，地球上发生了灾难性的陨石撞击。科学家甚至知道了导致恐龙灭绝的陨石撞击地点——就在尤卡坦半岛附近的希克苏鲁伯。

在之后的十几年里，地质学家了解到更多外太空对我们的星球所造成的灾难性影响。他们认为，月球就是在地球形成最初一亿年的时候，两个与火星和金星体积类似的星体撞击后形成的。在这次撞击之后，地球被包围在炙热的硅酸盐气体中，只有一些喜欢温热环境的有机物生活在距离地表0.5英里以下的岩石中。还有无数颗直径为5到10英里的小星体——与导致恐龙灭绝的星体类似——撞击地球，其影响稍小，但也是灾难性的。


1957年11月25日，华盛顿特区，爱德华•特勒博士对美国参议院军委筹备委员会说，他相信俄罗斯有能力使用洲际弹道导弹打击休斯顿和德克萨斯等遥远的目标，而且“他们很快就会采取行动”。特勒强力支持核科技和核武器，后来他努力推进利用核武器摧毁或者改变小行星的轨道，使其远离地球。

使用炸弹

查普曼和莫里森在1989年出版有关宇宙大灾难的书时，掩盖了一些事实，包括彗星、小行星和超新星。但是他们认为，小行星撞击的可能性最值得提出，因为人类在理论上有可能阻止其发生。1990年国会邀请莫里森讲述他的有关太空星体威胁的发现。一年之后，国会授权宇航局研究小行星，以及改变其运行轨道的方法。

查普曼和莫里森聚集起一批天文学、物理学和地质学专家来研究这个问题。这个团队的结论是，最危险的小行星直径为1英里左右。这样大的石块（相当于导致恐龙灭亡的小行星体积的十分之一）具有终结人类文明的威力，主要是通过撞击之后形成的尘埃造成气候变化，数十亿人将死于饥饿。于是他们建议找出符合这个体积标准的所有星体。

除了天文学家和地质学家，行星防御工作小组还招募了核武器设计人员。这些人在冷战结束之后几乎已经失业，但借此找到了可以应用他们专业知识的新场地，核武器是防御小行星威胁的一个选择。这其中就有奇爱博士——爱德华•特勒本人，他是美国核武器的先驱人物之一。还有热爱和平的萨根。两个人针对核武器问题发生了激烈的争吵，但他们都认为核武器可以拯救我们免于小行星灾难。


天文学家卡尔•萨根博士与戴夫•莫里森在第一次环恒星适居区会议上。作为萨根的第一个博士学生，莫里森在1989年与克拉克•查普曼合著的一本书《宇宙大灾难》中最先向公众警告小行星的威胁。

小王子行星

行星防御项目不仅仅依靠民间科学家。宇航局小行星项目负责人约翰逊是一名退休的上校，他曾经在空军部队中负责监控卫星。1994年，他发表了一篇有关空军在2020年需要具备的作战能力的论文，之后开始了小行星领域的研究。约翰逊在论文中主要关注小行星，论文题目就是“为行星防御做准备”——这个名词因此而来。在空军服役23年之后，约翰逊在2003年宣布退休，但是宇航局邀请他负责近地物体项目。

在卫星防御计划的发展中扮演关键角色的第三个人是拉塞尔•铁锈•施威卡特，他在阿波罗9项目中第一个在外太空行走。2002年，他创办了B612基金会（名字来源于安东尼•德•圣艾修伯里的《小王子》中行星的名字）。地质学家诺姆•斯里普的一次演讲启发了他，斯里普在演讲中讲述了33亿年前——远在恐龙时代之前——的小行星撞击给地球带来多么大影响，海水沸腾、新的物种形成。施威卡特几十年来致力于偏离导向的技术研究，他还试图邀请其它宇航员加入，发现了一些志同道合的宇航员。其中一个人就是目前B612基金会的负责人卢杰。

施威卡特游历各个国家，试图鼓励人们采取一致的行动。他说：“我很担心人们没有足够的勇气克服政治层面的阻挠，也就是说，我们遭遇小行星撞击灾难的原因并不是技术上的，也不是我们没有预见到。”

善恶大对决

当印度洋海啸在2004年12月26日袭击了14个国家，夺走了23万人生命的时候，这个事件引起全球关注的同时，也掩盖了另外一个几乎同时发生的——尽管是理论上的——与死神擦肩而过的大事件。印度洋海啸发生前的48小时，科学家计算出一个危险的结果：一块直径为885英尺的太空陨石正向地球飞来，在2036年有25分之一的几率与地球相撞，其能量相当于5.8万颗广岛原子弹。印度洋地震所造成的海啸，不及其影响的一半。

这颗呼啸而来的陨石随即被命名为“阿佩普”——古埃及破坏之神。在它被发现的6个月之后，小行星并没有引起小行星中心的关注。但是到了12月份，波多黎各和亚利桑那的天文学家已经在加利福尼亚帕萨迪那的喷气推进实验室收集了足够多的数据，认为阿佩普有2.4％的几率在2029年撞击地球，在2036年还有25分之一的机会与地球的运行轨道重合。这些数据让更多的科学家开始了紧张的研究，最终，他们把撞击几率锁定为令人比较放心的25万分之一。阿佩普离我们最近的时候，会在地球和月球之间穿过，人类肉眼可以看到。

对于行星防御者来说，这个事件是值得高兴的，暂时不需要警惕。

现实中的死星是行星防御者们迫切需要的一个东西，他们需要以此来唤醒政客和公众。但这样的信息是一柄双刃剑。公众脑海中的景象已经被好莱坞的灾难影片定型了（1998年的《天地大冲撞》和《绝世天劫》），也就是从天而降的灾难场景。现在，真正的末日劫难以印度尼西亚的海啸和小行星威胁的方式展现出来，公众肯定需要答案。但是所有人给出的诚实回答，仅仅是一个让记者和公众都无法接受的词。

那个词就是“不确定”。


欧洲太空船罗塞塔号在2010年7月11日拍摄到的小行星体司琴星。司琴星位于火星附近的小行星带，它已经围绕太阳运转了45亿年。我们对小行星体的认识，已经从一个瞄准我们飞来、带来巨大灾难的太空岩石——现在我们知道碰撞不可避免——演化为我们该如何应对。

“伦敦、巴黎，等着瞧吧……”

加拿大籍天文学家保罗•柯达斯对这句话了然于胸，每当小行星新闻见诸报端，记者们都知道能从他这里探听出有用的消息，他就经常会说这句话。留意小行星的动向是他在喷气推进实验室的工作之一，他在那里负责运作宇航局的近地物体项目办公室。他会给一个小行星设定很多参数——旋转幅度、质量、波段反射、辐射量，以及其它星体的引力——并输入一个超级计算机，然后得到预测的运行轨道。柯达斯和他的同事曾经计算出阿佩普有25分之一的几率与地球相撞。我问他，这些令人恐怖的数字让他害怕还是兴奋。他笑着说，的确有些兴奋。

他的预测中包含了很多变量，这些变量的微小差异所带来的结果可能相差十万八千里——即使用太空距离来衡量也是无法忽视的差距。柯达斯和他的同事们所纠结的那些不确定因素尽管都是微小的变量，但是会累计成巨大的数量级。例如，对于小行星体的重量估算的误差如果达到三颗葡萄的重量，其结果就可以决定它是否会击中地球。小行星旋转的速度收到热量的影响，而热量决定于岩石表面反射光线的能力。形态各异的小行星让这项工作变得更加复杂，有些是一堆游离的瓦砾，有些是坚硬的岩石，还有些是被引力聚集在一起的尘埃。

喷气推进实验室的研究人员不断提炼复杂的公式，他们试图把小行星运行轨道的决定因素简化为6个基准线。他们已经取得了一些进展。

喷气推进实验室在7年前有了一个机会来测试他们的公式。2008年10月的一个早晨，柯达斯送孩子去学校的时候，手机突然响了。哈佛大学小行星中心的人报告，一个近地天气正在加速向地球撞来。柯达斯把这块岩石的坐标输入计算机，很快预测出撞击的时间和地点——20小时之后的中东。喷气推进实验室立即联系到宇航局的约翰逊，约翰逊又告知国务院（根据当时的新闻秘书，有人还通知了乔治•W•布什总统）。约翰逊尤其纠结的是，是否要通知这个动荡地区的政府。他一本正经地说：“我们曾经认为撞击的地点是麦加，这太令人伤脑筋了。”

柯达斯和他的同事史蒂夫•切斯利进一步分析有关的数字，很快得到了一个精确的撞击点——苏丹沙漠地区荒无人烟的地带，整个地区的居民不超过10个人。切斯利通过GPS确定了装机店，而柯达斯使用的工具是地图。当他们把结果相比较的时候，柯达斯抑制不住内心的兴奋：“我发现地球上知道即将发生什么事的只有我们两个人。”

撞击发生之后，喷气推进实验室的科学家们指引一队喀土穆的大学生到现场调查。让柯达斯自己都很吃惊的是，苏丹学生们就在公式计算出的地点找到了撞击残余物。

尽管如此，科学家依然在与诸多不确定因素纠结。最近一次的小行星事件就没有被预测到。2013年，一个“仅相当于”一辆公交车大小的太空岩石在西伯利亚车里雅宾斯克上空爆炸，其威力相当于一颗原子弹。周边建筑物的窗户被震碎，1000多人住院治疗。俄罗斯很多司机在车上都安装了摄像头，所以科学家们在YouTube上看到了很多相关视频。先是一片夺目的亮光，之后是震耳欲聋的爆炸声，他们以此来确定该物体的运行轨道。

车里雅宾斯克让行星防卫者们意识到，即使一个体积较小的星体，与地球没有接触，而是在天空爆炸，会造成多么大的威胁。他们知道，纽约、伦敦、德里和东京发生这样的悲剧仅仅是时间问题。


阿波罗17号在执行最后一次登月任务时，拍摄的新月形地球照片。

把它们全部找到！

为了回应阿佩普的威胁，国会通过了乔治E布朗法案，布什总统在2005年签署了这项法案，要求宇航局侦查、跟踪、记录和详细描述直径大于460英尺的小行星的物理特征。（布朗是众议员科学委员会主席，声望很高，他最早发出气候变化和近地物体威胁的警告声。）换句话说，美国最终采纳了莫里森在15年前提出的建议：把它们都找到。

寻找小行星的工作包括三个部分：亚利桑那州和夏威夷的天文望远镜，和一个叫做NEOWISE的喷气推进实验室项目——近地物体光红外线探测仪，这是一种体积较小、架设在外太空望远镜上、捕捉红外波长的设备。2011年秋天，喷气推进实验室负责寻找小行星工作的艾米•曼泽尔宣布，他们已经获取了足够多的数据，可以判断地球——目前看来——不会成为任何巨大的、导致文明终结的物体的目标。但是还有无数未被发现的小型物体在我们的周围游动，曼泽尔说，他们只找到了百分之一直径超过60英尺的近地物体。这给我们带来了另外一种，或许是更令人无法接受的挑战，因为这些物体更难被找到，而且更容易击中我们。直径450英尺左右的小型物体会造成严重的区域性破坏，而被我们发现的这类物体大约只有25％。地质学家认为，平均每100年到300年会有一个直径150英尺到450英尺的物体击中地球，有些引发了巨大的灾难。宇航局正在考虑曼泽尔的建议，建造一座新的太空望远镜，以寻找更多的小行星体。如果这项计划得到批准，有机会在2020年开始运作。

“像白宫一样大”

阻挡一个小行星的技术手段尚处于萌芽阶段。基本上有三个方法可以使用，可以简单被称为炸、踢和拉。所谓炸，就是把一个炸弹（当然不是传统意义上的炸弹）——更可能是用很多炸弹——安置在小行星上。尽管它或许会呈现出好莱坞式的震撼效果，但行星防御委员会把它列为最后一个选择。

其它两个选择是“踢”（向小行星发射一个“动态抛射体”，将其稍稍撞离轨道）和“拉”（向小行星运行的轨道附近发射一艘无人驾驶飞船，使其具备足够的质量，利用引起让小行星脱离原有轨道）。

所有这三个方法都取决于人类飞船是否有能力飞向小行星。欧洲太空署的一个项目在去年11月做到了这一点，罗塞塔号飞船在一个彗星上放置了菲莱探测器，并且在电池电量耗尽前连续向地球发送了64个小时的数据。

这些技术从未被测试过，但是宇航局希望在2020年的“小行星转向任务”中测试“拉”的技术。一个机器人宇宙飞船将会从小行星上切下一块岩石并带走。作为这个项目的一部分，机器人宇宙飞船和它承载的货物将会在环绕小行星的轨道上运行100天。科学家相信，飞船加上货物的质量将会让小行星稍稍偏离原有轨道。之后，飞船会把那块切下的岩石送到月球轨道上，并留在那里，等待未来进一步的检测。

把一块岩石放到月球的轨道上，相当于给月球增加一个卫星，这个主意虽然听起来像是科学幻想，但是行星防卫者希望国家和太空科研机构可以真正贡献一些资金。政策制定者、记者和科学家可以理性、平静、现实地讨论这个威胁，既不陷入慌乱，也不要盲目乐观。

在未来，公众应该会更频繁地听到有关星体与地球擦身而过，甚至直奔我们飞来的消息。柯达斯和其他一些人建议宇航局像气象学家谈论飓风那样谈论小行星的风险，比如说通过新闻媒介每小时汇报事情的进展，以及与联邦应急管理局和负责疏散居民的地方当局合作。

实际上，在这个月晚些时候公众就会听到更多有关小行星的消息。一批包括天文学家、物理学家、摇滚乐明星和电影导演在内的人打算在6月30日的“小行星日”举办一系列活动。之所以选择这一天，是因为1908年6月30日，一颗小行星在遥远的西伯利亚丛林摧毁了数千平方英里的土地，被称为通古斯大爆炸。组织者和参与者包括皇后乐队吉他手、天体物理学家布莱恩•梅、英国皇家天文学家马丁•里斯、美国科学家比尔•奈伊、宇航员卢和施韦卡特。世界各地都将举办会议，伦敦和旧金山的规模最大。小行星日的组织者还在网络上发表了一份请愿书《100X声明》，呼吁为寻找和跟踪小行星追加大笔投资。文件中提到：“在我们的太阳系中有无数颗小行星可能会击中地球，导致城市被摧毁，但是我们目前只发现了不到1万颗——仅仅是1％。我们有足够的技术能力来改变这个局面。”


2013年2月16日，距离俄罗斯车里亚宾斯克80公里处的切巴尔库尔湖冰面上有一个大洞，人们认为是一个流星碎片造成的。

小行星之怒

经过8年的努力，一个联合国委员会终于在3月份宣布将建立一套全球预警系统，保护地球躲避那些可以摧毁城市、引发海啸，甚至终结文明的大型太空物体袭击。行星防御者们在4月中罗马的郊区进行了演习，他们的目的是，在面对一个体积相当于4个橄榄球场的小行星时，如何拯救地球。他们所测试的技术和标准极为贴近现实，以至于他们在网络实时报道中不得不用红色字体提示“演习！演习！并非真实事件！”

宇航局的约翰逊说，通过这次演习，他相信人类可以有效应对小行星的威胁，而且成本可控。在他们试图说服政客采取措施预防一生中有可能都不会发生的事件时，这是一个关键因素。他说：“每年全世界只需要几百名专家和几亿美元，就足够识别出潜在危险，并设计出防范措施。”

柯达斯为演习设置了一个真实的场景。一开始，参与者们了解到，科学家“发现了”一颗直径为460英尺到1300英尺之间的小行星，在7年之后，也就是2022年9月3日即将撞击地球。参与者被分为三个角色扮演小组——政策制定者、媒体和科学家，他们需要在5天之内规划出应对方案。

小行星被发现的第一年（会议的第一天和第二天），参与者中的科学家利用已知的信息预测出一个“危险走廊”，从东南亚延伸到土耳其。随着小行星逐渐接近，科学家不断调整预测结果，测算出更加精确的小行星体积和撞击地点。到了2019年8月（会议的第四天），全球政策制定者们一致同意发射6枚动态撞击设备，这些设备在6月之后抵达小行星。但是撞击所产生的碎片云让科学家和政策制定者在2021年1月才观察到撞击的结果如何（会议第五天）。他们宣布，两个动态撞击设备错过了目标，一个将小行星击碎，其中一大块碎片依然在飞向地球，而且隐藏在太阳的光线中。另外两个也击中了小行星，让最大一块碎片改变了轨道。

接下来的一年（会议第五天晚些时候），参与者发现小行星碎片依然直冲地球飞来，蕴含巨大的危险。它将在2022年9月3日撞击印度、孟加拉国或者缅甸的某地。在撞击之前的一个月，科学家精确测算出碎片的体积（直径大约261英尺）、撞击时间（上午9点50分）和撞击地点（孟加拉国达卡，人口1500万）。他们预测爆炸将释放出18兆吨能量，与西伯利亚1908年发生的通古斯大爆炸规模类似。

柯达斯说：“我们的第一个教训是，必须要有远红外线太空望远镜，对于物体的体积有详细的了解。”

在一个巨大、火红的巨石飞向一个人口密集、贫穷的亚洲城市的同时，演习结束了。行星保卫者们在付出了一切努力之后，挂起胸牌，收拾行李，离开酒店，前往机场，给地球留下的是一个形势严峻的预警信息。



原文：

Earlier this spring, as violence and chaos drove thousands of refugees onto rickety boats off Libya, with some drowning on the grueling voyage to Italy, scientists gathered on the outskirts of Rome to discuss another sort of catastrophe. Astronomers and physicists from some of the world’s top institutions grappled with a dire scenario: An asteroid possibly as large as 1,300 feet in diameter—big enough to cause epochal damage—was hurtling toward Earth, and the countries likely to be hit included some of the poorest and most unstable in the world. Policymakers bickered over whether to try to blow it up or move it, and nations nearly went to war over whether deflecting it would make the fiery rock more likely to land on them.

Relax. It was only a drill. Had it been a real emergency, you would have been instructed to kiss the world—or a large chunk of it—goodbye.

Watching this five-day asteroid war game from the wings were two Americans, one from the scientific world and one from the military. These elder statesmen of what’s called planetary defense have been responsible for reminding policymakers that the planet and all life on it have been shaped by big rocks from outer space slamming into it. Dave Morrison was one of the first researchers to suggest that, unlike the dinosaurs made extinct by an asteroid impact, we might be able to defend ourselves. Former U.S. Air Force Lieutenant Colonel Lindley Johnson was eventually put in charge of NASA’s Near-Earth Object (NEO) Program division after first suggesting in the 1990s that the Air Force track asteroids. These men, along with all the dedicated planetary defenders around the world, are proud (and relieved) that the Big Question has evolved from what if a cataclysm-inducing space rock is aiming for us—we now know an impact is inevitable—to what will we do about it.

That question was the main topic of that mid-April meeting held in a conference hall in Frascati, a pleasant suburb of Rome. The European Space Agency had invited astronomers, physicists, nuclear engineers and mathematicians to discuss the slim possibility of a space rock smashing into Earth and causing regional damage or maybe even the end of civilization. The goal was, as it has been for the last six Planetary Defense conferences, to share information about identifying asteroid threats and the methods for saving us all.

The focus this year was on exploring whether nations would collaborate in the face of such a threat. Scientists today can tell us, with various degrees of certainty, that an object is on track to smash into the planet in, say, 200 years, and they believe we probably have the technology to stop it. But nobody knows how human beings could or would cooperate to face a global peril. And in an age when many politicians deny man-influenced climate change, can we even count on them to believe the asteroid hazard is real?

Humans are the first species on Earth who can recognize the danger of asteroids and have the foresight to think about how to stop a planet-killer from striking the planet.

Huge Blind Spot

Morrison, astronomer Carl Sagan’s first doctoral student, was in 1989 one of the first scientists to warn the public about asteroids, with Cosmic Catastrophes, a book he co-wrote with astronomer Clark Chapman. “Thirty years ago, there was no research on near-Earth objects,” he says. “There weren’t that many known and hardly anything to study.”

Since then, the field has grown to include national space agencies, Congress, the United Nations and labs filled with mathematicians, physicists, engineers, rocket scientists and even designers of nuclear weapons. Thanks to their efforts, more than 150,000 asteroids are now registered with the Smithsonian’s Minor Planet Center. The defenders estimate there are tens of thousands to hundreds of thousands more out there that we cannot see, many in our blind spot—hidden by the sun. About 12,700 of the identified ones are categorized as NEOs, with orbits that come within 121 million miles of the sun. NASA estimates that about 1,000 NEOs are civilization-enders—larger than a half-mile in diameter. None of the behemoths seems to be a likely threat, but about 1,600 other mapped NEOs may be headed our way, and an impact could kill millions.

An artist rendering of an asteroid belt around Vega, a distant star in our Milky Way galaxy. Vega is unique for scientist as it is the most studied star outside of our sun.

Megadeath in the Yucatán

The first comet was discovered in the 17th century—although comet-like objects have been sighted throughout history, showing up in biblical and other ancient accounts. The first asteroids were identified in the 19th century, but not until the early 20th century did we realize that some of them cross Earth’s orbit. Scientists now know that there are thousands of “Earth crossers,” and that we’re more than a little fortunate that Jupiter and Saturn absorb many of the asteroids that might otherwise pummel Earth.

The late geologist Gene Shoemaker, a science prodigy who graduated from the California Institute of Technology at age 19, was examining lunar craters in the 1950s for the U.S. space program when he determined that they were caused by impacts. Eventually, he was appointed head of the U.S. Geological Survey’s Astrogeology Science Center in Flagstaff, Arizona, where he and his team began mapping asteroids and studying the mechanics of meteorite impacts. With another scientist, Edward Chao, he discovered coesite, a type of silica produced in a violent impact. But his most important find—in terms of planetary defense—was Comet Shoemaker-Levy 9, which smashed into Jupiter in 1994. It was the first extraterrestrial impact human beings had predicted and then observed in real time. This gave scientists confidence that similar calculations could be made for Earth.

Around the same time that Shoemaker was compiling his notes about unusual, impact-related silicon deposits around Meteor Crater, near Winslow, Arizona, geologist Walter Alvarez discovered a layer of iridium-infused clay at the geological strata separating the Cretaceous and Tertiary periods—in other words, between the era of the dinosaurs and our epoch. Iridium is extremely rare on Earth but common in meteorites. Geologists soon found a similar iridium layer at the same geological strata in other parts of the world. They then postulated that a catastrophic impact had occurred around the time the dinosaurs became extinct, and scientists even know where the asteroid that killed off the dinosaurs likely hit—just off the Yucatán Peninsula, at Chicxulub.

In the decades since, geologists have learned more about how catastrophic extraterrestrial impacts changed our planet. They believe our moon is a chip off a collision between two Mars- and Venus-size objects sometime during Earth’s first 100 million years. After that impact, Earth was enveloped in a hot silicate atmosphere, leaving only heat-loving organisms in rocks a half-mile or more beneath the surface, and from that all future life developed. Numerous smaller objects, with diameters in the 5- to 10-mile range, like the one that caused the dinosaur extinction, have also slammed into the planet, causing lesser but still catastrophic changes.

Dr. Edward Teller tells the Senate Armed Services Preparedness subcommittee he believes Russia has the ability to hit such a distant target as Houston, Texas with an intercontinental ballistic missile “or will have in a short time,” Nov 25, 1957, Washington, D.C. Teller was a big advocate for nuclear technology and weapons and would later push for their use as a method to destroy or push an asteroid away from Earth.

The Bomb Option

When Chapman and Morrison published their 1989 book about cosmic catastrophes, they covered a broad range of menacing events, including comets, asteroids and supernovas. But both men thought the asteroid impact scenario was the most intriguing because mankind could theoretically do something to prevent one. In 1990, congressional staffers invited Morrison to present what he and others were finding about space rock hazards. A year later, Congress authorized NASA to study asteroids and how to deflect them.

Chapman and Morrison gathered together experts in astronomy, physics and geology to study the problem. The team concluded that the most dangerous asteroids were about 1 mile in diameter. Such a rock (one-tenth the size of the one that erased the dinosaurs) could have civilization-ending effects, mainly because weather alterations, caused by impact-related dust, would result in the starvation of billions of people. So they recommended sky surveys to find all objects of that size.

Besides astronomers and geologists, the planetary defender community attracted nuclear weapons designers, about to be left unemployed by the end of the Cold War, who found a new market for their expertise in massive impacts and creating a nuclear option for asteroid defense. Among them was Dr. Strangelove himself, Edward Teller, one of the fathers of America’s nuclear weapons program. Peace-loving Sagan was also involved. The two men had argued bitterly over nuclear weapons, but they found common ground in the idea that nuclear weapons could save us from an asteroid.

Astronomer Dr. Carl Sagan with Dr. David Morrison, at the first International Conference on Circumstellar Habitable Zones. Morrison, Sagan’s first doctoral student, was in 1989 one of the first scientists to warn the public about asteroids, with 'Cosmic Catastrophes,' a book he co-wrote with astronomer Clark Chapman.

The Little Prince’s Asteroid

Planetary defense was not left to civilian scientists alone. The chief of NASA’s asteroid program, Johnson, is a retired colonel who started in the Air Force as a satellite tracker. He got into the asteroid business in 1994 when he wrote a paper on what capabilities the Air Force might need by 2020. Johnson focused on asteroids and called his paper “Preparing for Planetary Defense”—thereby coining the term. After 23 years in the Air Force, Johnson announced he was retiring in 2003, and NASA enlisted him to run its Near-Earth Object Program.

A third American played a pivotal role in the development of planetary defense. Russell “Rusty” Schweickart was the first Apollo astronaut to walk in space, on the Apollo 9 mission. In 2002, he founded the B612 Foundation (named after the asteroid in Antoine de St. Exupéry’s story The Little Prince). He was inspired by geologist Norm Sleep’s lecture about how massive asteroid impacts 3.3 billion years ago—long before the dinosaurs—had boiled the oceans and formed the building blocks of life as we know it. Schweickart devoted several decades to proselytizing for deflection and mitigation technology. He also urged fellow astronauts to get involved and found some like-minded space explorers, including former astronaut Ed Lu, who now heads B612.

Schweickart has traveled the world to encourage a coordinated global response. “I fear there’s not enough of a collective survival instinct to really overcome the centrifugal political forces,” he says. “That is, in a nutshell, the reason we’ll get hit. Not because technically we don’t know it’s coming, or we can’t do something about it.”

Armageddon

When an Indian Ocean tsunami killed 230,000 in 14 nations on December 26, 2004, it captured the world’s attention and obscured a nearly simultaneous, albeit theoretical, brush with Armageddon. Just 48 hours before the Indian Ocean disaster, scientists made an alarming calculation: An 885-foot-diameter hunk of dark space rock was heading our way with a 1 in 25 chance of smashing into Earth in 2036, an impact with the potential force of 58,000 Hiroshima A-bombs. The Indian Ocean earthquake that launched the tsunami released less than half that force.

The ominous spinning rock was soon renamed Apophis, after an Egyptian god, “the un-creator.” For six months after it was first discovered, the asteroid wasn't even deemed interesting by the Minor Planet Center. But by December, astronomers at telescopes in Puerto Rico and Arizona had gathered enough data to enable scientists at the Jet Propulsion Laboratory (JPL) in Pasadena, California, which tracks NEO orbits, to project that Apophis had a 2.4 percent chance of impact in 2029, and an alarming 1 in 25 chance of smashing the Earth on an orbital swing in 2036. That prompted more scientists to start working feverishly. Eventually, they refined the prediction down to a much more unlikely threat of 1 in 250,000. When Apophis makes its close approach, it will pass between us and our satellites and be visible to the naked eye.

For the planetary defenders, such an event is cause for glee, not alarm.

The potential death star was something planetary defenders desperately needed—an event to wake up politicians and the public. But the publicity it garnered was double-edged. The public imagination was already primed by a pair of Hollywood disaster movies in 1998 (Deep Impact and Armageddon) featuring annihilation from the skies. Now, with a real apocalyptic catastrophe in the form of a killer tsunami in Indonesia, as well as an asteroid threat, the public wanted answers. And no one could give an honest answer without highlighting a single word that journalists and the public don’t want to hear.

That word is “uncertainties.”

The asteroid, Lutetia, as photographed on July 11, 2010 by the European spacecraft Rosetta. Lutetia sits in an asteroid belt beyond Mars and is estimated to have been circling our sun for as long as 4.5 billion years. Our thoughts on asteroids have evolved from evolved from what if a cataclysm-inducing space rock is aiming for us—we now know an impact is inevitable—to what will we do about it.

‘Boom Goes London, Boom Paris…’

Canadian-born astronomer Paul Chodas knows that word only too well, not least because he’s often had to repeat it to journalists who know he’s the go-to man whenever an asteroid makes the news. Minding asteroids is part of his job at the JPL, where he manages NASA’s Near-Earth Object Program office. There, he feeds an asteroid’s many variables—spin, mass, the way it reflects and absorbs light and radiates heat, and the gravitational pull of other asteroids nearby—into a supercomputer that then spits out an orbit prediction. Chodas and his colleagues for a time had calculated Apophis had a 1 in 25 chance of colliding with Earth. When I asked him whether those frightening numbers scared or excited him, he smiled and admitted it was a thrill.

His predictions, though, are filled with variables that sometimes involve plus or minus 18 million miles—a fairly significant distance even by space standards. Chodas and his colleagues grapple with uncertainties that start out small and grow in huge orders of magnitude. For example, a loss in weight equal to that of three grapes can mean the difference between an Earth hit or miss. The speed of an asteroid’s spin is affected by heat, which is in turn affected by the reflectivity of the rock’s surface. The vast variety of asteroids complicates the task—some are flying rubble piles, some are solid rocks, some are dust held together by gravity, and many have satellites.

The JPL crew continually refine complex equations, attempting to predict orbits with as few as six sightings. And they are getting better all the time.

JPL got another real-time chance to test equations seven years ago. One October morning in 2008, Chodas’s cellphone rang as he was dropping off his son at school. It was the Minor Planet Center at Harvard, reporting that an object appeared to be speeding toward Earth. Chodas plugged the coordinates of the rock into the computer and was soon able to predict an impact time and location—just 20 hours hence, in the Middle East. JPL then contacted Johnson at NASA, who called the State Department (someone also called President George W. Bush, according to a memoir by his then-press secretary). Johnson was especially concerned that governments in the volatile region be notified. “For a while, we had it predicted heading toward Mecca,” he says, drily. “And that was a concern.”

At JPL, Chodas and colleague Steve Chesley drilled into the numbers and soon had a precise impact point, near a fly-specked outpost, population 10 people, deep in the Sudanese desert. Chesley identified the location on his GPS, while Chodas grabbed an atlas. When he and Chesley compared notes, they realized they had come up with the same exact impact location. Chodas was elated. “I realized we were the only two people on the entire planet who knew exactly where this thing was going to land,” he says.

After the impact, JPL scientists were able to direct a team of university students from Khartoum to the predicted impact point. Even Chodas was surprised when the Sudanese students found remnants right where his equations had led them to look.

Still, frightening uncertainties remain. The last significant asteroid event was one that no one saw coming. In 2013, a “merely” bus-size space rock blew up in the sky near the town of Chelyabinsk, Siberia, with a force similar to a nuclear bomb. Windows were blasted, and 1,000 people went to the hospital. Because many drivers in Russia mount video cameras on their dashboards, scientists had a plethora of YouTube images of a streaking light, followed by a blinding explosion in the sky, that they used to pinpoint the object’s trajectory.

Chelyabinsk gave the planetary defenders another lesson in what even a relatively small asteroid, bursting not on impact but in the air, can do. And they know it’s only a matter of time before something like that happens over New York, London, Delhi or Tokyo.

The crescent Earth rises above the lunar horizon in this photograph taken from the Apollo 17 spacecraft in lunar orbit during a final lunar landing mission in the Apollo program.

Find Them All!

Congress passed the George E. Brown Act in response to the Apophis threat, and President Bush signed it into law in 2005, instructing NASA to detect, track, catalog and characterize the physical characteristics of asteroids larger than about 460 feet across. (Brown had been a much-admired chairman of the House Science Committee and an early voice on climate change and near-Earth threats.) In other words, the U.S. was finally doing what Morrison had suggested 15 years earlier: trying to find them all.

The mapping program involved three main elements: telescopes in Arizona and Hawaii and a JPL project called NEOWISE, the Near-Earth Object Wide Infrared Survey Explorer—a fairly small, space-based telescope operating at infrared wavelengths. In fall 2011, JPL’s Amy Mainzer, who heads the mapping effort, announced that the project had collected enough data for experts to declare Earth is—for now—not a target for any huge, civilization-ending mass. But hundreds of thousands of unmapped smaller objects are winging around near our planet; Mainzer says only 1 percent of NEOs above 60 feet in diameter have been found. They pose a different and perhaps more vexing challenge because they are harder to find and more likely to hit us. Objects as small as 450 feet across would cause severe regional damage, and the mapping project has identified only an estimated 25 percent of them. Geologists believe objects between about 150 and 450 feet in diameter hit Earth every 100 to 300 years, and some have wreaked havoc. NASA is considering Mainzer’s proposal to build a new space-based telescope that will find and measure many more asteroids. If approved, it could be operational by 2020.

‘As Big as the White House’

Deflecting an asteroid is an embryonic science. There are three schemes, roughly classified as Nuke, Kick or Tug. The Nuke option would aim an explosive device (not a conventional bomb)—or, more likely, many devices—at an asteroid on a collision course. Despite its Hollywood-grade visual potential, the planetary defense community regards it as a last-ditch effort.

The other two options are the Kick (aiming a projectile called a “kinetic impactor” at an asteroid to knock it slightly off its orbit) and the Tug (shooting an unmanned spacecraft into the orbit of the asteroid to operate as a “gravity tractor” with enough mass to pull the rock off its natural trajectory).

All three schemes depend on man’s ability to navigate a craft to an asteroid. A European Space Agency project did that last November, when the Rosetta craft landed the Philae probe on a comet and sent data back to Earth for 64 hours before its batteries died.

None of the asteroid mitigation techniques have been tested, but NASA hopes to demonstrate the Tug method as part of the Asteroid Redirect Mission expected to take place in 2020, which would launch a robotic spacecraft to break off and grab a chunk of an asteroid. As part of the project, the robotic spacecraft, with its cargo, will remain in orbit around the asteroid for 100 days. Scientists believe the enhanced mass of the craft with its load of rock will eventually pull the asteroid slightly off its trajectory.

The device would then drag the chunk of the asteroid back to the moon’s orbit sometime in the 2020s and leave it there, allowing future experiments on it.

The idea of dragging a space rock into orbit around the moon, essentially giving the moon a satellite, still sounds like science fiction, but planetary defenders want nations and space agencies to put real money behind testing, and for policymakers, journalists and scientists to discuss the threat calmly and realistically, somewhere between the poles of mass panic and dubious hilarity.

To that end, the defenders have devoted hours to discussing questions like what and how to tell the public about the risk. Currently, scientists rely on an ad hoc system of news releases from NASA couched in earthbound analogies: Asteroids are “big as the White House” or “an SUV,” and their predicted impact effects are measured in numbers of “Hiroshimas.”

The public will be hearing with increasing frequency about objects veering relatively close or even speeding toward us. Chodas and others have suggested NASA find a way to talk about asteroid risks as meteorologists talk about hurricanes, with news releases that update tracking hourly, coordinated with a government department for disasters like the Federal Emergency Management Agency and local authorities who could oversee an evacuation.

The public will also be hearing more about asteroids later this month, when a motley crew of astronomers, physicists, rock stars and filmmakers get behind what’s being billed as the world’s first “Asteroid Day,” on June 30. The annual event’s date was selected because on June 30, 1908, an asteroid flattened thousands of square miles of remote Siberian forest, in what’s known as the Tunguska event. The organizers and participants include Queen guitarist and astrophysicist Brian May, U.K. Astronomer Royal Lord Martin Rees, American scientist Bill Nye and astronauts Lu and Schweickart. Events are planned in cities around the world, and live presentations will be beamed from London and San Francisco. The Asteroid Day organizers are also circulating an online petition called “The 100X Declaration” calling for a hundredfold increase in the mapping and tracking of asteroids. “There are a million asteroids in our solar system that have the potential to strike Earth and destroy a city, yet we have discovered less than 10,000—just one percent—of them,” the document states. ”We have the technology to change that situation.”

A hole, thought to be made by the fragment of a meteor in the ice of Chebarkul Lake is seen on February 16, 2013 some 80 kilometers from Chelyabinsk, Russia. The local government reported more than 1,100 people injured, mostly by flying glass broken by the shock wave of the meteor explosion.

Asteroid Rage

After eight years of deliberating, a U.N. committee in March finally announced the creation of a global early-warning system to protect the planet from a potentially city-destroying, tsunami-causing or, worse, civilization-ending large space object. The planetary defenders tested the concept in mid-April by playing the war game in the suburbs of Rome. Their mission: Save the planet from an asteroid possibly four times the size of a football field. The science and policy they tested were so realistic that their online daily press releases had to be emblazoned with bright red boxes proclaiming, “Exercise. Exercise. Not a Real World Event.”

NASA’s Johnson says the exercise proved to him that humans can mount an asteroid response—and it can be affordable, a key element when trying to sell politicians on preventing disasters that might very well not occur in our lifetime. “A worldwide effort of a few hundred experts and a few hundred million dollars per year would be quite sufficient to identify any potential impact threat and develop the means to prevent it,” he says.

Chodas created a realistic scenario for the game. At the beginning, the conference participants learned that scientists had “discovered” an asteroid estimated to be somewhere between 460 and 1,300 feet in diameter, apparently on course to smash Earth in seven years, on September 3, 2022. The participants had divided into three role-playing groups—national and international policymakers, the media and scientists—and played out over five days what humans might do.

In the first year after the asteroid’s discovery (days one and two of the conference), the participants found out that scientists had used available information to estimate a long “risk corridor” that stretched from Southeast Asia to Turkey. As the asteroid moved through its orbit, scientists continually refined their predictions and homed in on its size and likely damage point, and they advised policymakers on the options. By August 2019 (day four of the conference), the participants learned that global policymakers had agreed to fire six kinetic impactors at the asteroid, and they reached their target six months later. But a debris cloud from that impact prevented observers and policymakers from knowing what had worked until January 2021 (day five of the conference), when it was announced that two of the six KIs had missed, one hit and fractured the asteroid, and another hit and broke off a chunk that remained on a path toward Earth and was hidden from view by sunlight. Two others hit the remains of the now-broken asteroid, deflecting the largest piece of it.

The following year (later on day five), the participants found out that the broken fragment was still hurtling toward Earth and remained a significant hazard. It would slam us on September 3, 2022, somewhere in India, Bangladesh or Myanmar. About a month before its projected impact, scientists were able to pinpoint the object’s size (about 261 feet in diameter), as well as the likely time of impact (9:50 a.m.) and precise location (Dhaka, Bangladesh, population 15 million). They predicted the explosion would release 18 megatons of energy, similar to that asteroid explosion in 1908 that flattened thousands of miles of Siberian forest.

"The number one lesson I took away is that we need infrared, in-space telescopes that could tell us more about the sizes of these objects,” Chodas says.

The exercise ended on a cliff-hanger, with a massive, flaming rock closing in on a teeming, impoverished Asian city. Having done the best they could, the planetary defenders hung up their hero lanyards, packed their suitcases, checked out of their hotels and headed for the airport, leaving the planet forewarned.