追踪流星

Skyfalls

天幕坠落

Networks of cameras are making it easier to track meteors, and find the bits that actually reach the ground

摄像网络系统让流星观测更加便捷，并能准确追踪地面的陨石分布

Every day between 100 and 600 tons of rock hurtles into Earth’s atmosphere. The reason so little of this bombardment makes it to the planet’s surface is that much of it is burnt up by atmospheric friction, which creates the fireball that is the visible sign of a meteor’s arrival. As for the bits that do get through, once landed, they are known as meteorites.

每天，约有100至600吨的岩石冲破地球的大气层。在地球表面，我们很少注意到这样的轰炸现象发生，原因在于，流星体在冲破大气层的时候，大气阻力就已经将其烧毁了。因此，被烧毁的岩石变成了一个个火球，这就是为什么我们在天上可以看到流星划过。对于一些小的石块而言，它们确实会穿破大气层，来到地面，一旦着陆，这些岩石就变成了我们熟知的陨石。

Roughly 60,000 objects of meteoritic origin have been picked up and catalogued. Most are fragments from a much smaller number of individual falls. Of these falls, only 36 were observed as they arrived with enough fidelity to calculate the orbit of the original meteor before it entered the atmosphere. If more such data were available it could, by showing where the rocks came from, cast more light on the composition of the solar system. It might also help in moving orbiting spacecraft out of danger.

大约有60000件陨石的来源得到了科学家的复原和记录。大多数陨石都是流星下坠从而产生的碎片，在数量上，这些流星比陨石块少得多。在这些流星中，科学家只观测到36次，因为这些流星进入大气层以前，科学家就已经记录了其原始的轨道路径。这类数据如果大量存在的话，科学家就能够追踪其来源，它能够反映出太阳系物质组成。这些数据同样可以调整宇宙飞船的运行轨道，保证其免受撞击。

The tracking of meteors is carried out by arrays of cameras on Earth. The oldest of these is the European Fireball Network (efn), which dates back to 1951 and is operated by the Astronomical Institute of the Czech Academy of Sciences. When it launched its equipment was primitive— two groups of eight cameras capturing images on glass photographic plates using all- night-long exposures. Each camera group covered half the sky. Now, the network deploys 24 state-of-the-art digital cameras equipped with fish-eye lenses in 18 stations scattered across Austria, the Czech Republic and Slovakia. Two more stations, in Germany, are planned for later this year.

地球上有一系列摄像头在追踪流星的轨迹。最早的一套设备是欧洲流星网络（EFN），它于1951年投入使用，隶属于捷克国家科学院航空学院。这套设备最早推出的时候是十分简陋的，只有两组摄像头，每组八台，使用玻璃底片全天候曝光抓取图片。每一个摄像组负责一半天空。如今，这个摄像网络拥有24台尖端数码设备，并配有鱼眼镜头，在欧洲共有18个分站台，包括奥地利，捷克共和国和斯洛文尼亚。德国在建的两个分站台也将在今年投入使用。

The digital cameras take back-to-back photographs, with 35 second exposures, from dusk to dawn. Fish-eye lenses allow a single exposure to cover the whole sky immediately above each camera. If more than one camera sees the same fireball—which is usually the case—that meteor’s course can be triangulated, with a precision of about ten metres, by comparing the images. This yields two valuable pieces of information. Plotting the path backwards reveals the rock’s orbit before it slammed into Earth’s atmosphere. Projecting it forward suggests a potential landing site.

这些电子摄像头主要拍摄连排照片，每次曝光时间是35秒，并且从早工作到晚。鱼眼镜头使得每次曝光都能够瞬间抓拍每个镜头上方的全部天空图景。如果不止一台机器发现了流星（这种现象十分常见），流星的轨迹就可以被三角定位，并且通过比较影像，通常能够达到10米的精确度。两个十分重要的信息就此产生。回溯运行轨迹可以发现流星撞击大气层以前的运行轨道，而预测其轨迹就可以发现它可能的着陆点。

The efn’s cameras also contain radiometers that measure changes in a fireball’s luminosity 5,000 times a second. This reveals the rock’s entry speed, its probable mineral make-up, the amount of fragmentation and deceleration rate. If the data indicate anything is likely to have reached the ground, an alert is automatically emailed to the network’s operators.

EFN的摄像头还具有辐射计的功能，它可以在一秒钟内监测到流星光度发生的5000次变化。这一点揭示了岩石的进入速度，其可能的矿物组成，以及碎片化的数量和减速速率。如果数据表明地面受到岩石撞击的话，该网络的操作者就会收到机器发出的自动警告。

Dark flight

黑暗航程

To calculate an impact’s location, researchers take into account how wind affects the trajectory during 20km or so of “dark flight”, after a fireball has burned out. A decade ago, half of meteorites found as a result of the efn’s data were within 500 metres of the predicted spot. That figure has now shrunk to 100 metres. Pavel Spurný, the network’s co-ordinator, usually keeps the impact zone secret until his team, or trusted helpers, can search for it. Meteorites have commercial as well as scientific value. Giving the game away too early risks losing finds to professional collectors.

为了估计一次撞击的位置，研究者常常需要考虑风速对于流星轨迹的影响，比如20公里内风速影响下的流星轨迹，或是流星被烧毁后所经历的那段“黑暗航程”。10年以前，根据EFN的数据报告，其发现的一半陨石都能够在预计着陆点的500米内被找到。如今，这个数字已经减少到了100米。帕维尔·斯伯尼是这一组织的协调员，他常常需要对可能的撞击地点进行保密处理，直到他的团队或受到该组织信任的研究者能够找到其具体位置。陨石具有商业价值，也具有科研价值，对专业的收藏者而言，过早将坠落地点信息泄漏出去可能就丧失了一次重大发现的机会。

The efn’s hardware was not hugely expensive. The network’s cameras cost about $30,000 a piece. Operating the system adds $114,000 a year, according to Dr Spurný. But it has improved the success rate enormous- ly. Between 1951 and 2014, when the new cameras started to be rolled out, rocks from five falls were recovered. Since then, that total has doubled. Even so, cloudy skies can foil the instruments. And meteorites, many of which are small and dark, are not always easy to find in the vegetation and darkish soils of central Europe.

EFN的这些硬件设施并不是很昂贵。每一台摄像头的造价约为30000美元，每年的运行费用约为114000美元，斯伯尼博士表示，这一系统让流星定位成功的概率提高了不少。在1951年至2014年间，最初设备在投入使用时，研究者只能复原5次坠落的陨石来源，之后这个数字翻了一倍。但若是遇到多云天气，这些设备还是不能发挥最大的效用。至于散落的陨石，它们大多数都体积小，颜色深，在中欧地区的植被和黑土地上很难找寻其踪影。

For all these reasons, Phil Bland, a British meteorite expert, reckoned the pickings are better on the flat, brushless, lightly coloured deserts of Western Australia—a place where, as a bonus, the skies are mostly clear. Dr Bland, who works at Curtin University, in Perth, has therefore set up what he calls the Desert Fireball Network (dfn). This now sports 52 camera observatories, though the cameras themselves are, at $10,000 a pop, cheaper and less snazzy than the efn’s. These cameras keep a persistent eye on the western third of Australia’s night sky.

出于这样的原因，菲尔·布兰德，一位流星研究专家提出了这样的设想。他推断，采集陨石最好在西澳大利亚进行，因为这里地势平整，灌木稀少，并且是广袤的淡色沙漠，更具优势的一点是，这里的天气常年都是晴朗状态。布兰德博士工作于伯斯的科廷大学，他在此建立了沙漠流星网络系统（DFN）。这一系统目前配备了52台摄像天文观测台，虽然比EFN的数量多，但每一台设备的造价要更低，仅为10000美元。这些摄像头将时刻关注西澳大利亚的三分之一夜空。

The dfn has been a success. It has produced, Dr Bland says, a big data set “of gorgeous orbits” for incoming rocks. The number of meteorites believed to have landed has overwhelmed the team’s resources. They have recovered stones from four falls, but are in need of adventure- some volunteers to mount expeditions into the outback to gather the remains of more than 30 others.

DFN是一大创举。布兰德博士表示，这一系统产生了大量的流星数据，为研究其轨道提供了难得的资料。产生的陨石数量常常使这一研究团队感动困难重重，他们目前复原了四次陨石坠落的来源。他们可能得更加冒险一点，比如派遣志愿者深入内陆地区，收集其他30余次坠落的陨石情况。

In America, meanwhile, the nasa All- sky Fireball Network, run by America’s space agency, operates 18 cameras across the United States. Its goal is not to find meteorites, but to protect spacecraft from collisions. By studying fireballs, the agency’s Meteoroid Environment Office in Hunts- ville, Alabama, which operates this particular network, improves estimates of the number, size, speed and trajectory of space rocks in areas where satellites operate. The forecasts of Earth’s periodic peak bombard- ment by objects from a cloud of cometary debris called the Draconids, for example, has improved from an accuracy of about two hours in 2012 to just 30 minutes today, says Bill Cooke, who runs the project.

在美国，国家航空航天局（NASA）的空天流星网络系统也处于运行之中，主要由美国的空间机构负责操作遍布全美的18台摄像设备。这一系统的目标不是寻找流星，而是保证其航空器免受流星撞击的风险。美国研究流星的组织是隶属于空间机构的流星环境办公室，其设立于阿拉巴马州的亨茨维尔，它主要负责操作整个摄像系统，在预测航天器周围的陨石数量，规模，速率和轨迹等方面不断提高其运行精度。该系统的主要负责人比尔·库克表示，彗星碎片云团（较有名的比如天龙座流星群）会对地球产生周期性的峰值轰炸，该组织对于这一现象的预测精度已经从2012的2个多小时提高到30分钟左右。

Dr Cooke’s team use the data the network collects to calculate the risks faced by individual spacecraft. nasa publishes these numbers so that insurance underwriters can take them into account, as can mission operators. In areas with higher collision risks, controllers may temporarily shut down high-voltage subsystems that, if struck, might fry the spacecraft they are part of, or reorient a craft so that the narrow edges of its solar panels face any onrushing space rocks, minimizing the risk of impact.

库克博士的团队使用该网络系统的数据测算独立航空器可能面临的撞击风险。NASA将这些数据公开发表，希望各保险代理机构和航空任务执行机构能够将这些因素考虑在内。在高撞击风险区域，飞船控制者需要暂时关闭高电压系统的运转，因为一旦和彗星碎片撞击，飞船的该区域就会有炸毁的风险；控制员也可以重导飞行器的运行方向，确保其太阳能电池板的窄边面向碎片，从而减少撞击的风险。

Protective measures

保护性措施

Spacecraft engineers also use Dr Cooke’s data to design better “bumper shields”. These consist of layers of Kevlar and other materials spaced so that they gradually break apart an incoming meteor, depriving it of energy. To keep launch weights down, not all sides of a spacecraft are shielded equally, usually the rear is the most heavily armored part.

航空工程师也在库克博士提供的数据基础上设计出了更好的减震防护装置。这套设备包括多层凯夫拉纤维，以及其他整齐排列的材料装置，它们能够逐渐消解袭来的流星体，并削减其撞击能量。为了保证飞行器更小的发射重量，它各个部分的保护措施不是统一的，通常来说，尾部是最需要保护的部位。

To gauge a projectile’s destructive power, one must know its speed. A team at the University of Western Ontario, in Canada, clocks meteors smaller than grains of sand. Using high-frequency radar, the team fires

pulses into the sky 500 times a second, day and night. These detect not meteors themselves, but rather the trails of ions, generated by friction within the air, that they leave behind. The radar sees this as a “giant wire in the sky”, says Peter Brown, the team leader. An array of microphones sensitive enough to measure shock waves from meteors a centimeter or more across provides additional data. Dr Brown puts the average speed of such shooting stars at about 20km a second—significantly faster than many had thought.

测算物体发射的破坏力往往得知道它的速度。加拿大西安大略大学的一只研究队伍测量了比沙粒还小的陨石所产生的撞击速度。这个团队通过高频雷达向天空发射全天候脉冲信号，频率是500次每秒。这一试验并未检测到流星体本身，但是，研究者发现了流星体与空气摩擦产生的离子轨迹。该团队负责人皮特·布朗表示，这一雷达将这些轨迹视为一张“空中巨网”。一系列微波的存在也提供了更多的数据，这些微波的敏感性很高，它们能在一厘米或更大的范围内检测到流星体所产生的冲击波。布朗博士表示，换算过来，这样的流星体运行速度约在20公里每小时，这一发现比很多人想象中的要快得多。

That is bad news for satellites. But if the various meteor-monitoring networks around the world can help improve the forecasting of peak meteoric activity, then the number of spacecraft suddenly found to be in peril will be reduced.

这对航空器而言并不是什么好消息。但是遍布世界的流星体监控网络的存在能够切实提高流星体活动峰值的预测能力。众多飞行器突然陷入困境的的风险也要小的多。