The problem of space pollution
太空污染问题
Junk science
除脏科学Scientists are increasingly worried about the amount of debris orbiting the Earth
科学家们越来越担忧环地轨道上的那些太空垃圾 Aug 19th 2010
FEBRUARY 10th 2009 began like every other day in Iridium 33's 11-year life.One of a constellation of 66 small satellites in orbit around the Earth, it spent its time whizzing through space, diligently shuttling signals to and from satellite phones. At 3pm a report suggested it might see some excitement: two hours later it would pass less than 600 metres from a defunct communications satellite called Cosmos 2251. It did. A lot less. The two craft collided and the result was hundreds of pieces of shrapnel more than 10cm across, and thus large enough to track by radar—and goodness knows how many that were not. This accident came two years after the deliberate destruction by the Chinese of their Fengyun-1C spacecraft in the test of an anti-satellite weapon. That created over 2,000 pieces of junk bigger than 10cm, and an estimated 35,000 pieces more than 1cm across. Together, these incidents increased the number of objects in orbit at an altitude of 700-1,000km by a third (see chart).
2009 年2月10日，这天的开始，就像"铱星33号"11岁生涯中的任何一天一样，毫无异兆。由66颗小铱星组成的铱星群在绕地轨道上的太空高速运行，孜孜不倦地在铱星和卫星电话间来回发送信号，铱星33号是其中的一颗，今天是它生命的终结时刻。下午三时，一份报道提示，铱星33号可能会出现某种令人激动的景象：两小时后，它将以少于 600 米的距离从一颗已报废的叫作"宇宙 2251号"的通信卫星旁掠过。它确实掠过了，距离却少了很多。结果是两星相撞，产生了无数大于10 厘米的碎片，而且大到足以通过雷达来跟踪——只有老天才知道究竟有多少碎片。这种事两年前就有了，两年前，中国搞了一次反卫星武器试验，试验中有目的地摧毁了一颗中国自己的"风云—1号C "卫星。那次毁星行动产生了2000 多件大于10 厘米太空碎片，估计还四散了35000 片大于1 厘米的碎片。这些事故使离地700-1000公里的轨道上运行的太空物
增加了三分之一（见图表）。
Such low-Earth orbits, or LEOs, are among the most desirable for artificial satellites. They are easy for launch rockets to get to, they allow the planet's surface to be scanned in great detail for both military and civilian purposes, and they are close enough that even the weak signals of equipment such as satellite phones can be detected. Losing the ability to place satellites safely into LEOs would thus be a bad thing. And that is exactly what these two incidents threatened. At orbital velocity, some eight kilometres a second, even an object a centimetre across could knock a satellite out. The more bits of junk there are out there, the more likely this is to happen. And junk begets junk, as each collision creates more fragments—a phenomenon known as the Kessler syndrome, after Donald Kessler, an American physicist who postulated it in the 1970s.
这类低地轨道（或叫LEOs）是人造卫星最理想的轨道。这类轨道很容易通过发射火箭到达，这类轨道使地球表面的军事和民用目标得到非常详尽的扫描，这类轨道离地相当近，甚至如卫星电话等设备的微弱信号都可以捕捉到。因此，将卫星置入低地轨道安全运行这一可能性的失去，原本就是一件坏事。而这两次事件的威胁所引起的恰恰就是这种坏事。该轨道的运行速度约每秒八公里，即使碰上一个一厘米大小的物体，也会使一颗卫星毁灭。在该轨道上的碎片越多，就越有可能产生碎片。碎片生碎片，因为每次碰撞产生更多的碎片——这就形成一种叫"凯斯勒综合症"的现象，也就是美国物理学家唐纳德·凯斯勒在上世纪七十年代创立的一种假说。
According to the European Space Agency (ESA) the number of collision alerts has doubled in the past decade. Nicholas Johnson, the chief scientist for orbital debris at ESA's American equivalent, NASA, says modelling of the behaviour of space debris "most definitely confirms the effect commonly referred to as the Kessler syndrome". Even the National Security Space Office at the Pentagon is worrying about whether a tipping-point has been reached, or soon will be.
根据欧洲航天局(ESA)所述，卫星碰撞报警次数在最近十年增加了一倍。与ESA地位相当的美国国家航空和宇宙航行局（NASA）轨道碎片首要科学家尼古拉斯・约翰逊又说，太空碎片行为模拟试验"非常确凿地证实了平常所称的凯斯勒综合症所指的现象"。甚至美国五角大楼的国家太空安全办公室也在担心：[碎片灾难]的临界点是已经到达呢，还是将要到达。
Concerns like Dr Kessler's have caused launch agencies to take more care about what they get up to. In particular, accidental explosions in orbit have been reduced by depressurising redundant rockets after they have released their satellites. Also, the number of spy satellites launched has fallen since the demise of the Soviet Union. Nevertheless, there is still a problem.
与凯斯勒博士的假说相类似的担心实际已促使卫星发射部门更加注意其发射行为。特别是在轨道上的意外撞击事故已经实现了减少，这是通过在轨道释放卫星时减少不必要的箭体[残留]来实现的。此外，自苏联解体以来，间谍卫星的发射数量也下降了。不过，问题仍在。
Scouring the skies
太空大扫除The real threat now comes from collisions between things that are already up there—so much so that since the demise of Iridium 33, the normally secretive Strategic Command (Stratcom) of America's Defence Department has become rather helpful. Brian Weeden, an expert on space debris at the Secure World Foundation, a think-tank, says Stratcom now screens every operational satellite, every day, looking for close approaches, and notifies all operators. Even the Chinese? "Everybody," he says, "the Russians, the Chinese, even the Nigerians." This means that satellites' owners have better information with which to decide whether to use a small amount of their precious fuel reserves to avoid a collision.
现在，真正的威胁来自那些已经在轨道运行的物体间的碰撞——这个问题已非常严重，以致于像美国国防部那个通常神秘兮兮的战略安全处（战备通信司令部）自从铱星33号撞毁以来，也愿意多多少少提供一些帮助。世界基金会（系一个智囊团）的太空碎片问题专家布赖恩•威登说：战备通信司令部现在每天都在寻求更加有效的方法观察每颗业务卫星，并将观察结果通知每个卫星经营者。甚至还包括通知中国？他说："通知及于每个国家，俄国、中国、甚至尼日利亚"。这就意味着，卫星业主们获得了更好的信息，以决定是否使用[卫星上]不多的、宝贵的燃料储备来避免碰撞事故的发生。
But even this would not be enough. What is needed is a way to clean up the junk so that it is no longer a problem. Ideas for doing this are growing almost as fast as space debris. One proposal, originally made a decade ago by the American armed forces, would be to use ground-based lasers to change the orbits of pieces between 1cm and 10cm across by vaporising parts of their surfaces. This would produce enough thrust to cause the debris to re-enter the atmosphere. The proposal suggested a single laser facility would be enough to remove all junk of this size in three years.
但这还不够。真正需要的，是一种清除太空碎片的方法，使太空碎片不再是问题。清除太空碎片的主意迅速成熟起来，成熟的速度几乎像太空垃圾的增长速度一样快。有一个最初是由美国军方于十年前提出的主意——用地面激光汽化1至10厘米的碎片的表面部分，使之运行轨道改变。这种方式会产生足够大的推力使碎片重新进入大气层。该主意称：三年内，单束激光的能量将足以清除所有1至10厘米的太空碎片。
Another way of slowing junk down, and thus causing it to burn up in the atmosphere, was proposed this month by Alliant Techsystems, a firm based in Minneapolis. Alliant suggests building special satellites enclosed in multiple spheres of strong, lightweight materials. Debris hitting such a satellite would give up momentum—and thus velocity—with each collision. As a bonus, many objects large enough to cause damage would be shattered by the collisions into fragments too small to cause serious harm.
还有一种方法是使碎片的速度降低，进而进入大气层烧掉，这种方法是在本月由设在明尼阿波利斯能源公司Techsystems 提出的。能源公司建议，研制一种特殊卫星，这种卫星外面用各种强度好、质地轻的材料包裹起来。碎片撞击这种卫星时，动能减弱、其速度也因每次撞击而慢下来。其结果是，许多足以造成严重损害的碎片会因撞击而碎成很小的、不再对卫星造成损害的碎片。
However, many space agencies are considering a third option: robot missions that would dock with dead satellites and fire rockets either to boost them into "graveyard" orbits or to deorbit them completely, so they crashed into the sea. Jer-Chyi Liou, an expert on orbital debris at NASA, estimates that if such a mission started in 2020, and removed the five objects most likely to create future debris, it would more or less solve the space-junk problem.
不过，许多航天局正在考虑的是选择第三的类办法：把机器人派上天去，由它们去把那些报废卫星和已烧尽的火箭残体捉倒，然后将其推进"死亡轨道"或将其坠入大海。美国航空航天局轨道残骸专家Jer-Chyi Liou估计，如果在2020年开始这种机器人扫天，并消除五个很可能形成未来残骸的太空物的话，它也将或多或少解决了太空垃圾问题。
A knock-out blow?
ESA is thinking about this sort of solution, too. It is the owner of what has—perhaps unfairly—been termed "possibly the most dangerous piece of space debris" by a recent article in SpaceNews. The debris in question is Envisat, one of the largest Earth-observation satellites yet built. At the moment it is still working, but when its fuel runs out, sometime between 2016 and 2018, it will become a giant piece of junk—one that will remain in a crowded orbit for 150 years.
Or not. For even conservative estimates suggest there is one chance in four that it will be destroyed in a collision during that period.
欧空局也在考虑这种解决方案。《太空杂志》最近有篇文章说：欧空局是一个被称作"或许是最危险的太空废弃物"的主人（或许这种说法不公平）。这个热议中废弃物就是指的Envisat星，它是尚在运行中的最大的地球观测卫星之一。尽管它目前仍在工作，但其燃料会2016年至 2018的某个时刻耗尽，届时它成为一块巨大的太空垃圾且将在拥挤的轨道上存续150年。
或者不到150年。即使根据保守的估计，在此期间因撞而毁的机率将达到四分之一。
On the face of things, all this consideration of the problem is good. But this being space, where matters military are never far from the minds of those who think about it, there remains a serious question.
从形式上看，所有国家对太空碎片问题的关注都是善意的。但这些问题仍然非同小可，因为太空对军事有重要影响的想法从未远离那些想把太空用于军事目的人。
Satellites are crucial to modern warfare. They spy on battlefields and on even the peaceful activities of enemies, rivals and questionable allies. They provide communication links. Knocking them out—as the Chinese practised with Fengyun-1C—would be a useful military trick.
卫星是现代战争的关键。卫星监视战场，甚至监视敌人、竞争对手和可疑盟国的和平活动。卫星提供通讯联系。打掉卫星（就像中国在试验中击毁"风云—1号C "卫星一样）将是很有用的军事手段。
Any programme designed to remove satellites from orbit thus makes military types from other countries nervous. Some people, Mr Weeden among them, argue that such fears can be overcome if there is international co-operation over exactly which objects are removed and who is doing what. It would certainly be in everyone's interest to do so.
无论哪种从轨道上清除卫星垃圾的设计方案都使其他国家产生军事紧张情绪。一些人（其中有Weeden先生）坚持认为，如果在该清除哪些太空垃圾和由谁去清理上实行完全的国际合作，那么这类紧张情绪是可以克服的。这种合作会使人人受益。
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