好莱坞对世界的启示 太空垃圾并非耸人听闻

I don’t think I’m spoiling too many surprises when I reveal that the plot of the film Gravity, a low-orbit spectacular starring Sandra Bullock and George Clooney, involves spacecraft getting hit by space debris. It’s a less fanciful premise than it might seem: in 2009, two unmanned satellites hit each other without warning, nearly 800km above Siberia.

关于桑德拉?布洛克(Sandra Bullock)和乔治?克鲁尼(George Clooney)主演的、以近地轨道和壮观的太空为背景的电影《地心引力》(Gravity)，我透露一下该片情节与太空垃圾撞击太空船有关，想必不算严重剧透。不过，电影的故事背景并不像看上去那样不可思议——2009年，两个无人卫星就在毫无预警的情况下，在西伯利亚上空近800公里处相撞了。

That collision heralded a serious problem, first flagged in 1978 by Donald Kessler, then an astrophysicist at Nasa. The concern isn’t that space debris will rain down on us here on Earth: it’s that it will stay up there in space.

那次碰撞事故预示出一个严重问题。1978年，美国国家航空航天局(Nasa)的天体物理学家唐纳德?凯斯勒(Donald Kessler)第一次将这个问题带入了世人的视野。令人担心的不是太空碎片会砸到地球上，而是这些碎片会一直留在太空中。

The two satellites that collided, Cosmos-2251 and Iridium-33, weighed almost a ton and a half altogether. The result was at least a thousand fist-sized chunks of metal, any one of which could destroy a further satellite, and produce hundreds of further chunks. It takes time for these chunks to fall out of orbit.

这两颗相撞的卫星分别是Cosmos-2251和Iridium-33，总重近1.5吨。碰撞后留下了至少1000个拳头大小的金属块，其中任何一块都能再毁掉一颗卫星，并产生成百上千个新的碎块。这些碎块要过很长时间才会从轨道上掉下来。

What worried Kessler – and still does – was the prospect of a chain reaction. Too much debris in orbit would make it impossible to launch the satellites that have become an indispensable part of life back on Earth.

凯斯勒当时担心的问题（这个问题现在仍然存在）是，太空垃圾可能造成连锁反应。太空轨道上碎片过多可能让人类无法发射新的卫星，而卫星已成为地球上人类生活不可或缺的部分。

Nasa is tracking 21,000 pieces of junk 10cm across or bigger – like small cannonballs. In low Earth orbits, they are travelling at about 7km a second (25,200km/h). But space hasn’t been made impassable by debris just yet. There’s quite a lot of room up there, after all. Low Earth orbits are common but include a variety of altitudes, so objects have plenty of ways to fail to hit each other. Geosynchronous orbits, popular with communications satellites, must be exactly 42,164km from the centre of the Earth. But satellites that far out share more than 22bn sq km of space.

Nasa正在跟踪2.1万个直径10厘米或更大一点的太空垃圾，这些太空垃圾就像一个个小炮弹。在近地轨道，这些碎片的飞行速度大约为每秒7公里（合每小时25200公里）。不过，太空碎片还没有多到堵塞太空的地步。毕竟，太空的空间很大。近地轨道比较常见，不过近地轨道的海拔高度各不相同，因此人造天体避免相撞的几率很大。而通讯卫星常用的地球同步轨道，距地心高度必须正好是42164公里。不过，在这个距离的轨道上，卫星可使用的太空空域总面积超过了220亿平方公里。

Still, some orbits are more crowded than others; more collisions are surely just a matter of time. That was the opinion of a 2011 report from the National Academy of Sciences, “Limiting Future Collision Risk to Spacecraft”, which argued that there is already enough junk crashing into other junk that the problem will worsen even if there are no further launches.

不过，有的轨道仍更为拥挤一些，发生更多碰撞事件肯定只是个时间问题。2011年，美国国家科学院(National Academy of Sciences)发表的报告《限制飞船未来相撞风险》(Limiting Future Collision Risk to Spacecraft)就持这种观点。这份报告声称，太空垃圾相互碰撞的例子已经够多，就算人类现在停止发射新的卫星，这个问题也会不断恶化。

Deliberately moving the debris somewhere safer seems possible, but pricey. It’s expensive to tidy up a satellite – or to design one that tidies itself up – and while the benefits of doing so are widely shared, the costs are not. So the clean-up doesn’t happen.

主动将这些碎片移至安全地带看上去似乎可行，不过代价高昂。清理一颗卫星（或设计能自行清理的卫星）成本巨大，尽管这么做对大家都有好处，但这样做的成本却不会由所有人分摊。因此，“大扫除”方案是行不通的。

The regulation of satellites is no simple matter: Cosmos-2251 was launched by the Russian military; Iridium-33 by a US corporation. The single largest space-junk incident was in 2007, when the Chinese military blew up a satellite just to show that it could. The regulatory authority capable of dictating to all three of those parties does not exist. (The United Nations did issue voluntary guidelines in 2010.)

对发射卫星进行监管可不是简单的事，Cosmos-2251卫星是俄罗斯军方发射的，而Iridium-33卫星则是一家美国公司发射的。而最大的一起太空垃圾事件发生在2007年，当时中国军方炸掉了一颗卫星，只是为了展示中国有能力这么做。目前，能令以上三个国家全都俯首帖耳的监管机构还不存在。（不过，2010年联合国(UN)确实发布过一套由各国自愿遵守的准则。）

Economists such as Molly Macauley of Resources for the Future, a think-tank, have been pondering this problem for some time. The obvious economic solution, recently revived by three researchers, Nodir Adilov, Peter Alexander and Brendan Cunningham, is a tax on new satellite launches. Macauley has proposed linking the level of this tax to the design of the satellite – safer designs would attract a lower charge. Another possibility is that satellite operators would put down a deposit, to be refunded once the obsolete satellite had been pushed into a safer orbit.

来自智库“未来资源研究所”(Resources for the Future)的经济学家莫利?麦考利(Molly Macauley)曾对这问题进行过一段时间的思考。经济学上有一个显而易见的解决方案——对新发射卫星征税。该方案最近在三位研究人员——诺迪尔?阿迪洛夫(Nodir Adilov)、彼得?亚历山大(Peter Alexander)和布伦丹?坎宁安(Brendan Cunningham)——的努力下再次流行起来。对此，麦考利提议将卫星设计与征税多少挂钩，设计得更安全的卫星可以少收一点税。另外，还有一种可能是，卫星运营方支付一笔押金，这笔押金在过期卫星被移至更安全轨道后返还。

This is one of those all-too-common situations when it is easier for economists to announce the optimal policy than it is for politicians to implement it. As with climate change, there’s a burden to be shared here, a threat of uncertain magnitude, and plenty of opportunity for free riding.

不过，经济学家宣布一种最优政策容易，政治家实现起来可就难了——这样的事可谓屡见不鲜。太空垃圾问题和气候变化问题类似：需要大家共担责任，面临的威胁大小难以预料，有许多“搭便车”的机会。

Yet this is a far cheaper problem than climate change, with a smaller number of decision makers. It should be easier to reach an agreement on space junk than on greenhouse gases. Alas, that is a not a very encouraging comparison.

不过，解决这个问题比解决气候变化问题便宜多了，参与决策者也少得多。比起温室气体问题，太空垃圾问题应更容易达成共识。唉，这样的对比可不算令人鼓舞。