好萊塢對世界的啓示 太空垃圾並非聳人聽聞

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.

關於桑德拉?布洛克(Sandra Bullock)和喬治?克魯尼(George Clooney)主演的、以近地軌道和壯觀的太空爲背景的電影《地心引力》(Gravity)，我透露一下該片情節與太空垃圾撞擊太空船有關，想必不算嚴重劇透。不過，電影的故事背景並不像看上去那樣不可思議——2009年，兩個無人衛星就在毫無預警的情況下，在西伯利亞上空近800公里處相撞了。

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.

那次碰撞事故預示出一個嚴重問題。1978年，美國國家航空航天局(Nasa)的天體物理學家唐納德?凱斯勒(Donald Kessler)第一次將這個問題帶入了世人的視野。令人擔心的不是太空碎片會砸到地球上，而是這些碎片會一直留在太空中。

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.

這兩顆相撞的衛星分別是Cosmos-2251和Iridium-33，總重近1.5噸。碰撞後留下了至少1000個拳頭大小的金屬塊，其中任何一塊都能再毀掉一顆衛星，併產生成百上千個新的碎塊。這些碎塊要過很長時間纔會從軌道上掉下來。

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.

凱斯勒當時擔心的問題（這個問題現在仍然存在）是，太空垃圾可能造成連鎖反應。太空軌道上碎片過多可能讓人類無法發射新的衛星，而衛星已成爲地球上人類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正在跟蹤2.1萬個直徑10釐米或更大一點的太空垃圾，這些太空垃圾就像一個個小炮彈。在近地軌道，這些碎片的飛行速度大約爲每秒7公里（合每小時25200公里）。不過，太空碎片還沒有多到堵塞太空的地步。畢竟，太空的空間很大。近地軌道比較常見，不過近地軌道的海拔高度各不相同，因此人造天體避免相撞的機率很大。而通訊衛星常用的地球同步軌道，距地心高度必須正好是42164公里。不過，在這個距離的軌道上，衛星可使用的太空空域總面積超過了220億平方公里。

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.

不過，有的軌道仍更爲擁擠一些，發生更多碰撞事件肯定只是個時間問題。2011年，美國國家科學院(National Academy of Sciences)發表的報告《限制飛船未來相撞風險》(Limiting Future Collision Risk to Spacecraft)就持這種觀點。這份報告聲稱，太空垃圾相互碰撞的例子已經夠多，就算人類現在停止發射新的衛星，這個問題也會不斷惡化。

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.

主動將這些碎片移至安全地帶看上去似乎可行，不過代價高昂。清理一顆衛星（或設計能自行清理的衛星）成本巨大，儘管這麼做對大家都有好處，但這樣做的成本卻不會由所有人分攤。因此，“大掃除”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.)

對發射衛星進行監管可不是簡單的事，Cosmos-2251衛星是俄羅斯軍方發射的，而Iridium-33衛星則是一家美國公司發射的。而最大的一起太空垃圾事件發生在2007年，當時中國軍方炸掉了一顆衛星，只是爲了展示中國有能力這麼做。目前，能令以上三個國家全都俯首帖耳的監管機構還不存在。（不過，2010年聯合國(UN)確實發佈過一套由各國自願遵守的準則。）

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.

來自智庫“未來資源研究所”(Resources for the Future)的經濟學家莫利?麥考利(Molly Macauley)曾對這問題進行過一段時間的思考。經濟學上有一個顯而易見的解決方案——對新發射衛星徵稅。該方案最近在三位研究人員——諾迪爾?阿迪洛夫(Nodir Adilov)、彼得?亞歷山大(Peter Alexander)和布倫丹?坎寧安(Brendan Cunningham)——的努力下再次流行起來。對此，麥考利提議將衛星設計與徵稅多少掛鉤，設計得更安全的衛星可以少收一點稅。另外，還有一種可能是，衛星運營方支付一筆押金，這筆押金在過期衛星被移至更安全軌道後返還。

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.

不過，經濟學家宣佈一種最優政策容易，政治家實現起來可就難了——這樣的事可謂屢見不鮮。太空垃圾問題和氣候變化問題類似：需要大家共擔責任，面臨的威脅大小難以預料，有許多“搭便車”的機會。

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.

不過，解決這個問題比解決氣候變化問題便宜多了，參與決策者也少得多。比起溫室氣體問題，太空垃圾問題應更容易達成共識。唉，這樣的對比可不算令人鼓舞。