扫天
Mar 31st 2010 | From The Economist print edition
A satellite that tidies up after itself
死后自我收尸的卫星
ARISTOTLE believed that the heavens were perfect. If they ever were, they are no longer. The skies above Earth are now littered with the debris of dead satellites, bits of old rockets and the odd tool dropped by a spacewalking astronaut. Such is the extent of the detritus that the first accidental collision between two satellites has already taken place. It happened in February 2009, when a defunct Russian Coos ashed into a functioning American Iridium, destroying both and creating even more space junk. To stop this sort of thing happening again Vaios Lappas of the University of Surrey, in England, has designed a system that will remove satellites from orbit at the end of their useful lives—and as a bonus will scour part of the sky clean as it does so.
亚里士多德相信宇宙是完美的。如果宇宙曾经完美，现在也已经不完美了。地球上空现在遍布着垃圾——报废卫星的碎片和残骸，火箭残体，宇航员进行太空行走时偶尔掉落的工具。既然宇宙垃圾都已经到达如此程度了，那么，首次的卫星相撞事故肯定早已发生。那是在2009年，当时一颗报废的俄罗斯“宇宙”系列卫星猛撞在一颗正常工作的美国铱星上面，双方具毁，而且产生了更多的宇宙垃圾。为了防止这种事情的再次发生，英国萨里大学的沃欧斯·拉帕斯设计了一个系统，能在卫星服役期满之后将其移出轨道——并且在这个过程中还能附带将部分天空擦亮。
Dr Lappas’s satellite-removal system employs a solar sail. As light from the sun hits the sail, it imparts a minuscule but continuous acceleration. When a satellite is first launched, the sail is angled in a way that causes this acceleration to keep the satellite in orbit. (Orbits gradually decay as a result of collisions with the all number of air molecules found even at altitudes normally classified as “outer space”.)
拉帕斯的卫星移除系统有一个太阳帆。太阳光撞击帆的同时会带来一个细微但却持续的加速。卫星发射之初，太阳帆的角度会固定在某一位置，以便让这个加速把卫星固守在轨道之上。（轨道会渐渐衰减，这是由于卫星与为数甚少的空气分子之间相互碰撞所致。即使是在通常被划归为“外太空”的高度上也还找的到这些分子。）
Solar sails have yet to be used widely to propel spacecraft in this way—several earlier versions came unstuck when the sails failed to unfurl properly—but doing so is not a novel idea in principle. The novelty Dr Lappas envisages is to change the angle of the sail when the satellite has become defunct. Instead of keeping the derelict craft in orbit, it will, over the course of a couple of years, drag it into the atmosphere and thus to a fiery end. Not only that, but the sail will also act like a handkerchief, mopping up microscopic orbital detritus such as flecks of paint from previous launches. A fleck of paint may not sound dangerous, but if travelling at 27,000kph (17,000mph), as it would be in orbit, it could easily penetrate an astronaut’s spacesuit.
太阳帆这种推动宇宙飞船前进的方法尚未被广泛应用——几个早期的型号因为帆未能按要求打开而遭遇失败——但是这么做从根本上说不是一个新想法。而拉帕斯博士的新设想是在卫星报废以后也可以让帆改变角度。这个时侯，帆的作用不是让破旧的卫星继续待在轨道上，而是会用几年的时间把卫星拖入大气层，使其在烈火中走向终点。帆不只有这一点功用，它还会像手帕一样，收拾轨道上微小的残片，比如以前的发射留下的油漆碎片。一小片油漆听起来可能并不危险，但是如果以27000千米/小时（17000英里/小时）的速度运行——就像轨道中的漆片那样，宇航员的太空服就很容易被其穿透。
A prototype of Dr Lappas’s design, called CubeSail, will be launched late next year. It weighs just 3kg (7lb) and, when folded up, measures 30cm (12 inches) by 10cm by 10cm. Once unfurled, however, the sail will have an area of 25 square metres. If this prototype, which is paid for by EADS, a European aerospace company, proves successful, solar sails might be added to many future satellites. That would enable them to be removed rapidly from orbit when they became useless and would restore to the skies some measure of Aristotelian perfection.
拉帕斯设计了一个叫作“立方帆”的样品，将会在明年晚些时候发射。这个帆仅重3千克（7磅）；合起来时，体积30（12英寸）x10x10厘米 。但是一旦张开，其面积将会达到25平方米。如果这个样品——由欧洲一家叫作EADS的航空公司提供资金——试验成功，太阳帆可能会被添加到未来的许多卫星上面。这会让它们在报废之后能够很快地从轨道上移走，天空从而就会恢复几许亚里士多德的完美。
