经济学人下载:小行星探测 黎明号初试锋芒

Science and technology
科学技术

Investigating the asteroids
小行星探测

Dawn's early light
"黎明"号初试锋芒

A mission to the asteroid belt will visit leftovers from the solar system's formation
小行星带探测任务将造访太阳系形成时期的衍生物

Dimly seen through the mists of the deep
透过深层迷雾隐约可见

LAST week all eyes were on the Kennedy Space Centre in Florida, as NASA's space shuttle blasted off on its final mission.
上周，佛罗里达州肯尼迪航天中心成为全球目光的焦点，美国宇航局航天飞机呼啸升空，展开航天任务的收官之旅。

Meanwhile, hundreds of millions of kilometres away, another NASA spacecraft was approaching its destination.
与此同时，在数百万公里以外，另一艘美国宇航局的航天器正向目标进发。

If all goes to plan, then on July 16th Dawn, the largest robotic probe ever launched by America's space agency,
如果一切按计划进行，美国宇航局有史以来发射的最大的机器人探测器将与7月16日进入灶神星轨道飞行。

will drop into orbit around Vesta, the second-largest member of the asteroid belt.
灶神星是小行星带中体积居第二位的成员。

Though a mission to the asteroids may lack the glamour of sending probes to Mars and the moons of Saturn,
尽管小行星探测任务不如将探测器送入火星和土星那样令人瞩目，

these tiny planetlets have long fascinated astronomers, for they offer a window on the earliest years of the solar system.
但这些微小星体一直令天文学家们着迷，因为它们能为我们揭开太阳系形成初期之谜。

When the sun formed, some 4.5 billion years ago, it was surrounded by a disk of gas and dust.
约45亿年前，太阳形成之时曾被大量的气体和尘埃盘包围。

During the next few million years, lumps of that disk stuck together to form the familiar eight planets of the modern solar system.
在后续的几百万年间，这些盘状物质块相互结合形成了现代为人熟知的太阳系八大行星。

Some lumps, however, were left over. And a lot of them are concentrated in the asteroid belt, between the orbits of Mars and Jupiter,
但还是有些物质块被遗留下来。它们大部分集中于火星和木星轨道之间的小行星带内，

where the combined gravities of Jupiter and Saturn seem to have gathered them from other parts of the solar system.
木星和土星的组合引力似乎将它们集聚起来并与太阳系的其它部分分离。

There, they have crashed repeatedly into one another to form fragments of various sizes.
在那里，它们之间的反复碰撞形成了各种尺寸的碎片。

Small asteroids are often little more than piles of dust and chondrules, the spherical pebbles of rock that formed from dust which melted in the heat of the young sun.
体积较小的小行星只不过是一些尘埃和陨石球粒的堆积物，其中这些圆状岩石形成于太阳早期高热量下融熔的尘埃。

Larger bodies such as Vesta, though, are more interesting.
但是象灶神星这样的较大天体更能引起人们的兴趣。

Vesta's size and density make it massive enough for its gravity to keep it roughly spherical, like a proper planet.
与真正行星相仿，灶神星的尺寸和密度使其具备足够的引力保持基本的球体形状。

Another thing that makes Vesta planetlike is that it is split into distinct layers.
灶神星与行星的另一个相似点是它也分裂成不同的层。

The evidence suggests it has a nickel-iron core like the Earth's, overlain by a rocky mantle.
有证据表明它的地核也象地球一样由镍、铁元素组成，表面由岩石地幔覆盖。

Part of that evidence comes from its density and part from hundreds of chunks of rock, in the form of meteorites,
该证据的一部分来源于灶神星的密度，另一部分来源于数以百计以陨石形状存在的大块岩石，

that have been examined by Earth-bound scientists.
地球科学家们已对这些岩石进行过研究。

These rocks are believed to be the result of an asteroidal prang that happened many millions of years ago and left a crater 460km across, which dominates Vesta's southern hemisphere.
这些岩石据说是几百万年前的小行星碰撞的结果，并在灶神星的南半球形成了一个跨度为460公里的陨石坑。

The reason they are thought to come from Vesta is that the asteroid has an unusual and characteristic spectrum.
它们被认为来自灶神星是因为这颗小行星有着不同寻常的特征光谱。

It shares this with a number of smaller asteroids whose orbits suggest they were spalled off in the collision, and with about 5% of the meteorites which fall to Earth.
一些更小的行星也具有这样的光谱，从它们运行的轨道来看，它们是灶神星在碰撞中的脱落物，拥有的陨石数量约是落到地球表面陨石数量的5%。

Such meteorites look like igneous rocks from Earth—hence the belief that Vesta has a mantle.
这种陨石与地球上的火成岩相似—因此可以相信灶神星上有地幔存在。

The rocket's blue glare
火箭的蓝色眩光

After spending a year in orbit around Vesta, Dawn will perform a trick rare in space travel—it will reignite its engines and head off to orbit another body.
在绕行灶神星轨道一年后，黎明号将施展航天探索中少有的绝技——它将重新点燃发动机向另一个天体的轨道进发。

Ceres, at about 960km in diameter, is the largest asteroid. Dawn is due to arrive there in 2015.
谷神星直径大约为960公里，是小行星带里体积最大的成员。黎明号将于2015年到达谷神星。

Ceres, too, is spherical and probably divided into core and mantle, though the mantle seems to be wetter than that of Vesta.
谷神星也是球状天体，它可能也被划分成地核和地幔，但它的地幔似乎比灶神星的地幔更富含水质。

Indeed, Ceres may have ice caps and a thin atmosphere.
实际上，谷神星可能有冰层覆盖而且大气稀薄。

But it has been luckier than Vesta—and almost every other asteroid—in avoiding collisions, and has thus not yielded a huge crop of meteorites for Earth-bound scientists to examine.
但在避免碰撞方面，它比灶神星要幸运得多—几乎比其它任何小行星也都幸运，因此它没有产生可供地球科学家研究的大量陨石。

Dawn is able to perform the trick of moving from one asteroid to another thanks to its ion-rocket engines, pioneered on an earlier NASA mission called Deep Space One.
由于有了离子火箭发动机，黎明号能够施展自己的绝技从一个小行星飞行到另一个小行星。

Unlike conventional rockets, which use high-energy chemical reactions to force a stream of hot gas out of the engine,
这种发动机始创于美国宇航局早期的太空一号任务。与传统火箭不同，离子火箭发动机利用电场对飞船底部燃料中的带电粒子进行加速，

ion rockets employ electric fields to accelerate charged particles of fuel out of the back of the spacecraft.
而传统火箭是采用高能量化学反应将热气流排出发动机。

Ion engines give a pretty feeble kick.
离子发动机产生的振动极其微弱。

Dawn's produce 92 millinewtons of thrust, something like a fiftieth of the amount in a smallish firework rocket.
黎明号产生的推进力仅为0.092牛顿，是小型烟花火箭推进力的五十分之一。

The exhaust velocity, though, is enormous—more than ten times that of a chemical rocket—and this makes ion propulsion extremely efficient.
但其排气速度却异常惊人，大约为化学燃料火箭的十倍—这使得离子推进器的效率非常高。

Though an ion engine could never lift a spacecraft out of Earth's gravity well,
尽管离子发动机不能使航天飞机很好地摆脱地球引力，

once that craft is in deep space the futuristic-looking blue glow of its exhaust can take it to parts that chemical engines find much harder to reach.
但航天器一旦进入太空深处，尾部喷射出的极具超现实光彩的蓝色辉光可以将它带到化学燃料发动机难以到达的地方。

Dawn started off with 450kg of propellant, and even at maximum throttle its engines use only a quarter of a kilo a day.
黎明号初始装载了450公斤推进剂，即使在全速行进的情况下，其发动机每天消耗的燃料也仅为0.25公斤。

The arrival of Dawn at Vesta also marks another significant achievement.
黎明号到达灶神星也将标志着太空探索取得了另一项重大成就。

If the craft does manage to go into orbit it will mean that working man-made satellites are circling and scrutinising eight bodies in the solar system:
如果它能够成功进入轨道，那将意味着在轨人造卫星已经能够对太阳系的八个天体:

the sun, Mercury, Venus, Earth, the moon, Mars and Saturn, as well as Vesta.
太阳、水星、金星、地球、月球、火星、土星和灶神星—进行绕行和探查。

That gives comfort to those who fear that the end of the shuttle programme might mean a wider loss of interest in the exploration of space.
对于那些担忧航天计划终结意味着对太空探索兴趣渐失的人们来说，这将会带来一些安慰。

Whether it does—and the new record proves to be the high-water mark of unmanned space exploration.
是否黎明号能够完成任务—若完成将表明无人飞船创造了太空探索的最高记录,

or whether Dawn's arrival proves merely a staging post on the road to greater things, remains to be seen.
或者是否黎明号的到达仅是完成了宏伟目标中的一项阶段任务，对此我们都将拭目以待。