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Science and technology
科學技術
Investigating the asteroids
小行星探測
Dawn's early light
"黎明"號初試鋒芒
A mission to the asteroid belt will visit leftovers from the solar system's formation
小行星帶探測任務將造訪太陽系形成時期的衍生物
Dimly seen through the mists of the deep
透過深層迷霧隱約可見
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，佛羅里達州肯尼迪航天中心成為全球目光的焦點，美國宇航局航天飛機呼嘯升空，展開航天任務的收官之旅。
Meanwhile, hundreds of millions of kilometres away, another NASA spacecraft was approaching its destination.
與此同時，在數百萬公里以外，另一艘美國宇航局的航天器正向目標進發。
If all goes to plan, then on July 16th Dawn, the largest robotic probe ever launched by America's space agency,
如果一切按計劃進行，美國宇航局有史以來發射的最大的機器人探測器將與7月16日進入灶神星軌道飛行。
will drop into orbit around Vesta, the second-largest member of the asteroid belt.
灶神星是小行星帶中體積居第二位的成員。
Though a mission to the asteroids may lack the glamour of sending probes to Mars and the moons of Saturn,
盡管小行星探測任務不如將探測器送入火星和土星那樣令人矚目，
these tiny planetlets have long fascinated astronomers, for they offer a window on the earliest years of the solar system.
但這些微小星體一直令天文學家們著迷，因為它們能為我們揭開太陽系形成初期之謎。
When the sun formed, some 4.5 billion years ago, it was surrounded by a disk of gas and dust.
約45億年前，太陽形成之時曾被大量的氣體和塵埃盤包圍。
During the next few million years, lumps of that disk stuck together to form the familiar eight planets of the modern solar system.
在后續的幾百萬年間，這些盤狀物質塊相互結合形成了現代為人熟知的太陽系八大行星。
Some lumps, however, were left over. And a lot of them are concentrated in the asteroid belt, between the orbits of Mars and Jupiter,
但還是有些物質塊被遺留下來。它們大部分集中于火星和木星軌道之間的小行星帶內，
where the combined gravities of Jupiter and Saturn seem to have gathered them from other parts of the solar system.
木星和土星的組合引力似乎將它們集聚起來并與太陽系的其它部分分離。
There, they have crashed repeatedly into one another to form fragments of various sizes.
在那里，它們之間的反復碰撞形成了各種尺寸的碎片。
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.
體積較小的小行星只不過是一些塵埃和隕石球粒的堆積物，其中這些圓狀巖石形成于太陽早期高熱量下融熔的塵埃。
Larger bodies such as Vesta, though, are more interesting.
但是象灶神星這樣的較大天體更能引起人們的興趣。
Vesta's size and density make it massive enough for its gravity to keep it roughly spherical, like a proper planet.
與真正行星相仿，灶神星的尺寸和密度使其具備足夠的引力保持基本的球體形狀。
Another thing that makes Vesta planetlike is that it is split into distinct layers.
灶神星與行星的另一個相似點是它也分裂成不同的層。
The evidence suggests it has a nickel-iron core like the Earth's, overlain by a rocky mantle.
有證據表明它的地核也象地球一樣由鎳、鐵元素組成，表面由巖石地幔覆蓋。
Part of that evidence comes from its density and part from hundreds of chunks of rock, in the form of meteorites,
該證據的一部分來源于灶神星的密度，另一部分來源于數以百計以隕石形狀存在的大塊巖石，
that have been examined by Earth-bound scientists.
地球科學家們已對這些巖石進行過研究。
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.
這些巖石據說是幾百萬年前的小行星碰撞的結果，并在灶神星的南半球形成了一個跨度為460公里的隕石坑。
The reason they are thought to come from Vesta is that the asteroid has an unusual and characteristic spectrum.
它們被認為來自灶神星是因為這顆小行星有著不同尋常的特征光譜。
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這樣的光譜，從它們運行的軌道來看，它們是灶神星在碰撞中的脫落物，擁有的隕石數量約是落到地球表面隕石數量的5%。
Such meteorites look like igneous rocks from Earth—hence the belief that Vesta has a mantle.
這種隕石與地球上的火成巖相似—因此可以相信灶神星上有地幔存在。
The rocket's blue glare
火箭的藍色眩光
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.
在繞行灶神星軌道一年后，黎明號將施展航天探索中少有的絕技——它將重新點燃發動機向另一個天體的軌道進發。
Ceres, at about 960km in diameter, is the largest asteroid. Dawn is due to arrive there in 2015.
谷神星直徑大約為960公里，是小行星帶里體積最大的成員。黎明號將于2015年到達谷神星。
Ceres, too, is spherical and probably divided into core and mantle, though the mantle seems to be wetter than that of Vesta.
谷神星也是球狀天體，它可能也被劃分成地核和地幔，但它的地幔似乎比灶神星的地幔更富含水質。
Indeed, Ceres may have ice caps and a thin atmosphere.
實際上，谷神星可能有冰層覆蓋而且大氣稀薄。
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，它比灶神星要幸運得多—幾乎比其它任何小行星也都幸運，因此它沒有產生可供地球科學家研究的大量隕石。
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.
由于有了離子火箭發動機，黎明號能夠施展自己的絕技從一個小行星飛行到另一個小行星。
Unlike conventional rockets, which use high-energy chemical reactions to force a stream of hot gas out of the engine,
這種發動機始創于美國宇航局早期的太空一號任務。與傳統火箭不同，離子火箭發動機利用電場對飛船底部燃料中的帶電粒子進行加速，
ion rockets employ electric fields to accelerate charged particles of fuel out of the back of the spacecraft.
而傳統火箭是采用高能量化學反應將熱氣流排出發動機。
Ion engines give a pretty feeble kick.
離子發動機產生的振動極其微弱。
Dawn's produce 92 millinewtons of thrust, something like a fiftieth of the amount in a smallish firework rocket.
黎明號產生的推進力僅為0.092牛頓，是小型煙花火箭推進力的五十分之一。
The exhaust velocity, though, is enormous—more than ten times that of a chemical rocket—and this makes ion propulsion extremely efficient.
但其排氣速度卻異常驚人，大約為化學燃料火箭的十倍—這使得離子推進器的效率非常高。
Though an ion engine could never lift a spacecraft out of Earth's gravity well,
盡管離子發動機不能使航天飛機很好地擺脫地球引力，
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.
但航天器一旦進入太空深處，尾部噴射出的極具超現實光彩的藍色輝光可以將它帶到化學燃料發動機難以到達的地方。
Dawn started off with 450kg of propellant, and even at maximum throttle its engines use only a quarter of a kilo a day.
黎明號初始裝載了450公斤推進劑，即使在全速行進的情況下，其發動機每天消耗的燃料也僅為0.25公斤。
The arrival of Dawn at Vesta also marks another significant achievement.
黎明號到達灶神星也將標志著太空探索取得了另一項重大成就。
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:
如果它能夠成功進入軌道，那將意味著在軌人造衛星已經能夠對太陽系的八個天體:
the sun, Mercury, Venus, Earth, the moon, Mars and Saturn, as well as Vesta.
太陽、水星、金星、地球、月球、火星、土星和灶神星—進行繞行和探查。
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.
對于那些擔憂航天計劃終結意味著對太空探索興趣漸失的人們來說，這將會帶來一些安慰。
Whether it does—and the new record proves to be the high-water mark of unmanned space exploration.
是否黎明號能夠完成任務—若完成將表明無人飛船創造了太空探索的最高記錄,
or whether Dawn's arrival proves merely a staging post on the road to greater things, remains to be seen.
或者是否黎明號的到達僅是完成了宏偉目標中的一項階段任務，對此我們都將拭目以待。
