2.3.5 小行星撞击 原文很难懂 翻译了下。。。

LJWISC1OL

Yet the rubble-pile hypothesis is conceptually troublesome. The material strength of an asteroid is nearly zero, and gravity is so low you are tempted to neglect that, too. What’s left? The truth is that neither strength nor gravity can be ignored. Paltry though it may be, gravity binds a rubble pile together. And anyone who builds sand castles knows that even loose debris can cohere. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, electrostatic attraction and so on. (In fact, charged particles from the sun can cause dust at the surface to levitate.)



The size of an asteroid should determine which force dominates. One indication is the observed pattern of asteroidal rotation rates. Some collisions cause an asteroid to spin faster; others slow it down. If asteroids are monolithic rocks undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators. If nearly all asteroids are rubble piles, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours (depending on its bulk density) would fly apart. Alan Harris of the Jet Propulsion Laboratory in Pasadena, Calif., Petr

Pravec of the Academy of Sciences of the Czech Republic in Prague and their colleagues have discovered that all but five observed asteroids obey a strict rotation limit [see illustration on page 48]. The exceptions are all smaller than about 150 meters in diameter, with an abrupt cutoff for asteroids larger than about 200 meters.


The evident conclusion—that asteroids larger than 200 meters across are multicomponent structures or rubble piles—agrees with recent computer

modeling of collisions, which also finds a transition at that diameter. A collision can blast a large asteroid to bits, but those bits will usually be moving slower than their mutual escape velocity (which, as a rule of thumb, is about one meter per second, per kilometer of radius). Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile [see illustration above]. Because collisions among asteroids are relatively frequent, most large bodies have already

suffered this fate. Conversely, most small asteroids should be monolithic, because impact fragments easily escape their feeble gravity.





瓦砾堆的假设在概念上很复杂。构成行星的物质的力几乎为0，而引力更是小得可以忽略。那么还有啥？真相是物质的力和引力都不可以忽略。虽然这些引力真的很小，但是重力使瓦砾粘在一起。玩过用沙做的城堡的人都知道，即使是松散的沙都有粘力。忽略的东西可能很重要：滑动摩擦力，化合粘力，动力阻尼，电子引力或者其他,(事实上，从太阳发出的带电的粒子可以导致表面的尘埃悬浮起来)

行星的体积决定了什么力量是它的主导。被观察到的行星转动速率可以作为一个参考指标。有些碰撞使行星转得更快；有的则减慢。如果行星是整块的石头，如果通过任意撞击，它的转动速率分布图应该是一个BELLshape的分布，尾部的分布属于速度快的概率。如果所有的行星是瓦砾状的，这个分布的尾部部分就会不见了，因为任何瓦砾状的东西都会因为转得快而分离。一些专家发现，所有行星（除了5个外）都严格遵守转动的这一速率的限制。其他的都是直径小于150米的，而且都是从直径大于200的大行星分离出来的。


这个现象表明，直径大于200米的行星是多结构的或者是瓦砾状的，和电脑的模拟的关于行星破装结果一致。碰撞可以把行星炸成小块，但是这些小块的速度通常都会比脱离速度更慢。几小时后，重力就会把那些速度慢的碎片集中而形成瓦砾堆。因为小行星的碰撞经常发生，所以大体积的行星都遇到过这些情况。相反，很多小行星是整块的石块（不是瓦砾堆），因为撞击它们的碎片很容易逃脱它们的引力。

翻译得不好，别见怪。。。顺便寻找本月食物号同考的广州的朋友 PS:我下午考

XYXB

all but five observed asteroids。。。。

这句话应该理解为所有的小行星遵循，除了5个

all but five  直译的话是全部但扣除5个

LJWISC1OL

谢谢，已经更改

sdcar2010

It's better to read and analyze the passage in English without converting it into Chinese.  GMAT is not a test for one's skills in translation.

LJWISC1OL

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luoxlucy88

谢谢楼主呀。这样会帮助理解呢。

luoxlucy88

ps：楼主的签名和头像图片很给力呀！！！

cat62158

哈哈 我上午考哦~

cat62158

广州场~~搞不好会撞到LZ呢

LJWISC1OL

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