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2.3.5 小行星撞击 △★
第一段：
Yet the rubble-pile碎石, hypothesis is conceptually troublesome. The material strength物质力量 of an asteroid小行星 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微小的; 不重要的 though it may be, gravity binds约束 a rubble pile together. And anyone who builds sand castles knows that even loose debris can cohere. Oft-ignored忽视 details of motion begin to matter要紧, 有关系: sliding friction滑动摩擦, chemical bonding, damping of kinetic energy, electrostatic attraction and so on. (In fact, charged particles带电粒子 from the sun can cause dust at the surface to levitate(使)升空, (使)漂浮.) We are just beginning to fathom理解…的真意, 搞懂, 弄清楚 the subtle细微的 interplay相互作用 of these minuscule forces.
碎石堆叠的小行星质量相对其他大行星体质量可以忽略，但这些小行星通过各种方式（摩擦力之类的）抱成团，就不能忽略了。
第二段：
The size of an asteroid should determine which force dominates.
小行星的大小决定哪种力占优势
One indication指示, 象征 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独块巨石 undergoing经历, 承受 random collisions, a graph曲线图 of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators.
独块巨石行星：大部分都集中在钟形分布速度中等的这一区域，少数（只有在尾巴 tail 会出现）过快或者过慢的
If nearly all asteroids are rubble piles碎石堆叠, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours (depending on its bulk density容积密度) would fly apart飞散,粉碎.
碎石堆叠行星：rubble（不是一整块而是很多碎片）的时候，这个转的很快的小尾巴会消失
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几乎, 差不多 five observed asteroids obey a strict rotation limit. The exceptions are all smaller than about 150 meters in diameter, with an abrupt突然的cutoff停止for asteroids larger than about 200 meters.
科学家发现大部分小行星都遵守这样的旋转定律，只有少数例外
第三段：
The evident conclusion—that asteroids larger than 200 meters across are multicomponent多成分的,多组分的 structures or rubble piles—agrees with recent computer modeling of collisions, which also finds a transition过渡; 转变; 变迁 at that diameter.
科学家的猜想和计算机模拟结果不谋而合
A collision can blast炸, 炸掉 a large asteroid to bits, but those bits will usually be moving slower than their mutual 共同的, 共有的escape velocity逃逸速度 (which, as a rule of thumb凭感觉的方法; 单凭经验的方法, is about one meter per second, per kilometer of radius).
体积较大的行星（貌似是独块巨石行星）被撞后成为小片时，那些碎片的旋状速度慢于他自己的速度
Over several hours, gravity万有引力; 地心引力; 重力will reassemble重新组装all but the fastest pieces into a rubble pile.
形成的小碎片又再次聚拢成为碎石堆叠行星
Because collisions among asteroids are relatively frequent时常发生的, 常见的, most large bodies天体have already suffered this fate命运. Conversely相反地, 颠倒地, most small asteroids should be monolithic, because impact冲击(力), 碰撞fragments碎片; 片断easily escape their feeble虚弱的, 衰弱的 gravity万有引力; 地心引力; 重力.
许多大型星体都难逃相互撞击的命运。然而，大多数的行星应该为独块巨石行星，因为那些碎片很容易逃脱相互的引力（意思貌似是不是那么容易形成碎石堆叠行星） |
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