求助阅读狗的疑问---小行星撞击，NN一定要来看看呀

sirenamx

正在看小行星的那篇文章。暂且采用这个版本整理：

搜索 dumdumface （已确认）
...自己读了一下,有3段ms是原文

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.) We are just beginning to fathom the subtle interplay
of these minuscule forces.

（考过的狗主人说第二段重点考，请大家注意！）
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.

内容很绕，看得头都晕了。借用一狗主人的总结大致是

（一）临界转速：1次 / 2-3小时
       （1）超过临界转速且size>200，引力拉不住，fly apart，能吸收周边的行星碎片，成分复杂
       （2）超过临界转速且size<200，引力可能拉住，不一定fly apart，无法抓住其它行星碎片，成分单一
（二）没有超过临界转速，那么无论size大小都能留下，不会fly apart

选项问题是

1. 还有一个，绕了一大圈，问哪些旋转的速度快得足以让行星fly apart的一定是以下哪一种
答：rubble pile spinning faster than once every two or three hours would fly apart.

2. 问哪些旋转的速度快得足以让行星不fly apart的小行星一定是以下哪一种
答：那5个直径小于150米的行星。

3. 一个问削弱最后一段的第一句里的evident的
答：any rubble pile spinning faster than once every two or three hours (depending on its bulk density) would fly apart取反

不知对否？实在看晕了！谢谢大家了！