小行星的补充阅读

dumdumface

找到了一个和小行星撞击差不多的文档,里面有一个图文的解释...很直观,有兴趣的同学可以看看.
在文档的最后一页...
http://www.miracosta.edu/home/kmeldahl/articles/asteroids.pdf

dumdumface

没有人ding...自己读了一下,有3段ms是原文, 请同学鉴定.

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.

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.

rachel19

我来顶

serenashu

谢谢~~

yydetiankong

我也来丁~~~~~~~

手塚弥生

昨天考的确认是 我都欲哭无泪叻两篇天文… 差不多就是这个的缩写 大家快点看懂它…

谷雨书

ameliewoo

谢谢~~~

aquatic

谢谢楼主和确认的同学！

simpleru

比起文来，我更喜欢图