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[破事水水] [2006.06.18] 〖天文生物探索部〗（天文组）阳光使小行星以奇怪的方式快速旋转[原创翻译

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发表于 2006-6-18 10:40:32 |只看该作者 |倒序浏览

阳光使小行星以奇怪的方式快速旋转
SOUTHWEST RESEARCH INSTITUTE 新闻稿
发布时间: 2003 年9月 11 日
在Southwest Research Institute (SwRI) 和 Charles University (Prague) 的研究员的一项新研究里发现阳光竟然能转动重要影响小小行星。研究指出阳光可以发挥更重要的作用,确定自旋率与小行星碰撞的比,原先被认为是小行星旋转控制率。研究结果会在9月11日的自然出版物中发表。
David okrouhlicky(查尔斯大学) ，David Nesvorny 和William Bottke( 两人均在 SwRI 空间学部门) 进行了研究, 被吸收而且再发出的数百万年的阳光能快速旋转一些小行星甚至可以将它们摧毁。在其他情况下,几乎可以完全阻旋转。研究人员指出,阳光效益,加上地球的引力,可以慢慢迫使小行星向同一个方向旋转.
直到最近，研究员指出小行星的碰撞影响着漂浮在太空中的小行星旋转的速度和方向。 10个被麻省理工科技系的研究员Stephen Slivan观察的旋转不规则的直径为15至25英里的小行星,正是在所谓的 Koronis 小行星家庭, 一个在10亿年前被高速度撞击后产生的小行星碎片群。Slivan 发现这4颗小行星的旋转速度不仅相同，连旋转轴都指向同一个方位。
" 数据清楚的现实这些旋转矢量是真实的，但是它们是如何做到这点的却是个迷”Slivan说。 "我是高兴的是其它人也认为这是一个有趣的问题"。
" 这些图片现实了这些小行星是如此古怪, 想像你当你正要搭乘航天飞机发射正如被传递顶端旋转的盒子，把所有的震动都集中到发射上,当你到达了轨道的时候 , 你会猜测顶端有不同的旋转速度和方位,"Bottke说。 "再者,想象你打开一个顶部在以同样的速度旋转而且全部指向仙后座星群的盒子。现在增加的数量和十亿多年小行星碰撞的数量几乎相等。这是我们找到我们自己奇怪的情形。"
Slivan研究的其余6个小行星要么旋转的速度异常慢，比钟表的时针走得还慢，要么就是旋转的特别快，以至于在它们表面上的一些物质要被甩出来。
"一个撞会可能会随机打破这些旋转率。这是一个很意外地发现一批小行星有多种的自旋方式。” Nesvorny说。
为了解释Koronis家族的小行星的旋转方式， Vokrouhlicky Nesvorny 和Bottke调查了这些小行星是如何反射吸收太阳光以及将这些能量以热的方式再发射出去。他们发现,虽然由于再辐射阳光产生的反作用力极少,但如果有足够的时间，仍然能大大的改变小行星的旋转轴和选择速度。"和故事《龟兔赛跑》一样，速度慢但稳定的阳光赢得了这张比赛虽然比不上小行星之间的碰撞带来的影响。在太空中的阳光从来没有停止过,"Bottke说," 而且最大多数的小行星因为年龄已经遭到影响。"
通过使用计算机进行模拟, 研究人员显示日光已经在2到3亿年前就开始慢慢增减Koronis 家族的小行星的旋转速度。更不可思议的是,他们发现一些模拟小行星形成特殊自旋方式,迫使原来的小行星的旋转轴(由来自太阳引力干扰)"攻击"在同频率旋转的小行星的轨道(制作来自地球的引力干扰). 这种状态称为自旋共振轨道,能改变小行星的运动的速度和旋转轴。
"这些结果让我们以一种新的方式看待这些小行星"Vokroulicky说. "我们希望这方面的工作能促进不同地区对主要小行星带的观测研究. 我们现在只是接触到这个有趣的问题的表皮. "
NASA,NationalResearchCouncil[美国航空暨太空总署],捷克共和国资助机构提供的研究报告.

Sunlight makes asteroids spin in strange ways
SOUTHWEST RESEARCH INSTITUTE NEWS RELEASE
Posted: September 11, 2003
A new study by researchers at Southwest Research Institute (SwRI) and Charles University (Prague) has found that sunlight can have surprisingly important effects on the spins of small asteroids. The study indicates that sunlight may play a more important role in determining asteroid spin rates than collisions, which were previously thought to control asteroid spin rates. Results will be published in the Sept. 11 issue of Nature.
David Vokrouhlicky (Charles University), David Nesvorny and William Bottke (both of the SwRI Space Studies Department) conducted the study, which showed that sunlight absorbed and remitted over millions to billions of years can spin some asteroids so fast they could potentially break apart. In other cases, it can nearly stop them from spinning altogether. The team even noted that the effects of sunlight, combined with the gravitational tugs of the planets, can slowly force asteroid rotation poles to point in the same direction.
Until recently, researchers thought asteroid impacts controlled the rotation speed and direction of small asteroids floating in space. The unusual spin states of 10 asteroids observed by Stephen Slivan, a researcher at the Massachusetts Institute of Technology, however, have cast doubt on this idea. Slivan's asteroids, the first in the 15- to 25-mile-diameter range to have their spins extensively studied, are in the so-called Koronis asteroid family, a cluster of asteroid fragments produced by a highly energetic collision billions of years ago. Slivan found that not only do four of these asteroids rotate at nearly the same speed, but they also have spin axes that point in the same direction.
"The data clearly show that the spin vector alignment is real, but how they got that way has been a big puzzle," says Slivan. "I'm delighted that others find this to be an interesting problem."
"To picture just how weird these asteroids really are, imagine you were handed a box of spinning tops just as you were about to launch aboard the space shuttle. Given all the shaking produced by the launch, you would expect the tops to have different spin speeds and orientations by the time you reached orbit," says Bottke. "Instead, imagine your surprise upon opening the box if the tops were all spinning at the same speed and had their handles pointing toward the constellation Cassiopeia. Now increase the size of the tops by a factor of a million and pretend that the bouncing during launch is equivalent to billions of years of asteroid collisions. This is the strange situation we find ourselves with."
The remaining six asteroids studied by Slivan either have extremely slow spin rates, such that they rotate slower than the hour hand of a clock, or very fast spin rates, such that they are near the limit beyond which loose material on the surface of an asteroid would fly off.
"One would expect that collisions would have randomized these rotation rates. It was a big surprise to find a cluster of asteroids with such odd spin states," says Nesvorny.
To explain the spin states of Koronis family asteroids, Vokrouhlicky, Nesvorny and Bottke investigated how asteroids reflect and absorb light from the sun and reradiate this energy away as heat. They found that while the recoil force produced by the reradiation of sunlight is tiny, it can still substantially alter an asteroid's rotation rate and pole direction if it has enough time to act.
"Like the story about the tortoise and the hare, slow and steady sunlight wins the race over the fast-acting, but less effective, jolt of collisions between asteroids. Sunlight in space never stops," says Bottke, "and most asteroids have been exposed to a lot of it because of their age."
Using computer simulations, the team showed that sunlight has been slowly increasing and decreasing the rotation rates of Koronis family asteroids since they were formed 2 to 3 billion years ago. More remarkably, they found that some simulated asteroids were captured into a special spin state that forced the wobble of the asteroid's spin axis (produced by gravitational perturbations from the sun) to "beat" at the same frequency as the wobble of the asteroid's orbit (produced by gravitational perturbations from the planets).
"These results give us a new way to look at the asteroids," says Vokroulicky. "It is our hope that this work will stimulate observational studies into many different regions of the main asteroid belt. We have only scratched the surface of this interesting problem."
NASA, the National Research Council and the Grant Agency of the Czech Republic funded the study.

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