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New Spin on Asteroids
Asteroids in the Koronis family intrigue scientists because they move in sync despite billions of years of collisions. （写这篇文章的背景，科学家研究K family 小行星为什么同步旋转尽管相互碰撞了数十亿年）
Sunlight can have surprisingly important effects on the spins of small asteroids. A recent 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.（新理论的提出，小行星为什么同步旋转的原因，日光的作用大于碰撞产生的）
David Vokrouhlicky (Charles University), David Nesvorny and William Bottke (both of the Southwest Research Institute) conducted the study, which showed that sunlight absorbed and reemitted 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.（DV,DN,WB对上面理论的进一步研究说明）
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.（对老学说（既行星之间的碰撞控制小行星旋转速率和方向）的置疑，学者SS通过观察K family小行星提出了置疑的evidence，说明碰撞后产生的小行星碎片同一个轴心，同一个方向，相同的速率旋转）
"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."（S的数据说明小行星碎片是同步旋转的，但是是怎样形成的？）
"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."（对上面的现象做了一个比喻说明）
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.
（S研究的10颗小行星剩下的6颗要么旋转都很慢，要么都很快,进一步说明小行星是同步旋转的）
"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.（解释一簇小行星旋转速率为什么相同，受日光的影响）
"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."（做了一个比喻解释上面一段话）
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). This state, called a spin-orbit resonance, can drive an asteroid's rotation rate and spin axis to particular values.（用计算机仿真演示了日光对K family小行星的旋转的影响）
"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."（这些研究结果对以后研究其它行星的作用）
NASA, the National Research Council and the Grant Agency of the Czech Republic funded the study.
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发表于 2004-8-26 15:49:00
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