THE ONE GREAT QUESTION left unanswered by the manned landings of the Apollo project in the1960s was the moon' s origin. Now a growing consensus¹ among astronomers² favors the"giant impact"theory，an idea that was discouned by most when it was first put forward in1975.It rcquires no magic，no special pleading，no extra twiddling"says H.J.Melosh，professor of planetary sciences at the University of Arizona." It just works."

　　The moon may have gotten its start 4.5billion years ago when，according to the theory，a planerary projectile³ about one一seventh the earth' s mass collided with the earth.The energy of collision crushed and vaporized major parts of the two globes，sending out a high-velocity jct of material at temperatures as high as 12，000 degrees Fahrenheit⁴. Within Just a few hours，some of it came back together far enough away from the earth to remain in orbit.The earth itself reformd as a combination of the old planet and the bulk of the projectile.

　　Strong support for this scenario⁵ has come recently from simulations on some of the world's most powerful computers，primarily in a project at Los A1amos national laboratory involving astrononer Alastair Cameron，one of the first proponents⁶ of the theory.

　　One reason for the appeal of the giant-impact scenario is that it seems to explain all the chemical findings from Apollo.For example，the moon rocks lack water，sodium and other volatile⁷ materials-precisely the substances that would boil away in the rapid vaporization after impact.

　　Scientists believe that the giant-impact theory may also solve some other outstanding puzzles of the earth'makeup-一for example，why veins⁸ of heavy metals such as gold and platinum⁹ lie shallow enough to be mined."The impact scenario explains that all in one shot."says Melosh." The gold in your wedding ring came from the projectile，not the original earth".

　　月球起源的再思考

　　目前，尚未被始于1960年的阿波罗登月计划解决的最大的一个问题就是月球的起源问题。而现在在天文学家中间越来越趋于统一意见的"大爆炸"理论，在 1975年提出之初却被大大低估了。正像美国亚利桑那大学研究行星科学的教授H.J.麦路斯所说：不必变魔法，不需特别的借口，更无需玩弄什么手法，只有靠扎实地工作。

　　月球也许是4.5亿年前诞生的。根据有关理论，相当于地球1／7质量的行星碎块与地球碰撞。其碰撞的能量挤压和蒸发掉两个星球的主要部分。以华氏12000度的高温高速喷出的物质，只几个小时便到达远离地球的轨道，而地球则作为一个老行星和大块碎块的结合物而存在了下来。

　　世界上最大的电子计算机的一次"大爆炸"模拟演示，对"大爆炸"理论是一个强有力的支持。在美国洛斯阿拉莫斯国家实验室计划参与者中有天文学家阿拉斯泰·卡麦伦，他是"大爆炸"理论的第一个支持者。

　　这个"大爆炸"理论演示实验吸引人的一个原因是它似乎解释了从月球带回来的所有化学发现物，例如，月球上缺水，钠和别的易挥发物质——精确地讲，这些物质在碰撞之后在迅速的蒸发中被烧干了。

　　科学家们相信大碰撞理论也许解决了另外一些关于地球起源的难题，例如，为什么像金子和铂这样的矿藏埋藏在足够浅的地表而容易被人采掘，"碰撞的演示在一组画面里就解释了这一切。"麦路斯说道："你的结婚戒指所含的金子就是从碰撞后的碎块来的，而非来自地球本身".