这个理论认为，在早期的时候，地球曾经被一颗火星般大小(相当于地球体积的一半)的行星撞击。
An object about the size of mars-half the size of earth-slammed into our planet very early in its formation, the theory says.
现有理论认为在大约40亿年前，一个火星大小的天体撞击了原始地球，星球被装出的部分最终形成了现在的月球。
Current thinking is that some 4 billion years ago, a Mars-sized object smashed into the proto-Earth, knocking off a chunk of planet that eventually formed into the moon.
主流的理论假设认为，月球是大爆炸时期一火星大小的物体猛烈撞击地球而形成的。
The prevailing hypothesis is that the moon was formed from a giant impact event, when a Mars-sized object slammed into Earth.
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