[roshan]

Set a collision distance, when 2 masses are closer than this to each other,
they are assumed to collide, and become 1 body, with mass and momentum
equal to the sum of the pre-collision values.
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还有，当两个运行的星球由于万有引力碰撞在一起后，就合二为一，这一步怎么做？
用到的动能公式：(m1+m2)v=m1v1+m2v2