载人飞船要发射到月球上,必须按登月轨道飞行。因为月球没有大气层,飞船飞抵月球后,必须用发动机减速,才能实现软着陆。登月轨道从理论上说,可以有直接登月、环地登月、环月登月3种。直接登月轨道是一种最简单的登月轨道。直接登月轨道从理论上说,可以是直线、椭圆、抛物线、双 Manned spacecraft to be launched on the moon, lunar orbit must fly. Because the moon has no atmosphere, the spacecraft must be decelerated with an engine to achieve a soft landing after it has flied to the moon. In theory, the lunar orbit can have 3 types of lunar landing, lunar landing and lunar landing. The direct lunar orbit is one of the easiest lunar orbit. In theory, the lunar orbit can be straight, elliptical, parabolic, double