航天器
How many spacecrafts are orbiting the moon? 有多少航天器绕月球轨道飞行？
The rotation dynamic and quaternion kinetic equations of spacecrafts were educed according to the principles of variable-structure control. 根据滑动模态变结构控制原理，导出了航天器三轴动力学和四元数姿态运动学方程。
