Trajectory optimization of the lunar soft landing problem can be treated as a nonlinear optimal control problem with free terminal time and control constraints. The difficulties of this optimization problem are how to deal with the free terminal time and how to convert the nonlinear control problem into a tractable optimization problem. This paper surveys different kinds of lunar soft landing models and the optimization approaches for determining the optimal trajectory of the lunar detector. Specifically, we make a general classification for the current optimization methods, and then compare the relative merits of different methods. At last, according to the essential characteristics of the trajectory optimization problem for the lunar soft landing, we summarize several effective processing methods, which provide a guidance for the China's lunar soft landing plan.%月球软着陆轨道优化问题是一非线性、终端时间自由且带有控制约束的最优控制问题.月球探测器软着陆轨道优化的难点在于怎么处理自由的终端时间,如何把非线性的控制问题转换为易于处理的优化问题.本文介绍了月球探测器的几种软着陆方式和月球探测器轨道的多种优化方法,对各种优化算法进行了一个大概的分类,并对各种优化算法的优缺点进行了分析比较.进而,总结了解决月球探测器软着陆轨道优化问题难点的几种有效方法,为未来我国的月球软着陆工程提供了参考.
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