astronaut是元音读音开头，所以用an。可数名词前面用a还是an，主要看后面的单词的读音，元音读音开头的就用an，辅音读音开头的用a。astronaut：n.宇航员;航天员

例句：

China's astronaut didn't find great wall of china?

太空中看不到长城？

Human dynamics model of astronaut action based on computer animation simulation

航天员活动的计算机仿真中人体动力学模型

The Research on Astronaut Performance in Modules by Ground Simulated Experiment

航天员舱内活动地面模拟实验研究