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主要目标载人航天风险很大;迄今已有17人死在美国的航天器上,另有4人丧生于俄罗斯的飞船上。美国航天飞机每60次飞行就有一次以发生灾难性事故而告终。那么什么样的目标
The main goal of manned space flight is high; so far 17 people have died on U.S. spacecraft and another 4 have been killed by Russian spacecraft. Every 60 flights, the U.S. spacecraft ended in a catastrophic accident. So what kind of goal