They radiate heat, they're very expensive to keep alive and unlike robotic missions, they have a natural desire to come back, so that anything involving human beings is enormously expensive.

他们辐射热量，需要高昂的代价来保障生命，不像机器人，他们还需要返回地球，因此只要涉及到人类，花费必然不菲。

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"There is a very keen sense of desire to exercise sovereignty, and so there is a natural impulse, " he says.

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