—What do you think of travelling in space?—It was _____...

根据句子中的‘It was ________ in the past. However, it is no longer a dream now.’，意思是过去太空旅行是‘不可能的’，但现在不再是梦想。所以正确答案是‘impossible’，表示过去太空旅行不可能实现，而现在则有可能。