What Do You think the Holy Grail of Space is?

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The Holy Grail of space, according to the TV program Naked Science, is life beyond our world. I might agree, but I'm not sure. I think it's in the top 5 without fail, but I think finding another inhabitable planet might beat that. Since we seem to be screwing up this one rapidly. We'll need a new place to go when the worst happens. What do think the Holy Grail of Space is? And why?