2001: A Space Odyssey
The infinite unknown of space is more terrifying than any grotesque monster. It's real. It's a threat for our belief. Threatening our own beliefs of known existence. Unlike our beliefs in entities whose existence we can't prove space is there to prove us wrong. In this film the unknown is the ultimate adversary. Not harming us but making us questioning ourselves.
Man made tools to conquer this earth. But also tools that may conquer him. The thinking machine as a potential doom bringer. A machine that will threaten man because it falls victim to what humans also fear most: death. Nothing is perfect yet the realisation can cause frustration and fear.
Space can feel grand through making us small. Conquering it the stuff of infinite tales. The findings enlightening of terrifying. The terror might be something too complex for the human mind. Or worse: Something familiar in a place and context devoid of (human) logic.
There is nothing more frightening than something the mind cannot comprehend and the body can't escape.