Well, the scariest elements often include the idea of being helpless. In space, if something goes wrong with your equipment, like your oxygen supply or your spaceship's propulsion system, there's not much you can do. You can't just call for help and expect immediate assistance. Also, the vastness of space itself is terrifying. It makes you feel so small and insignificant, and the thought that you could be lost in that infinite expanse forever is really scary. And then there are the extreme conditions, like radiation and temperature fluctuations that can kill you in an instant if your protection fails.
The isolation is one of the scariest elements. In space, you're so far from Earth and other humans. It's like being alone in the vast universe. Another is the unknown. We don't fully understand what's out there, and that lack of knowledge can create a lot of fear. For example, the possibility of encountering unknown alien life forms that could be hostile.
The scariest elements in scary space stories are the threats to one's existence. Take the threat of asteroids for example. An asteroid could come out of nowhere and smash into your spacecraft. There's no way to dodge it in time. And then there's the fear of the unknown entities. Maybe there are creatures out there that are so different from what we know on Earth that they could pose a threat we can't even fathom. Plus, the idea of the cosmos being so indifferent to human life is also very scary. There's no mercy in space, and any mistake can lead to a horrible death.