One common element is the isolation. Astronauts are often alone or with a small group in the vastness of space, which makes them easy targets for unknown terrors. Another is the malfunction of technology. When things like communication systems or life - support go wrong in space, it's a terrifying situation. Strange, unexplained phenomena like strange lights or voices are also common. These things can't be easily explained by science as we know it, and that adds to the scariness.