Tom Godwin's short story "The Cold Equations" is realistic and believable. The use of space travel in the story is for colonization, exploration and trade. The consequences of the extra weight of the girl on the ship's deceleration is explained in terms that are understandable and logically in sync with physics. The time frame of how long the girl can stay on the ship is explained in detail. The decision to jettison the girl is the only one that could possibly be made; the survival of the colony outweighs the life of the girl. It is also consistent that a young girl would make an impulsive decision to stow away on the ship. One of the few things that is not realistic is the length of time verbal messages take to travel to and from the girl's brother in the colony.
Consider the differences between this and other, unrealistic space travel stories. There is usually a plot device that (as far as we know) is impossible, such as time travel, or faster-than-light travel. Various alien species usually inhabit these universes; there are no known alien species. We are constantly on the lookout for some, though!