The central difference, which of course relates to the title of this chilling short story, is that life in the space frontier is much harsher and crueller than life on earth. Note how the narrator tells us, as Barton contemplates the way he must eject his stowaway into space to face a certain death, that this was the "law" and there was no "appeal":
It was a law not of men's choosing but made imperative by the circumstances of the space frontier.
The narrator goes on to explain how fuel is measured out exactly and even a slight addition in the weight of space shuttles would result in their destruction:
The stowaway had signed his own death warrant when he concealed himself on the ship; he could not be permittted to take seven others with him.
Such "cold equations" rule the space frontier, causing Barton, even though he would love to be able to save the life of Marilyn, to actually have to be the agent of her death. The realities of life in the space frontier call for such hard and unyielding rules that result in such difficult consequences for people such as Marilyn. This is the biggest central difference between life on earth and life in space.