This is the Wiki summary for the Cold Equations:
Summary
The story takes place entirely aboard an Emergency Dispatch Ship (EDS) headed for the frontier planet Woden with a load of desperately needed medical supplies. The pilot, Barton, discovers a stowaway: an eighteen-year-old girl. By law, all EDS stowaways are to be jettisoned because EDS vessels carry no more fuel than is absolutely necessary to land safely at their destination. The girl, Marilyn, merely wants to see her brother, Gerry, and is not aware of the law. When boarding the EDS, Marilyn sees the "UNAUTHORIZED PERSONNEL KEEP OUT!" sign, but thinks she will simply have to pay a fine if she is caught. Barton explains that her presence dooms the mission by exceeding the weight limit, and will result in the deaths of the colonists. No cargo can be jettisoned, and the presence of the captain is required on ship, so Barton cannot sacrifice himself. After contacting her brother, Marilyn willingly walks out of the airlock and is ejected into space.
The story, first published in the August 1954 issue of Astounding, has been widely anthologized and even dramatized.