Skagway was a moon of Klondike VI located between the planet's inner and outer rings.
In 2270, the rings of Klondike VI began to destabilize causing debris to hit the Federation mining colony on the moon. The USS Enterprise was dispatched to help protect the colony and determine what was destabilizing the rings. Eventually, the Enterprise determined an alien machine that allowed the rings to exist was located on Klondike VI and was malfunctioning. Soon after, the Enterprise was able to fix the device and Skagway and its Federation colony were saved. (TOS novel: The Rings of Time)