Io is a moon in orbit of the planet Jupiter located in the Sol system. The transfer point on Io is an important outpost for Earth since the primary jumpgate of the Sol system orbits the moon. Space Station Io also orbits nearby; the base is second in size only to the colony on Mars. Io functions both as a civilian and military way station for most out-of-system traffic, especially those ships too small to have their own jump engines. The area is always heavily patrolled since any damage to the jumpgate could cripple or seriously harm Earth's interests.