Io is one of Jupiter's moons. In the Babylon 5 universe it is the location of the Mafaa-3 Station.
Theres at least 2 known locations at Io, theres Mafaa-3 station in Trinity and then there is the Io Jumpgate in the Babylon 5 universe.
Mafaa-3 was originaly a United African Nations (UAN) exploratory mining stations off of Jupiter's moon Io. It became a scientific research station with terawatt laser array mounted on platforms at Io's Lagrange points. The station is also the site of the discovery of Olaminium.
In Babylon 5 lore Io was where the Solar System Jumpgate was located.
- Io NASA