The International Space Agency (or ISA) was a cross-national space organization of Earth, which was active in the early 21st century. It was founded sometime prior to 2032.
Among the greatest achievements of the ISA was Ares IV, a joint mission with NASA and the first manned mission to Mars. (VOY: "One Small Step")
It seems the ISA worked with other already existing space agencies, as the diagrams aboard
Ares IV read "ISA/NASA" and
Ares IV sent their telemetry to NASA in Houston.