The outer planet region of the Solar System offers us natural laboratories for planetary formation and evolution, references for comparative study with exoplanetary systems, and the search for a second genesis in the ocean worlds. Yet, the outer planet region beyond Mars remains a destination only for legacy space agencies with large launch capabilities, RTG technology, multi-billion-dollar budgets, and multi-generational commitments. In addition, recent advances in telescopic observations of the outer planets from the Earth's vicinity provide more frequent and shorter turnaround scientific outcomes than decade-long spacecraft missions. As an innovative solution, JAXA/ISAS has introduced a program concept called OPENS (Outer Planet Exploration by Novel Small Spacecraft) and recently selected "OPENS-0" as the first mission for a newly created "Eco and Fast" small-class category. In the OPENS program concept, the OPENS-0 technology verification and operational lessons will be applied to subsequent missions for other outer planet destinations through various flight opportunities, such as a solo rendezvous mission, a sample return mission combined with a larger orbital transfer vehicle, and daughtership provision for an international flagship mission. tested, we aim to realize a higher frequency of outer planet exploration at less cost.
