The Atmosphere and Plume Explorer Mission (APEX) is a novel nano-satellite architecture that will place a CubeSat onboard NASA’s proposed Europa Multiple Flyby Mission in the late 2020s. This CubeSat has been designed to perform atmosphere and plume science as well as radiation measurements around both Europa and Io for relatively low cost. The spacecraft described here is a 6U, solar powered CubeSat which will be sent to the Jovian system offering a significant scientific extension of the Europa Clipper mission at a relatively low cost. This represents a flexible mission architecture which can be adjusted to accommodate different mission science priorities as well as changes in the mission of the parent spacecraft. This concept focuses on utilizing only components and technologies that are presently available or will be available in the near-term with minimal development required.