<p>This paper uses a motion primitive approach to automatically generate constrained spacecraft trajectories for Neptunian system exploration. Motion primitives are generated as smaller building blocks of motion that summarize periodic orbits and arcs along stable and unstable manifolds of selected orbits in the Neptune-Triton circular restricted three-body problem. The sequential composability of these motion primitives is represented by a graph that also incorporates path and maneuver constraints. This graph is searched using a k-best paths algorithm to generate multiple motion primitive sequences. These sequences are transformed into an array of geometrically diverse initial guesses. After corrections and optimization, the resulting tradespace of continuous, constrained trajectories with impulsive maneuvers is analyzed. This approach is applied to two planar trajectory design scenarios in the Neptunian system: high-energy insertion into a Neptune-centered science orbit after interplanetary arrival and low-energy transfers between science orbits centered around each of Neptune and Triton.</p>