Europa Clipper Mission Design: Design of the 21F31 Reference Tour

Europa has been a prime target for space exploration ever since 1997, when the Galileo spacecraft's magnetic measurements of Europa suggested the moon might hide a vast, salty ocean beneath its icy surface. To investigate its habitability, NASA developed Europa Clipper, which was launched on October 14, 2024, on a Falcon Heavy rocket from Cape Canaveral. After a 5.5-year interplanetary journey that includes one Mars gravity assist and one Earth gravity assist, Europa Clipper will reach Jupiter in April 2030 and insert into a 200-day orbit in the Jovian system. Subsequently, a complex gravity assist trajectory (tour) will commence. This paper will present the reference tour, 21F31, which consists of 53 flybys of Europa, 9 flybys of Callisto, and 7 flybys of Ganymede, culminating in a Ganymede impact to meet planetary protection requirements. The principles, design process, and evolution of tour designs -- which leverage more than two decades of research in astrodynamics and multiple interactions with planetary scientists and engineers -- as well as the science objectives and mission constraints that drive the design of the tour, will be covered in this paper. This paper is part of a series of papers describing the Mission Design and Navigation analysis of Europa Clipper.