Efficient design techniques for cislunar-space VLBI spacecraft formation in GTOC8

The 8th edition of the Global Trajectory Optimization Competition (GTOC8) presented a novel concept of a space-based very-long-baseline interferometry (VLBI) telescope in cislunar space for observing selected radio sources in cosmos. It requires designing a three-spacecraft triangular formation with changeable sizes and orientations such that observation can be scheduled as efficiently as possible. We first review the problem, and then describe the methods employed by representative teams participating in the competition. Subsequently, we present the design techniques employed by the team from the Chinese Academy of Sciences, which are primarily based on orbital-geometry analysis. Two efficient trajectory patterns are summarized: million-kilometer triangular formations with symmetric circular orbits, and consecutive-lunar-flyby trajectories with Moon-to-Moon transfer orbits. These two trajectory patterns enable establishing and reconfiguring the triangular formation with sufficiently different sizes so that a number of radio sources can be observed, thus maximizing the performance index. Finally, we present a solution with the best currently known score of $J=158$ million km.