Navigation Captures
If uncorrected, small altitude errors in gravity-assist flybys cause larger errors in the trajectory after the flyby. Since triple-satellite-aided capture sequences involve three close flybys that are separated by intervals of less than one day, they are difficult to navigate using conventional techniques. Since chemical triple-satellite-aided capture sequences also have a Jupiter orbit insertion maneuver that is usually sandwiched between two flybys, they are even more difficult to navigate since the Jupiter orbit insertion (JOI) maneuver dispersion adds additional uncertainty to the trajectory.
It is possible to model this difficult triple-satellite-aided capture problem by assuming an advanced AutoGNC (autonomous guidance, navigation, and control) system and realistic ephemeris errors for Jupiter and its moons. In particular, a Callisto-Io-JOI-Ganymede chemical triple-satellite-aided capture sequence is modeled that arrives at Jupiter in February 2025.
The AutoGNC system is assumed to be able to determine the spacecraft’s trajectory to a suitable precision and calculate a trajectory correction maneuver (TCM) before the periapsis of the first flyby (of Callisto). TCM 1 is assumed to be executed at the periapsis of the Callisto flyby with small execution errors. At the second flyby (of Io), the spacecraft’s trajectory is determined again, but this time the nominal JOI maneuver is modified to correct for the errors instead of adding a second TCM at Io’s periapsis. The nominal JOI maneuver occurs a few hours later, and it is also modeled with realistic dispersion errors.
The spacecraft’s flyby of Ganymede is modeled next with only orbit determination, and a JOI cleanup maneuver is modeled 3 days later that targets the nominal apojove position of the capture orbit. The error in the apojove velocity vs. the nominal apojove velocity is modeled as a fourth maneuver. The total mission ΔV is the sum of the ΔV values for TCM 1, JOI, the JOI cleanup maneuver, and the apojove cleanup maneuver.
The model was simulated in a Monte Carlo simulation with over 4000 runs. The median mission ΔV was 261.996 m/s, the 99th percentile value was 288.036 m/s, and the absolute maximum value was 307.027 m/s. The B-plane dispersions were also rather small due to the AutoGNC targeting. The Callisto, Io, and Ganymede B-plane dispersions are plotted in Figure 4 with 1σ and 3σ error ellipses.
The results indicate that even the hardest to navigate chemical triple-satellite-aided capture sequences can be safely navigated for a reasonable amount of statistical ΔV if AutoGNC technology makes reasonable advances over the next few years. SEP triple-satellite-aided capture sequences would require hydrazine thrusters to navigate, but their lack of a JOI maneuver reduces dispersion errors compared to the chemical sequences. Both SEP and chemical double-satellite-aided captures with JOI maneuvers after both flybys are easier to navigate than triple-satellite-aided capture. The two flybys could plausibly be navigated ballistically with the possibility of a canned JOI maneuver after the flybys. A JOI cleanup maneuver would then fix the dispersions caused by the flybys and JOI maneuver.