Employing multiple gravity-assist flybys of Jupiter's Galilean moons can save a substantial amount of ΔV when capturing into orbit about Jupiter. (Saving ΔV means that less of the spacecraft’s propellant mass would be needed to capture into Jupiter orbit.) An example double-satellite-aided capture sequence is depicted in Figure 1. A spacecraft arrives at the Jupiter system from an interplanetary trajectory from Earth, performs gravity assists of Jupiter’s moons Callisto and Ganymede, and completes the capture with a Jupiter orbit insertion (JOI) maneuver at perijove (its closest approach distance to Jupiter).
Table 1 shows estimates of the ΔV costs of Jupiter capture at various perijoves. Double-satellite-aided capture can potentially save 200-300 m/s of ΔV over single-satellite aided capture, while triple-satellite-aided capture can save an additional 100-200 m/s. The lower perijoves in Table 1 have reduced ΔV costs, but they are also deeper with in Jupiter’s radiation environment. Furthermore, double- and triple-satellite-aided capture sequences are more difficult to navigate than single-satellite-aided capture sequences. Thus, the ΔV costs, radiation exposure, and navigation difficulty must be carefully balanced when designing Jupiter capture sequences.
| Closest Approach Distance to Jupiter | 13 Jupiter Radii (below Ganymede) | 9 Jupiter Radii (below Europa) | 5 Jupiter Radii (below Io) | 1.01 Jupiter Radii |
|---|---|---|---|---|
| Unaided Capture ΔV, m/s | 1317 | 1101 | 825 | 371 |
| Best Single-Satellite-Aided Capture ΔV, m/s | 863 | 771 | 556 | 308 |
| Best Double ΔV, m/s | 498 | 529 | 330 | 228 |
| Best Triple ΔV, m/s | — | 330 | 202 | 190 |
Double flybys occur once every 2 to 13 days (depending on the sequence) in a Jupiter-centered frame, but only a small fraction of the available double flybys can be used as a practical double-satellite-aided capture sequence. The spacecraft needs to arrive at Jupiter using an efficient interplanetary trajectory from Earth in order for the ΔV savings to actually benefit the mission.
A broad-search methodology was developed that searches every double flyby sequence and tests whether it can be used as a double-satellite-aided capture in concert with an efficient interplanetary trajectory. The interplanetary trajectories are permitted to have deep space maneuvers (DSMs), but these DSMs should be small enough that the spacecraft does not lose the ΔV benefits of double-satellite-aided capture. The optimal double-satellite-aided capture trajectories are found for each practical double flyby by using MATLAB’s fmincon, a Lambert solver, and Gauss’s f and g functions.
A broad range of dates from 2020–2060 were searched using the above methodology for Callisto-Ganymede-JOI, Ganymede-Europa-JOI, and Ganymede-Io-JOI double-satellite-aided capture trajectories. These particular sequences have both flybys before the JOI maneuver, which reduces the navigational errors, risks, and statistical ΔV costs associated with double-satellite-aided capture. Some of the trajectories found by the broad search included several efficient double-satellite-aided capture sequences that had Earth launches in 2022 and 2023 at the nominal and backup launch dates for NASA’s Europa Mission. These trajectories were numerically integrated using NASA’s GMAT (General Mission Analysis Tool) software in order to obtain higher-fidelity solutions than the patched-conic solutions available using the low-fidelity MATLAB model. These trajectories would be excellent candidates for use by the Europa Mission.