Successful Earth flyby improves Juice’s course to Jupiter

The European Space Agency's Jupiter Icy Moons Explorer (Juice) skimmed the very outer edge of Earth's atmosphere on 28 September, using Earth's gravity to alter its route to Jupiter with very little fuel. The flyby deflected Juice by 20° compared with its pre-flyby path and increased its velocity by 3.5 km/s. In the four weeks before the flyby, spacecraft operators needed only one tiny nudge out of six available opportunities to put Juice on exactly the right approach trajectory. They tracked the spacecraft extra carefully from 17 August and planned to continue until 10 October. Closest approach came at 13:45 CEST (11:45 UTC) on 28 September, when Juice flew 8,640 km above the Indian Ocean and snapped a series of images with its onboard monitoring cameras.

Angela Dietz, Juice's Spacecraft Operations Manager, said the flyby required ultra-precise navigation in real time and that careful planning meant only a small amount of the propellant reserved for the flyby was used. That leaves more propellant for use at Jupiter to observe the planet's icy moons. While the main goal was to alter Juice's trajectory, the flyby also provided a third opportunity to test the spacecraft's science instruments on a real surface in space, after the first during the 2024 lunar-Earth gravity assist and the second during 2025 observations of Comet 3I/ATLAS.

Colleagues from ESA's spacecraft operations, science operations and technical centres spent months working with external teams on Juice's 10 science instruments to plan which instruments to switch on and when. Their efforts squeezed as much operational time as possible out of tight spacecraft resources. Juice project scientist Claire Vallat, who led the effort, noted that the journey to Jupiter offers only a few opportunities to calibrate and validate instruments under well-understood environmental conditions. Given limited time and operational constraints, instrument activities sometimes had to be prioritised, such as when Juice was in Earth's shadow and relying on battery power alone. During the flyby, calibration activities were prioritised based on their relevance to preparing the instruments for work at Jupiter and on whether an opportunity was unique or could be scheduled later.

The Earth flyby is relatively straightforward compared with Juice's upcoming 35 flybys of Jupiter's giant moons, including Ganymede and Europa. For the Europa flybys, discussions on instrument operations began 10 years ago and will continue until Juice flies past Europa in the early 2030s.