On the 28th of September this year, ESA’s Juicy Icy Moons Explorer (Juice) mission, which launched back in April 2023, completed a remarkable flyby manoeuvre in which the flight path passed within just 8,640km of Earth. This marks the second Earth flyby of the spacecraft following its Lunar-Earth flyby in August 2024 and its Venus flyby in August 2025. In accordance with the flight path, Juice should pass Earth for a final time in January 2029 before its arrival to Jupiter in July 2031.
The mission itself will make detailed observations of the gas giant planet and three of its largest icy moons: Ganymede, Callisto and Europa. These moons will be investigated as planetary bodies and potential environments capable of supporting life, while also providing new insights into Jupiter’s complex atmosphere, magnetosphere and surrounding system. By studying Jupiter and its moons in unprecedented detail, Juice will help scientists better understand gas giant planets throughout the Universe.
Why is the ESA Juice Mission relevant to ESTL?
The European Space Tribology Laboratory (ESTL) has played a key role within this mission. We conducted a detailed examination of tribological components, including the inspection and analysis of gears, bearings and other spacecraft mechanism components, including motor disassembly and detailed component evaluation.
A significant aspect of ESTL’s work focused on the tribological challenges associated with operating spacecraft mechanisms across the full Juice mission temperature range. While the spacecraft’s Solar Array Drive Mechanism (SADM) is required to function at temperatures approaching -70°C in the Jovian system, the mission profile includes several years within the inner Solar System, where mechanisms are subjected to elevated thermal exposure. ESTL evaluated the suitability of candidate lubrication strategies, including solid lubricant and hybrid lubrication approaches, considering factors such as low-temperature lubricant mobility, wear performance, evaporation losses, lubricant migration and long-term reliability. This work contributed to the understanding of how critical drive system components could maintain performance throughout the mission’s extended operational life.
ESTL’s support for the Juice programme also included a detailed lifetime assessment of solid-lubricated gears within the spacecraft’s SADM. The drive system incorporated a multi-stage spur gear train lubricated with a physical vapour deposited thin film molybdenum disulphide (MoS2) coating. ESTL evaluated how operating conditions such as contact pressure, gear alignment, temperature, material hardness and accumulated operating cycles could influence the life of this coating. Existing tribology and gearbox test data were used to develop a cumulative damage assessment covering successive phases of operation in dry nitrogen, vacuum and low-temperature conditions. This work supported the technical evaluation of the mechanism’s ability to maintain reliable performance throughout the demanding mission trajectory.
What next?
As JUICE continues its journey towards Jupiter ahead of its planned arrival in July 2031, ESTL will be following the mission with great interest. The spacecraft’s successful operation over this extended mission will further demonstrate the importance of reliable tribological and lubrication solutions in supporting deep-space exploration. ESTL is proud to have contributed to hardware associated with the mission and looks forward to the scientific discoveries and engineering achievements still to come.