The unexpected arrival of interstellar object 3I/ATLAS in our Solar System in July 2025 presented a unique challenge for space exploration. ESA's Juice spacecraft, designed to study Jupiter's icy moons, found itself in the right place at the right time, offering a fleeting opportunity to observe this rare visitor. The race against time began as scientists rushed to determine 3I/ATLAS's trajectory and plan observations. The geometry of the orbit predicted that Juice would be the closest spacecraft to 3I/ATLAS during its perihelion in November 2025, making it an ideal candidate for observation.
The preparation process was intense, with only four months to plan and execute a new observation campaign. The mission manager and the Juice Spacecraft Operations Manager, Angela Dietz, streamlined the workflow by bypassing the Science Operations Centre (SOC) to work directly with instrument teams, allowing for faster decision-making. The NEOCC and flight dynamics teams at ESOC collaborated to refine trajectory solutions, ensuring the instruments could be accurately pointed at 3I/ATLAS.
However, challenges arose during the mission. A communication failure during the Venus flyby in August 2025 required round-the-clock troubleshooting, which was quickly resolved. After the flyby, Juice entered a hot-cruise phase, remaining close to the Sun to act as a heat shield, extending until after the observation campaign. This phase presented thermal constraints for the spacecraft's instruments.
The encounter with 3I/ATLAS began on November 2, 2025, and lasted until November 25. The closest approach was on November 4, at approximately 0.4 AU (60 million km). Juice's scientific instruments, including JANUS, MAJIS, UVS, SWI, and PEP, were utilized to measure the interstellar object. The observations were limited to six 45-minute slots and one 4-hour slot, resulting in 126 science files with 11.18 Gbits of data.
Due to the spacecraft's hot configuration, the data could not be transmitted back to Earth immediately. Instead, all observations were stored onboard the mass memory (SSMM). It wasn't until January 2026, when Juice entered the cold-cruise phase, that the high-bit rate downlink became possible. The data was finally downlinked in February 2026, revealing valuable insights about the interstellar object.
The mission team's efforts were rewarded with a wealth of scientific data, which is now being analyzed by the science teams. The success of the 3I/ATLAS campaign has boosted confidence in Juice's ability to quickly achieve scientific objectives with short warning times. The team's dedication and quick thinking have paved the way for future achievements, such as the double lunar-Earth flyby, which will further enhance our understanding of the universe.
