No matter how much planning goes into space missions, one must always expect the unexpected. But luck favors the prepared. In July 2025, an unexpected visitor entered our Solar System: the interstellar object 3I/ATLAS. The European Space Agency’s (ESA) Jupiter Icy Moons Explorer, known as Juice, happened to be in the right place, equipped with the right tools, at the right time to observe this celestial anomaly. This unique vantage point offered a fleeting opportunity, leading to a race against time for the mission team.
An Unexpected Visitor
On July 1, 2025, a telescope in Chile confirmed the presence of an interstellar object, later designated as 3I/ATLAS, marking it as one of only three known interstellar objects to have passed through our solar neighborhood. Scientists quickly mobilized to determine 3I/ATLAS’s orbital trajectory to establish observation options. “Almost since the time of discovery, we realized that the geometry of the orbit would allow observations from the Juice spacecraft, which would observe the comet from a completely different angle than what we can do from Earth,” stated Marco Fenucci, a mathematician at ESA’s Near-Earth Object Coordination Centre (NEOCC).
Calculations indicated that Juice would be the closest spacecraft to 3I/ATLAS shortly after it reached perihelion in November 2025. “Preparations for things like payload pointing campaigns or flybys typically take about nine months,” explained Angela Dietz, Juice Spacecraft Operations Manager. “When ATLAS came, we knew there was not a lot of time.” With only four months to prepare for observations of an uncharacterized interstellar object, the team had to act swiftly.
Challenges in Deep Space
As planning for the 3I/ATLAS observations progressed, Juice faced several challenges. Leading up to its Venus flyby on August 31, 2025, a communication failure just days before the maneuver necessitated round-the-clock troubleshooting. Fortunately, the issue was resolved in time for the flyby, keeping the spacecraft on course for the 3I/ATLAS encounter. Due to its proximity to the Sun, after the flyby, Juice remained in a hot-cruise phase with its high-gain antenna pointed at the Sun to serve as a heat shield.
By early October, the NEOCC provided final information on 3I/ATLAS’s position, allowing the Juice flight control and dynamics teams to finalize observation plans under the spacecraft’s thermal constraints.
The Encounter
Juice officially began its observations of 3I/ATLAS on November 2, 2025, continuing until November 25. The closest approach occurred on November 4, at approximately 0.4 AU (about 60 million km). This distance was ideal, as Juice’s instruments were specifically designed to study icy bodies. The spacecraft utilized five scientific instruments—JANUS, MAJIS, UVS, SWI and PEP—to capture measurements of 3I/ATLAS.
Thermal constraints limited the observations to six 45-minute slots and one final 4-hour slot, resulting in a total of 126 science files comprising 11.18 Gbits of data. However, the team had to wait to analyze the results, as Juice remained in its hot configuration and could not yet transmit the data back to Earth. Instead, all observations were stored onboard the mass memory system.
Data at Last
After Juice transitioned to its cold-cruise phase in mid-January 2026, the anticipated data downlink occurred during two 11-hour passes on February 17 and 20, 2026, via ESA’s ESTRACK antennas in New Norcia and Malargüe, respectively. The first pass operated smoothly, while the second began early on February 20. As the 16-minute signal delay ended, the navigation camera images finally reached the control room, bringing smiles to the mission team.
By 19:21 CET, the observation campaign for 3I/ATLAS officially concluded, leaving a significant impact on the mission team. “That’s the nice thing in our job – We see always a team effort of many parties involved. I perceive the fact that we could optimize this campaign in little time and maximize the output is something to be proud of!” Angela reflected.
Looking ahead, the team is eager to analyze the data to uncover more about this interstellar object. As preparations ramp up for Juice’s upcoming Earth flyby in September, Angela noted the historical importance of the mission. “In the future, we will remember Juice for the double lunar-Earth flyby and 3I/ATLAS. Nobody’s ever done a lunar-Earth flyby or been so close to an interstellar object’s perihelion. These are very rare and remarkable things!”
