Earth as a rehearsal stage

Redirecting the trajectory was the main goal, but the flyby also gave scientists a rare chance to test Juice's 10 scientific instruments on a real planetary target. This was the third such opportunity of the mission, following the 2024 lunar-Earth gravity assist and the spacecraft's 2025 observations of Comet 3I/ATLAS.

Teams from ESA's spacecraft operations, science operations and technical centers spent months coordinating with the outside groups responsible for each instrument, scheduling when each one could operate and squeezing out as much calibration time as possible from limited spacecraft resources.

"Juice's journey to Jupiter provides only a few opportunities to calibrate and validate the instruments under well-understood environmental conditions," explains Juice project scientist Claire Vallat, who led the effort.

Time was tight. Some instrument activities had to be prioritized — for instance, when Juice passed through Earth's shadow and had to run on battery power alone. "During this flyby, calibration activities were prioritized based on their relevance to preparing the instruments for their work at Jupiter, while also considering whether an opportunity is unique or can be scheduled later in the journey," Vallat says.

The logic behind this rehearsal is straightforward: instruments always behave slightly differently in space than they do in laboratories on Earth. The Earth encounter lets scientists learn each instrument's quirks now and refine the analysis tools they will rely on later, when the stakes are far higher.

And they will be higher. The flybys of Europa, for example, have been in discussion for roughly a decade and will continue to be refined until Juice finally passes the moon in the early 2030s. Each of the 35 moon flybys will be scientifically demanding, with little margin for wasted observations.

A bonus: probing Earth's magnetic tail

The flyby also delivered an unplanned-looking bonus. Juice spent several days traveling through Earth's magnetotail — the long extension of our planet's magnetic field that stretches away from the Sun, shaped by the solar wind pushing against it. There, the spacecraft measured magnetic fields and electrically charged particles far from Earth.

At the same time, the European-Chinese Smile mission observed the northern lights while measuring magnetic fields and particles much closer to the planet. Combining the two datasets could give scientists an unusual way to link activity deep in the magnetotail with visible events around Earth's polar regions.

Researchers expect to release images and spectra from some instruments in the coming weeks, once the data reach Earth and instrument teams have had time to evaluate them. Among the anticipated results are high-resolution images of Earth and the Moon taken by JANUS, Juice's scientific camera.

One more pass before the giant

Juice is not done with Earth. In January 2029, the spacecraft will return for a third and final flyby of our planet, which will place it on the trajectory needed to reach Jupiter in 2031.

Once there, Juice will study Jupiter's complex magnetic, radiation and plasma environment and how it interacts with the moons, treating the Jupiter system as an archetype for gas giant systems across the Universe. Its larger ambition is to characterize Ganymede, Callisto and Europa — three worlds thought to hide subsurface oceans — as potential habitats for past or present life.