The European Space Agency’s Juice spacecraft returns to Earth on September 28, 2026, for a gravity assist. Skimming 8,640 kilometers above the Indian Ocean, the maneuver deflects its trajectory by 20 degrees and adds 3.5 km/s of velocity en route to Jupiter.
Reaching the outer Solar System demands more than raw rocket power. Mission planners confronted a major propulsion obstacle when the Jupiter Icy Moons Explorer—widely known as Juice—lifted off in April 2023 on an Ariane 5 rocket from Europe’s Spaceport in French Guiana under the direction of the European Space Agency. A direct route to Jupiter requires an immense amount of fuel both to accelerate out of Earth’s gravity well and to brake hard enough upon arrival to avoid shooting past the gas giant entirely.
To overcome this challenge, operators at the European Space Operations Centre situated in Darmstadt, Germany, devised a sophisticated, multi-year path through the cosmos. Rather than burning heavy propellant for every speed change, the €1.5 billion mission relies on planetary gravity assists. By skimming past Earth, the Moon, and Venus, the spacecraft borrows a fraction of planetary orbital momentum for free.
Orbital Mechanics and the Multi-Planet Journey
The journey to Jupiter’s moons follows a carefully timed sequence across the inner Solar System. Following its launch carrying nearly 3,000 kilograms of chemical propellant, Juice executed a first-ever lunar-Earth double flyby in August 2024, using a lunar pass to brake and redirect toward Venus before swinging back toward home.
Next week’s encounter on September 28, 2026, marks a gravity assist of the mission. The closest approach occurs over the Indian Ocean at 13:45 CEST (11:45 UTC), bringing the robotic explorer within 8,640 km of the surface. Amateur astronomers equipped with the right telescopes or binoculars may spot it as it passes over Australia from northeast to northwest between 15 and 30 minutes before closest approach.
Navigation Campaigns and Battery Power in Earth Shadow
Executing a flyby of this precision leaves no room for error. The navigation campaign began on August 17, establishing six dedicated timeslots—including intervals at four weeks, two weeks, one week, and three days before closest approach—to execute tiny trajectory corrections.
“All flybys are risky, requiring very careful planning and continuous monitoring.”
Claire Vallat, Juice Project Scientist
Angela Dietz, Juice Spacecraft Operations Manager, noted the precision required for the adjustments. Until now, only one manoeuvre at minus four weeks was needed,
Dietz explained, adding that the initial adjustment was relatively small yet effective enough to place the spacecraft on target.
The encounter also brings a severe operational hurdle. From 21:24 CEST on September 27 to 06:03 CEST on September 28, Juice plunges into Earth’s shadow, losing direct sunlight and running entirely on internal battery power. To safeguard power reserves, operators are prioritizing essential science observations while temporarily scaling back secondary instruments.
Instrument Calibration and the Road to Ganymede
Beyond its mechanical purpose, the flyby serves as an invaluable testing ground. Between September 23 and October 3, researchers are actively collecting data on Earth and the Moon using the spacecraft’s ten onboard science instruments. This active window allows instrument teams to calibrate hardware and refine data analysis tools ahead of their ultimate arrival in the Jovian system.
Claire Vallat pointed out that the current Earth encounter is much simpler operationally than the 2024 lunar-Earth maneuver, where a lunar pass set up an Earth flyby the next day. This operational simplicity lets mission controllers turn Juice in various directions to point its instruments at different parts of Earth and the Moon.
Following the 2026 encounter, a final Earth flyby in January 2029 will set Juice on its optimum path to intercept Jupiter in July 2031. Once inside the Jovian system, the spacecraft will focus on Ganymede and Callisto, while gathering data on Europa—which scientists consider one of the most promising places to look for life in the solar system—before entering orbit around Ganymede in 2034.