ESA's Jupiter Icy Moons Explorer, known as Juice, launched in April 2023 with nowhere near enough velocity to fly straight to Jupiter. Reaching the outer solar system directly would require roughly 11 kilometers per second of speed; Juice left Earth with an escape velocity of about 2.5 km/s. The gap between those two numbers is why Juice's eight-year cruise to Jupiter is not a straight line but a looping, multi-planet itinerary — and this month it completes the second of three planetary gravity assists needed to close that gap and finally reach its destination.
Rather than carry enough propellant to make up the difference by brute force, mission planners route Juice past Venus and Earth multiple times, using each planet's gravity to bend the trajectory and add speed for free. The sequence so far has included a Venus flyby in August 2025, and this month's Earth flyby is the second time Juice has passed its home planet since launch. One more Earth flyby, scheduled for January 2029, will complete the gravity-assist chain before Juice finally arrives at Jupiter in July 2031, more than eight years after it first left the ground.
Why It Matters
Gravity assists are not a workaround for a poorly designed mission — for a spacecraft as large and instrument-laden as Juice, they're the only practical way to reach Jupiter at all without an impractically massive rocket or propellant load that no current launch vehicle could realistically carry. Each flyby is calculated years in advance and executed with a precision that leaves little room for error: too close, and the spacecraft risks atmospheric or debris hazards; too far, and the trajectory correction needed afterward can cost propellant the mission can't spare over an eight-year cruise with no opportunity to refuel.
The mission hasn't been entirely uneventful in the run-up to this flyby. In July 2025, ESA's operations team lost contact with Juice during a routine ground station pass. The cause turned out to be a software timing bug in the spacecraft's signal amplifier: an internal timer that resets every 16 months could, under the wrong conditions, leave the amplifier switched off, making the spacecraft's signal too faint to detect from Earth. Engineers spent roughly 20 hours issuing commands “blind” — without confirmation they were being received — from a spacecraft roughly 200 million kilometers away, before successfully reactivating the amplifier through a backup low-gain antenna. No lasting damage resulted, but the episode was a reminder of how much can go wrong on a mission held together by software running autonomously at planetary distances, far beyond the reach of a quick manual fix.
Once Juice reaches Jupiter in 2031, its real work begins: an extended tour of Jupiter's icy moons Ganymede, Callisto, and Europa, all suspected to harbor subsurface oceans of liquid water beneath their ice shells. Ganymede in particular is the mission's ultimate destination — Juice will eventually enter orbit around it, becoming the first spacecraft ever to orbit a moon other than our own. Instruments aboard the spacecraft are designed to probe the thickness of these moons' ice shells, search for evidence of the oceans beneath them, and assess whether the conditions there could support the chemistry of life, even in environments far colder and darker than anything found in Earth's own oceans.
That's still five years away. For now, Juice's task is simpler and more mechanical: use Earth's gravity one more time to shave a few more months and a fraction of a kilometer per second off the long haul to Jupiter, and arrive at the January 2029 flyby with its trajectory — and its still-recovering communications system — in good enough shape to make the final push toward the outer solar system.
Mission controllers at ESA's operations centre in Darmstadt, Germany, will spend the days around closest approach confirming the flyby geometry matched predictions to within the tight margins the mission's long-term trajectory design demands, since even a small error compounds over the years remaining before Jupiter orbit insertion. Small navigation errors caught early are cheap to correct; the same errors discovered only after the January 2029 flyby would leave far less time, and far less fuel, to fix before the final approach to the Jupiter system begins in earnest.
