A spacecraft that left Earth more than three years ago on a mission to Jupiter has just returned. The European Space Agency’s Jupiter Icy Moons Explorer — known as Juice — swept past Earth on Monday, Sept. 28, using our planet’s gravity to change its speed and direction on its long journey to the giant planet.

Juice made its closest approach at 7:45 a.m. EDT, passing about 5,370 miles (8,641 kilometers) above Earth over the Indian Ocean. The encounter is the latest in a complex series of planetary flybys that will ultimately deliver the spacecraft to Jupiter in July 2031.

So why, more than three years after blasting off for Jupiter, is Juice back at Earth? In orbital mechanics, the quickest or most practical route between two planets is rarely a straight line.

Why Juice Came Back To Earth

Juice launched on an Ariane 5 rocket from Europe’s Spaceport in French Guiana in April 2023, but its rocket couldn’t provide enough energy to send the spacecraft directly to Jupiter. Even if it could, Juice would need an enormous amount of additional fuel to slow down enough to enter orbit when it got there.

Instead, Juice is taking the scenic route, repeatedly using planetary gravity to alter its speed and trajectory while conserving propellant. Monday’s encounter was designed to deflect Juice’s trajectory by about 20 degrees and increase its velocity by around 3.5 kilometers per second.

It’s effectively borrowing a tiny amount of Earth’s enormous orbital energy. The change to our planet is immeasurably small, but for a spacecraft it can provide a substantial boost without using precious fuel. That fuel will be critical when Juice finally reaches Jupiter and begins navigating the giant planet’s moons.

Juice’s Eight-Year Journey To Jupiter

Monday’s encounter wasn’t Juice’s first return to Earth. In August 2024 , the spacecraft completed the first-ever lunar-Earth gravity assist, flying past the moon and then Earth in quick succession. That double encounter slowed Juice down, putting it on course for Venus.

Juice then flew past Venus in August 2025, using the planet’s gravity to bend its trajectory before returning towards Earth for Monday’s encounter. It’s not finished with its home planet. Juice will return for one final Earth gravity assist in January 2029, putting it on a trajectory to intercept Jupiter in July 2031.

It means a spacecraft launched towards Jupiter in 2023 will have encountered Earth three times before finally reaching its destination eight years later.

Why Juice Is Going To Jupiter

Juice is one of Europe’s most ambitious planetary missions ever attempted. Once at Jupiter, it will investigate the giant planet and three of its four large Galilean moons — Ganymede, Europa and Callisto — all of which are thought to harbor oceans of liquid water beneath their icy crusts.

Those hidden oceans make the moons some of the most compelling destinations in planetary science and astrobiology. Juice will investigate their surfaces and interiors while studying Jupiter’s powerful magnetic, radiation and plasma environment and how it interacts with the moons.

The mission’s ultimate destination is Ganymede — the largest moon in the solar system. Bigger than Mercury and Pluto, it’s the only moon known to generate its own magnetic field. Scientists also think an enormous saltwater ocean may lie beneath its icy surface, potentially containing more water than all of Earth’s oceans combined.

During its initial tour of the Jupiter system, Juice will perform 21 flybys of Callisto and two flybys of Europa. It’s scheduled to enter orbit around Ganymede in 2034. If successful, it will become the first spacecraft ever to orbit a moon other than Earth’s.

Europa Clipper Will Get There First

Juice won’t have the Jupiter system to itself. NASA’s Europa Clipper launched on a SpaceX Falcon Heavy rocket in October 2024 and is also taking an indirect, gravity-assisted route to Jupiter. After flying past Mars in March 2025, it will return to Earth for a gravity assist on Dec. 3, 2026, according to NASA . It will arrive at Jupiter in April 2030.

Europa Clipper will focus, naturally, on Europa, the smallest of Jupiter’s four Galilean moons and arguably the most tantalizing of its ocean worlds. Beneath its fractured icy exterior is thought to be a vast global ocean, making Europa one of the prime locations in the solar system for investigating whether environments beyond Earth could support life.

Rather than orbit Europa, Europa Clipper will orbit Jupiter and make 49 close flybys of the moon. Its instruments will investigate Europa’s ice shell, ocean, composition and geology to determine whether it has conditions that could support life.

Juice Gets Some Bonus Science At Earth

Monday’s flyby wasn’t merely about getting Juice to Jupiter. Its 10 science instruments will observe Earth and the moon between Sept. 23 and Oct. 3, giving scientists another chance to calibrate equipment and refine techniques that will eventually be used hundreds of millions of miles away.

Juice is also spending several days in Earth’s magnetotail, the enormous region of our planet’s magnetic field stretched away from the sun by the solar wind. The spacecraft can make measurements there that will help scientists test its instruments ahead of their eventual work inside Jupiter’s much more extreme magnetic environment.

Its navigation camera is also observing the lunar horizon to test optical navigation techniques that could eventually help Juice navigate around Jupiter’s moons.

Wishing you clear skies and wide eyes.