ESA's Juice probe swings past Earth on 30 September in a crucial step towards Jupiter

One of the most ambitious space missions of our time is about to make a brief but important return visit to Earth. The European Space Agency's Juice spacecraft, on its way to explore Jupiter, will swing past our planet at the end of September in a carefully planned manoeuvre.

Although it may sound like a detour, this close pass is actually a clever piece of celestial navigation. In this article we look at what will happen during the flyby, why the spacecraft needs Earth's help, the long road it is travelling and what awaits it once it finally reaches the giant planet.

A slingshot past Earth

According to the mission details, Juice is scheduled to fly past Earth on 30 September 2026. Rather than simply passing by, the spacecraft will use our planet's gravity as a kind of free fuel stop, gaining speed and changing direction without burning precious onboard propellant.

During the encounter the probe is expected to pass within a few thousand kilometres of Earth. The goal is to reach what engineers call the Jupiter transfer velocity of around 11 kilometres per second, the speed it needs to continue its long outward journey across the Solar System.

Why Juice needs a boost

The reason for this elaborate route lies in the limits of rocket power. When Juice launched, its Ariane 5 rocket could only provide an initial speed of about 2.5 kilometres per second, far short of what is required to travel all the way to Jupiter directly across such vast distances.

To make up the difference, mission planners rely on a series of gravity assists. As the spacecraft sweeps past a planet, that planet's gravity accelerates it, and the geometry of the pass means Juice leaves faster than it arrived, effectively borrowing a tiny amount of the planet's motion.

This technique is a well established part of deep space travel and has been used by many previous missions to reach far off destinations. It allows spacecraft to save enormous amounts of fuel, at the cost of following a longer and more complex path that can take several years to complete.

A long and winding road

Juice launched on 14 April 2023 and has already been busy building up momentum. In August 2024 it carried out a double flyby of the Moon and Earth, followed by a flyby of Venus on 31 August 2025, each encounter carefully shaping its path through the inner Solar System.

The flyby on 30 September 2026 is the next step in this sequence, with one final Earth flyby planned for January 2029. Only after all these manoeuvres will the spacecraft have enough energy to coast out to its destination, arriving at Jupiter in July 2031.

Destination Jupiter's icy moons

The ultimate aim of the mission is to study Jupiter and three of its largest icy moons, namely Ganymede, Callisto and Europa. These distant worlds are thought to hide vast oceans of liquid water beneath their frozen surfaces, making them fascinating targets in the search for habitable environments.

In a historic finale, Juice is planned to eventually enter a permanent orbit around Ganymede. If successful, it would become the first spacecraft ever to orbit a moon other than our own, marking a genuine milestone in the exploration of the outer Solar System.

A mission to watch

Taken together, the upcoming flyby is a reminder that great discoveries often depend on patience and precise planning. As Juice quietly slips past Earth once more, it moves another step closer to unlocking some of the biggest questions about our Solar System's giant planet and its moons.