On 19–20 August 2024, ESA’s Jupiter Icy Moons Explorer (Juice) will achieve a world first: using the gravity of the Moon and then Earth to bend its path through space, bringing it one step closer to Jupiter. This infographic presents the key milestones before, during and after this lunar-Earth gravity assist. Credit: ESA, CC BY-SA 3.0 IGO or ESA Standard Licence
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It's a double world first. The first-ever lunar-Earth flyby, and the first-ever double gravity assist maneuver. It will change Juice's speed and direction to alter its course through space, but it's a daring feat; the slightest mistake could take Juice off course and spell the end of the mission.

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Jupiter is on average "just" 800 million km away from Earth. Without an enormous rocket, sending Juice straight to the giant planet would require an impossible 60,000 kg of onboard propellant. And then Juice would need to be carrying an enormous additional amount of propellant to slow itself down enough to go into orbit around Jupiter once it arrives, rather than simply zipping straight past and off into outer space.

So Juice is taking the scenic route, using the gravity of other planets to carefully adjust its trajectory through space and ensure it arrives at Jupiter with the right speed and direction. This incredibly complex, constantly evolving route has been carefully planned out by Juice's dedicated mission analysis team over the last 20 years.

Somewhat counterintuitively, using the lunar-Earth flyby to slow Juice down at this point in its journey is actually more efficient than using the flyby to speed it up. If we had instead used this flyby to give Juice a boost towards Mars, we would have had to wait a long time for the next planetary flyby. This first braking maneuver is a way of taking a shortcut through the inner solar system.

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