Next week, the Jupiter Icy Moons Explorer (JUICE) probe will fly past both the Moon and Earth on its way to space in a bold and previously unattempted double fly-by maneuver.

The European Space Agency (ESA) mission is one year into its complex eight-year journey and will eventually visit three of Jupiter's moons. The probe will use the gravity of the Earth, the Moon and Venus to reach Jupiter with as little fuel as possible.

On August 19th and 20th, the mission will fly past the Moon and Earth in succession and perform the first ever dual gravity maneuver. JUICE will reach the moon first, use the moon's gravity to brake and change course, and then swing around the Earth a day later to further change its speed and direction. There is no plan to adjust the flight path between flybys.

Mond-Erde-Fly-by. Diagramm. Route der JUICE-Sonde am Mond und an der Erde zur ersten doppelten Gravitationsunterstützung.

The moon is usually considered a disturbance that must be taken into account when tossing space probes around Earth, but harnessing its gravity can save fuel. The lunar gravity assist technique, combined with the timing of JUICE's launch in April last year, will be enough to save enough fuel for the probe to orbit Jupiter's moon Ganymede in just 200 kilometers at the end of the 2035 mission. “This is great news in terms of science,” said JUICE mission analyst Arnaud Boutonnet from the ESA European Space Operations Center (ESOC) in Darmstadt, Germany, at a press conference last Friday.

The double gravity assist operation is risky because each flyby will magnify errors in the spacecraft's trajectory. "It's like passing through a very narrow corridor, very, very quickly: pressing the accelerator pedal to the maximum when the edge of the road is only millimeters," ESOC mission manager for spacecraft operations Ignacio Tanco said in a statement. But conducting the maneuver close to home will be an opportunity to test whether JUICE's science instruments work as planned, in an environment well known to researchers, Claire Vallat, mission operations specialist at ESA in Madrid, said at the briefing. “This is a unique opportunity to study these instruments.”

The circular route of the spacecraft is planned. The Earth-Moon flyby will slow JUICE and divert its course on a shortcut to Venus. JUICE will gain energy as it swings around Venus, and two more gravity boosts from Earth - in 2026 and 2029 - will eventually catapult the spacecraft out to Jupiter.