• ESA’s JUICE spacecraft accomplished the first-ever lunar-Earth flyby and gravity assist manoeuvre.
• This was in service of saving up to 150kg of fuel as JUICE moves through the solar system toward the Jovian Moons.
• The manoeuvre required ultra precise, real-time navigation and if unsuccessful, could have been lost to space.

The behemothic presence of Jupiter in our solar system has long been a candle toward which scientists have been drawn to. The oldest planet in the solar system is so large that it has as many as 95 known Moons in its orbit and those Moons are the subject of an European Space Agency (ESA) mission.

The Jupiter Icey Moons Explorer, or JUICE is a mission where the ESA hopes to gain a better understanding of the Jovian Moons and whether these satellites could harbour life. The mission launched in 2023, with some complications along the way, but this week, there was a surprising development. The spacecraft is taking a shortcut to the gas giant.

This shortcut is made possible by the first-ever lunar-Earth flyby and the first-ever gravity assist manoeuvre. By utilising the gravity of Earth and the Moon, the ESA is able to “brake” the spacecraft and redirect it in service of another flyby, this time of Venus in August 2025.

This was a high-risk manoeuvre that could have put the spacecraft on the wrong trajectory entirely. But that didn’t happen.

“The gravity assist flyby was flawless, everything went without a hitch, and we were thrilled to see Juice coming back so close to Earth,” Ignacio Tanco, Spacecraft Operations Manager for the mission said in a statement.

The flyby of the Moon increased JUICE’s speed by 0.9km/s relative to the Sun while the Earth flyby reduced the speed by 4.8km/s relative to the Sun. This braking puts JUICE on course to flyby Venus thanks to a 100° angle correction.

Perhaps most impressive is that this entire manoeuvre required ultra precise, real-time navigation. The upside is that thanks to these flybys, JUICE has saved around 100 – 150kg of fuel. This fuel will be used to get JUICE to Jupiter once it gets a boost from the Venus flyby next year. The spacecraft will however, pass by Earth twice more on the journey to Jupiter.

“Thanks to very precise navigation by ESA’s Flight Dynamics team, we managed to use only a tiny fraction of the propellant reserved for this flyby. This will add to the margins we keep for a rainy day, or to extend the science mission once we get to Jupiter,” said Tanco.

During the flyby, the ESA also got to test all 10 of JUICE’s science instruments. Data and photos from those instruments will be published by the ESA in coming weeks. The agency says that the photos include “high-resolution images on the Moon and Earth” from the JANUS scientific camera.

While, as we mentioned, Jupiter has over 90 Moons, the satellites the ESA is most interested are Ganymede, Callisto and Europa, all of which are ocean bearing Moons. The mission hopes to characterise these moons using remote sensing, geophysical and in situ instruments to determine whether life could exist in such close proximity to a gas giant.

“Juice will monitor Jupiter’s complex magnetic, radiation and plasma environment in depth and its interplay with the moons, studying the Jupiter system as an archetype for gas giant systems across the Universe,” the ESA said.

JUICE is only expected to arrive at the Jovian Moons in 2031.