After a Boost from Earth and the Moon, Juice is On its Way to Venus and Beyond

The first spacecraft to use gravity assist was NASA’s Mariner 10 in 1974. It used a gravity assist from Venus to reach Mercury. Now, the gravity assist maneuver is a crucial part of modern space travel.

The latest spacecraft to use gravity assist is the ESA’s JUICE spacecraft.

The European Space Agency (ESA) launched its JUICE spacecraft on April 14, 2023. Its eventual destination is the Jovian system and its icy moons, Europa, Callisto, and Ganymede. But it’s a long journey, and the spacecraft took a shortcut by travelling close to Earth and the Moon and using their gravity to gain momentum and change trajectory.

It’s the first spacecraft ever to use the Earth and the Moon for a gravitational slingshot, and it captured some images to share with us.

JUICE stands for Jupiter Icy Moons Explorer, and it’s on a mission to study three moons with suspected oceans buried under layers of ice. It’s got a long way to go, and on long-duration missions, economical use of propellant is critical. This Earth-lunar slingshot maneuver is all about saving propellant.

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

At its closest approach to Earth, JUICE passed overhead of Southeast Asia and the Pacific Ocean at only 6840 km (4250 miles) altitude. It was a risky maneuver but one that saved the mission between 100 and 150 kg of propellant.

This lunar-Earth flyby isn’t JUICE’s only gravity-assist maneuver. Next August, it will slingshot past Venus, and on September 26th and January 2029, it will slingshot past Earth. All these gravity-assist maneuvers will give JUICE momentum for its journey to Jupiter. JUICE will reach Jupiter in 2031, and because of all of these maneuvers it will have more propellant left when it gets there.

JUICE has completed its first gravity-assist maneuver and, in one year, will perform another one with Venus. Credit: ESA. Acknowledgements: Work performed by ATG under contract to ESA. Licence: CC BY-SA 3.0 IGO

“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 Ignacio Tanco, Spacecraft Operations Manager for the JUICE mission.

Modern orbiters bristle with science instruments, antennae, and cameras. JUICE is no exception. Among all its instruments and science cameras, it carries two monitoring cameras called JMCs, or JUICE Monitoring Cameras. They’re 1024×1024 pixel cameras with different fields of view. Their job is to monitor the spacecraft’s booms and antennae, and their job was especially critical when they were deployed after launch.

The ESA’s Jupiter Icy Moons Explorer has two Juice Monitoring Cameras, or JMCs, to provide snapshots with different fields of view. Their main job is to monitor components of the spacecraft, but they captured images of Earth and the Moon during the recent flyby. Image Credit: ESA (acknowledgement: work performed by ATG under contract to ESA) LICENCE: CC BY-SA 3.0 IGO

During the flyby, JUICE used its JMCs to capture images of the Earth and the Moon.

JUICE Monitoring Camera 2 captured this image of the Moon as it flew past it on August 10th. “A closer look reveals a casual ‘photobomber’ – Earth shows itself as a dark circle outlined by a light crescent at the top centre of the image, peeking out from behind the spacecraft structure (look just above the fuzzy blue blob, which itself is a ghost image caused by the reflection of sunlight),” the ESA writes. CREDIT
ESA/Juice/JMC. ACKNOWLEDGEMENTS: Simeon Schmauß & Mark McCaughrean. LICENCE: CC BY-SA 3.0 IGO

It also used eight of its ten instruments to collect scientific data from Earth and all ten for the Moon.

“The timing and location of this double flyby allows us to thoroughly study the behaviour of Juice’s instruments,” explains Claire Vallat, Juice Operations Scientist.

JMC 1 captured this image of the Moon during the lunar flyby. CREDIT: ESA/Juice/JMC. ACKNOWLEDGEMENTS: Simeon Schmauß & Mark McCaughrean. LICENCE: CC BY-SA 3.0 IGO

JUICE’s main science camera is JANUS, a high-resolution optical camera. Its role is to capture detailed images of the surface of Ganymede, Callisto, and Europa. The JUICE team used JANUS to capture more than 400 preliminary views of the Earth and the Moon.

“After more than 12 years of work to propose, build and verify the instrument, this is the first opportunity to see first-hand data similar to those we will acquire in the Jupiter system starting in 2031,” says Pasquale Palumbo, a researcher at INAF in Rome and principal investigator of the team that designed, tested and calibrated the Janus camera.

The Moon’s pockmarked surface as revealed by JANUS. Image Credit:

“Even though the flyby was planned exclusively to facilitate the interplanetary journey to Jupiter, all the instruments on board the probe took advantage of the passage near the Moon and Earth to acquire data, test operations and processing techniques with the advantage of already knowing what we were observing,” said Palumbo.

Earth was imaged at dawn on August 20, 2024, by the JANUS optical camera aboard JUICE. The image shows the island of Hawai’i (the dark spot on the left), the largest island in the Hawaiian archipelago in the central Pacific of the United States. The view is very low, after a short while the Earth left the field of view of JANUS. Credits: JANUS team (INAF, ASI, DLR, CSIC-IAA, OpenUniversity, CISAS-Università di Padova and other international partners)

These early-mission images are whetting our appetite for when the real fun starts in seven years. JUICE will reach the Jovian system in July 2031 and will do 35 flybys of the gas giant’s icy moons. Then, in December 2034, it will enter orbit around Ganymede.

There is growing evidence that Europa, Ganymede, and Callisto have warm, salty oceans buried under thick layers of ice. These are prime targets in our search for life. But, maddeningly, we don’t know for sure if they could support life or even if the oceans are real.

Hopefully, JUICE can tell us. But it can’t do that without these risky, early-mission maneuvers.