As launch approaches, JUICE project manager discusses trajectories and science

by Haygen Warren

The European Space Agency’s (ESA) Jupiter Icy Moons Explorer (JUICE) spacecraft recently arrived in French Guiana for its upcoming launch, which is currently set for April 13, 2023. JUICE is currently undergoing launch preparations in the Payload Preparation Facility at the Centre Spatial Guyanais (CSG) in Kourou, French Guiana, and will soon be mounted atop the upper stage of the Ariane 5 rocket it will ride into orbit.

While the launch of JUICE will certainly be an exciting and critical event in the mission timeline, what occurs after the launch is, perhaps, some of the most important events of the mission. Following the launch, JUICE will spend eight years traveling through the inner solar system, performing four gravity assists to raise its aphelion (the farthest point from the Sun in its orbit) to Jupiter’s orbital plane. What’s more, when at Jupiter itself, JUICE will perform several flybys of three Jovian icy moons — Ganymede, Callisto, and Europa — to uncover the secrets of these potentially habitable celestial bodies.

With JUICE’s launch and the start of the spacecraft’s eight-year coast phase quickly approaching, NASASpaceflight sat down with Cyril Cavel, JUICE project manager of Airbus Defence and Space, to learn more about the upcoming mission, its eight-year coast phase, and the science it will gather when at Jupiter.

JUICE’s Trajectory

When JUICE launches from French Guiana in April, it will be equipped with some of the latest and greatest planetary science instrumentation to investigate the characteristics of Ganymede, Callisto, and Europa. However, before it can use any of these instruments, the spacecraft has to fly out to Jupiter, doing so through the use of four flybys.

The first of these four flybys will see JUICE perform a first-of-its-kind flyby of the Earth-Moon system called a Lunar-Earth Gravity Assist (LEGA). The maneuver will take place in August 2024 and will have JUICE fly past both the Moon and Earth, utilizing the gravity of both celestial bodies in a single flyby maneuver. If successful, the LEGA flyby will save JUICE a significant amount of propellant, potentially providing mission teams with more opportunities for flybys at Jupiter or a mission extension.

The second flyby, planned for August 2025, will see JUICE fly past Venus, utilizing the planet’s gravity to increase its aphelion height. The final two flybys, planned for September 2026 and January 2029, will be of Earth, with the fourth flyby increasing the spacecraft’s aphelion height to the orbital plane of Jupiter and placing the spacecraft on a trajectory to intercept the planet’s immense gravity well.

The four flybys are often referred to as “gravity assists.” During each flyby, JUICE will harness the gravity of either the Earth-Moon system, Venus, or Earth itself to increase its velocity around the Sun. When the spacecraft’s orbital velocity is increased, the height of its orbit is also increased. By performing multiple gravity assists rather than one, single burn that would place the spacecraft on a direct trajectory to the Jovian system, mission teams can reduce the amount of propellant on the spacecraft, reducing spacecraft mass and costs.

What’s more, following the fourth and final gravity assist flyby of Earth, JUICE could potentially perform a flyby of an asteroid while traveling out to Jupiter. If mission teams choose to perform the flyby of the asteroid, named 223 Rosa, the flyby will serve as a dress rehearsal for the spacecraft’s first flyby of Ganymede following its arrival at the Jovian system in July 2031.

“[The flyby of 223 Rosa] can effectively be used as a dress rehearsal for the very first flyby of Ganymede, which will take place just before Jupiter orbit insertion,” Cavel said. “We perform our first Ganymede flyby before performing the orbital insertion maneuver at Jupiter in order to do an initial reduction of the spacecraft’s energy and to reduce the amplitude of the maneuver that we have to do when arriving at Jupiter. And so a flyby of an asteroid on the way to Jupiter could be used as a rehearsal of this first Ganymede flyby that we do when arriving in the Jovian system,” Cavel said.

The decision to fly past the asteroid will be important for JUICE teams. However, as Cavel explained, they have plenty of time to assess their options and make the final decision to fly past the asteroid.

“The opportunities that flight dynamics and mission analysis at ESA have found for a flyby of an asteroid are typically after the last Earth gravity assist, so on the final trajectory arc to Jupiter. JUICE will be on that trajectory at least five to six years after launch. So there is some time to think about that, meaning the decision would need to be made in the first few years after launch. Then the trajectory would be fine-tuned at the expense of a few meters per second of additional delta-v, which we can accommodate, in order to really target these asteroids on the way to Jupiter.”

Mission milestones for JUICE. (Credit: ESA)

JUICE’s Instruments

As mentioned, JUICE will carry a suite of state-of-the-art instruments to Jupiter, providing mission teams and planetary scientists with the most powerful remote sensing, geophysical, and in situ payload component ever flown to Jupiter and the outer solar system.

In total, JUICE will carry 10 instruments to Jupiter. Each of the 10 instruments can be separated into three instrument packages: a remote sensing package, a geophysical package, and an in situ package.

The remote sensing package is comprised of the Jovis, Amorum ac Natorum Undique Scrutator (JANUS), Moons and Jupiter Imaging Spectrometer (MAJIS), UV imaging Spectrograph (UVS), and Sub-millimeter Wave Instrument (SWI). The instrument package will serve to image the surface of the icy moons, providing imaging capabilities in the ultraviolet and sub-millimeter wavelengths.

The geophysical package is comprised of the GAnymede Laser Altimeter (GALA), Radar for Icy Moons Exploration (RIME), and Gravity & Geophysics of Jupiter and Galilean Moons (3GM). GALA and RIME are expected to provide laser and radar technology for use when exploring the surfaces of the icy moons, with 3GM providing radio technology that will allow scientists to probe the atmospheres of Jupiter and its icy moons to measure their gravitational fields.

The last of the three instrument packages is the in situ package, which is comprised of the Particle Environment Package (PEP), JUICE-Magnetometer (J-MAG), and Radio & Plasma Wave Investigation (RPWI). PEP will study the particle environment surrounding Jupiter and its icy moons, J-MAG will study the magnetic field interaction between Jupiter and the moons, and RPWI will study radio and plasma waves around Jupiter. What’s more, the in situ package will feature electric and magnetic field sensors and four Langmuir probes.

JUICE’s instruments and their locations on the spacecraft. (Credit: ESA/ATG medialab)

In addition to the three instrument packages on JUICE, the Planetary Radio Interferometer & Doppler Experiment (PRIDE) will utilize ground-based very-long-baseline interferometry to produce precise position and velocity measurements for JUICE teams on Earth.

With three instrument packages and one experiment on a single spacecraft bus, JUICE’s electronics and software have to manage quite a few instruments, especially given that many of the instruments were developed by different agencies across the world.

“It’s definitely been one of the more difficult design and assembly challenges for JUICE,” Cavel said when discussing instrument integration onto the spacecraft.

“All of the instruments come with different constraints, from field of view, to radiation tolerance, tolerance to EMC perturbations, to electromagnetic perturbations coming from the spacecraft or other instruments, and more. So it was a design challenge to find a solution that would accommodate all of these instruments on the spacecraft and then put them on board and operate them from the central software of the spacecraft.”

JUICE’s massive ~85 square meter solar arrays are set to become the largest solar arrays ever built for an interplanetary spacecraft and will serve to produce high amounts of power for JUICE and its instruments. Despite the high amount of instruments on the spacecraft, JUICE will have the ability to run all of its instruments simultaneously.

“On JUICE,  we have decided together with ESA to power all instruments in parallel. And there’s a good reason for that. We will have only two close flybys at Europa, and we want to be able to operate all our instruments all together at the closest approach of the Europa flybys, during which we will fly at 400 kilometers of altitude over Europa,” Cavel explained.

“We have only two chances to do that, so we don’t want to have a staggered approach where we would power on various instruments and then switch them off to give power to other instruments. We really want to be able to have cross-observations and all instruments working together to make the most of these two particular flybys that we will have at Europa.”

“And there are other opportunities or situations during the mission where we will repeat this kind of behavior and have all instruments powered on.”

“During the cruise to Jupiter, there is very little science activity planned. We will do regular check out of all the payloads every six months or so. However, the flybys at Venus and Earth are great opportunities to perform calibrations of the instruments. In particular, we will calibrate our magnetometers when traveling throughout the magnetic field of Earth during the Earth flybys, so that’s one particular example of what we will do during Earth flybys.”

However, if scientists opted to attempt to perform science at Venus, they would not be able to do so, as the orientation JUICE must be kept at during its eight-year coast phase will limit the instruments that can be used.

“There is a particular constraint that we have to respect at all times during the inner cruise of the solar system, which is to keep JUICE constantly sun-pointed with our high gain antenna facing the sun. This is to optimize the thermal control subsystem design of the spacecraft. So, because of this, when flying by Venus, we will not be able to rotate the spacecraft in the direction scientists would want for scientific operations. They will be able to switch on instruments and to do observations, but under the limitation that the spacecraft remains pointing in the same direction and is constantly facing the sun with the high gain antenna.”

JUICE’s Scientific Goals and Activities

The primary purpose of JUICE’s mission is to characterize and explore the three largest icy moons of Jupiter — Ganymede, Callisto, and Europa. Each of the moons features surfaces that are primarily comprised of ice, and it is thought that subsurface oceans may exist underneath the icy crust. Given the location of the moons in the solar system and the environments in which they exist, there is a possibility for life existing within these subsurface oceans — making them a significant area of interest for planetary scientists and astrobiologists.

In addition to investigating and characterizing the Jovian icy moons, JUICE will explore the complex environment around Jupiter and study the Jovian system as an archetype for other gas giant exoplanets in solar systems across the universe. Furthermore, JUICE will look for signs of life-sustaining habitats in and around the icy moons, and attempt to answer the question of whether or not life is unique to Earth.

As part of ESA’s Cosmic Vision program, JUICE will address two key themes of the program: “What are the conditions for planet formation and the emergence of life?” and “How does the Solar System work?” JUICE teams are aiming to address these questions by exploring the habitable zone, characterizing the oceans, icy shells, compositions, surfaces, environments, and activity of the Jovian icy moons, and the Jovian system as a whole and the effects Jupiter has on its surrounding environment.

A few of the many environments JUICE will endure while in orbit around Jupiter. (Credit: ESA)

One of the most intense and prominent effects Jupiter places on its surrounding environment is its immense magnetosphere and radiation.  Given that JUICE will be flying directly through Jupiter’s magnetosphere, significant radiation shielding is needed for JUICE and its instruments to survive.

“Tolerance to radiation is one of the other most stringent design requirements that we faced when designing JUICE. Jupiter is the harshest environment that we can find in that respect. Radiation is nothing new in space, as standard telecommunication satellites also face that kind of environment, but not to the same extent. So, when designing JUICE, we found that there can be many different ways to design a spacecraft that needs to be tolerant to radiation.”

“One way is to redesign components and ask ‘do you triple this part?’ This is not the choice we ended up making. We decided to use standard parts in order to limit cost and development risk. But then, when you decide to use standard parts to build your pieces of electronics and all your hardware, you need to protect them effectively. And this is the choice we made. We decided to shield all the hardware and pieces of electronics against the radiation environment at Jupiter.”

“The spacecraft is equipped with two radiation shielding vaults which are built along the central cylinder. It’s a confined piece of structure, inside which we accommodate all of the sensitive pieces of our electronics. The common shielding, which is applied on these vaults, is made of foils of lead. So, we have glued foils of lead that are a few millimeters thick onto the spacecraft, which is good enough to decelerate the particles and reduce the amount of energy that is hitting the sensitive components of our electronics inside these vaults. And this is a way for us to protect the pieces of electronics and avoid redesigning everything.”

“Overall, we have approximately 150 kilograms on JUICE of lead, just lead, to protect our hardware from radiation,” said Cavel.

Polar aurora caused by Jupiter’s magnetosphere. (Credit: NASA/ESA/J. Clark/Hubble Space Telescope/STIS)

Although the icy moons all orbit around Jupiter and are susceptible to the planet’s intense magnetosphere, the radiation environments surrounding the moons vary.

“Regardless of the differences that you can find, for example, at Europa and Ganymede ⁠– the radiation environment at Europa is different to the one you will find at Ganymede ⁠– but for all those related effects, it does not matter so much. You just accumulate damage over life, so you just have to make sure that at the end of life, the spacecraft is still able to operate, your pieces of electronics are still alive, and you can still run your mission.”

“But then there are other effects produced by radiation that are more instantaneous, like latch-up single events. These types of events will be worse at Europa compared to Ganymede. So there are ways to operate the spacecraft or to operate various components which can be very specific to where you are in the Jovian system, and in particular at Europa. Europa is the place in the Jovian system in which we will be the closest to Jupiter and the deepest inside the radiation belts of Jupiter, making it the most dangerous place the spacecraft will be throughout the mission. So for the two flybys at Europa, we have designed some special features at the software level and at the operation level in order to make sure that we are protected against these instantaneous effects that can be produced by a high radiation environment.”

As previously mentioned, JUICE will study the Jovian system as a whole and will look for specific differences in the icy moons to determine how they formed and evolved into the moons we know today.

“In terms of the formation of a planetary system, Jupiter and its moon system can be regarded as a good example of how planetary systems and their moons form. On Europa and Ganymede, we know from previous missions that there is liquid water. There are oceans underneath the icy crust of Europa and Ganymede. But there are likely differences in the structure of these oceans. This is what we want to verify with the mission. On Europa, it is thought that the ocean is located directly on top of the rocky mantle of the core of Europa. This is not the case on Ganymede, where we believe that on top of the rocky mantle of the rocky core, there is an initial layer of ice, then the ocean, and then the icy crust of Ganymede’s surface. So the inner composition of the moons and the place where we find the oceans and the interactions between this ocean, the core, and the surface are presumably different at Europa and Ganymede. This is what the scientists want to verify using JUICE, because their results could have implications on the potential for life.”

The icy moons that JUICE will investigate. (Credit: ESA)

While the existence of these subsurface oceans has not yet been 100% confirmed, there are many different indicators that scientists have used to justify the existence of these oceans. One such indicator is the presence of a second magnetic field around Ganymede that is likely formed by its subsurface ocean.

“We will first confirm the presence of these oceans, and then we will better characterize these oceans by measuring the magnetic field produced by these oceans, which will allow the scientists to better constrain the models governing the presence and the characteristics of these oceans.”

“We know that these oceans, in particular, Ganymede’s ocean, have a magnetic signature. They emit their own magnetic field, which is very faint. So this means we have three levels of magnetic fields at Jupiter: the field produced by Jupiter itself, the field produced by Ganymede (Ganymede is the only moon in the solar system which has its own magnetic field as well); and then we believe Ganymede’s ocean, which is saline, also produces a magnetic field. And this is what we want to sense with magnetometers onboard the spacecraft. To do that, we need to be able to sense very low-level fields, meaning that we needed to build a very clean spacecraft from an electromagnetic point of view. We don’t want to measure the magnetic field produced by the spacecraft. We want to sense what is locally produced at Ganymede and, in particular, by the ocean of Ganymede. And to do that we have placed the most sensitive magnetometers of the mission on a deployable boom that is 11 meters long. We will deploy the boom in the first few weeks of the mission after launch to place these very sensitive magnetic sensors far away from the spacecraft such that we do not measure the field produced by the spacecraft.”

The science JUICE collects at Jupiter will be of significant importance to planetary scientists who are attempting to determine the circumstances under which these moons and their oceans formed, as well as to astrobiologists who are actively looking for habitable locations in our solar system.

The nominal science phase of JUICE’s mission will begin around six months before the spacecraft enters Jupiter’s sphere of influence. JUICE is set to arrive at Jupiter in July 2031 and will perform a flyby of Ganymede before performing its orbital insertion around the gas giant, which will occur approximately 7.5 hours after the Ganymede flyby.  JUICE’s initial orbit around Jupiter will be highly eccentric, or elongated, with its orbital height and eccentricity being lowered over time.

The first flyby of Europa will take place in July 2032, with regular flybys of Ganymede, Europa, and Callisto beginning soon after. In 2035, JUICE will exit its orbit of Jupiter orbit and enter into orbit around Ganymede, becoming the first-ever spacecraft to orbit a moon other than Earth’s. During its time in orbit around Ganymede, the spacecraft will use its suite of instruments to study Ganymede with extreme detail, producing entire maps of the moon’s surface and internal structure.

After JUICE’s mission, it will use the remaining propellant onboard the spacecraft to deorbit itself, crashing into Ganymede a short time after.

(Lead image: artist’s concept of JUICE spacecraft at Jupiter. Credit: ESA)

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