The JUICE Fly-By, as Told By Retired Mission PM Giuseppe Sarri

Editor’s note: The JUICE spacecaft made headlines with its recent rendevous with Earth. Retired ESA JUICE programme manager Giuseppe Sarri has taken the time to give us an in-depth look at the JUICE fly-by and its importance.

The JUICE spacecraft made its first fly-by

On the late evenings of 19th and 20th August the European Space Agency’s (ESA) Jupiter Icy Moons Explorer (Juice) was back to Earth and performed the first of its four gravity assist manoeuvres which will eventually bring it to Jupiter in 2031. The spacecraft flew by the Moon on 19th August at 21:15 UTC at an altitude of about 700 km and it passed by the Earth one day later at 21:56 UTC at an altitude of 6840 km. This is the first time ever that a spacecraft performed a double fly-by (the mission team calls it LEGA, which stands for Lunar-Earth Gravity Assist).

Gravity assist manoeuvres are quite normal for planetary missions: they allow the spacecrafts to reach the end destination, a planet or a moon in our solar system, by optimizing the propellant consumption. The energy (speed) required is coming from the celestial body they are flying by and not from the propellant on board.

Why does Juice need fly-by manoeuvre?

In the case of Juice, to go directly to Jupiter after launch, it would require a very high escape velocity, which could not be provided by Ariane 5, the rocket which launched it. Therefore, the ESA mission team worked out a cruise with four fly-bys (the one just completed plus a Venus fly-by in summer 2025, then an Earth fly-by in 2026 and again in 2029). Without these fly-bys, the spacecraft would need to carry a much higher propellant than it would be able to launch. However, the gravity assist manoeuvres are not only used to increase the speed of the spacecraft but also, if necessary, to slow it down and change the direction of travel. This is exactly what happened to Juice: it slowed down a bit and its direction was changed by 100 degrees to ensure that it will move inward to the Sun and encounter Venus next year. The full sequence of the four gravity assists will bring about the insertion into the orbit of Jupiter in July 2031.

The double fly-by saved around 100-150 kg propellant compared to a single Earth Juice fly-by, but it carried a higher inherent risk since it required ultra-precise real time navigation. A small mistake during the lunar fly by could have been amplified by the Earth’ s gravity, thus potentially risking the spacecraft entering and burning up in the atmosphere of the Earth.

The lunar Juice fly-by was preceded by a 32-minute-long eclipse with the spacecraft relative to Earth and powered by the on-board batteries instead of the solar arrays. The eclipse ended few minutes before the close approach allowing the operation team on ground to have real time monitoring of the activities.

Following the lunar fly-by the ESA Flight Dynamic Team analysed the new trajectory and confirmed that the manoeuvre was perfect therefore no fine-tuning correction before the Earth fly-by was necessary.

Earth Fly-by Operations: Preprogrammed Activities

The Earth fly-by was out of ground visibility since it happened at a time and place when the spacecraft had no coverage by the three deep space antennas (Cebreros in Spain, New Norcia in Australia and Malargue in Argentina) nominally used during the mission. All planned activities were therefore preprogrammed.

While the space around the Moon is reasonably clean of debris there is always a non-negligible risk of a space debris collision for a spacecraft orbiting Earth. Therefore, before starting the gravity assist the ESA space debris office, using the latest predicted trajectory of Juice, made a check that fortunately showed a clean path ahead. Therefore, no collision avoidance manoeuvre had to be planned.

Even though the objective of the Juice fly-by was to redirect the spacecraft to Venus and not to do science, the opportunity was grasped to check the various instruments and to let them work together in a pre-planned sequence like what will happen when in Jupiter. They were switched on and operated taking images, spectra, space environment data and communicating engineering information to ground to allow a deep assessment of their health status. The close approach to the Moon and Earth did provide a unique opportunity to test the RIME ground penetrating radar, which was in a representative configuration for the first time. This will allow to assess the actual in-flight conditions of the instrument, as early check-outs revealed that it will pick up some interference from the spacecraft power conditioning and distribution system.

Space Debris and Instrument Testing: Preparations for Jupiter

Several live images were captured using the spacecraft’s two monitoring cameras which provide black-and-white snapshots, that can be processed in colour. These cameras were designed to monitor the status of the spacecraft shortly after launch, as it unfolded its solar panels, antennas and booms in space. Having done flawlessly their job, the ESA operation team decided to switch them on again to take images at a rate of roughly two per minute as Juice made its closest approach to the Moon and Earth surface.

Other images were taken by the Navigation Camera (which will be used, once in the Jovian system, to support autonomous navigation of the spacecraft during the fly-by of the moons of Jupiter) and by the scientific instruments, including a high-resolution optical camera. These will be published soon by ESA.

The next Juice fly-by assist manoeuvres will increase the energy of the spacecraft which will reach a top speed of 140000 km/h with respect to the Sun. Once in Jupiter, now 7 years and 750 million km away, the spacecraft will have to slow down its speed to achieve the orbit insertion to the giant planet; it will be done by firing its main engine for two hours and burning almost one ton of propellant. This will conclude the cruise of Juice, and the “Jupiter Tour” will begin. For four years Juice will roam around in the Jovian system completing 35 fly-bys of the three icy moons and ending with the insertion in an orbit around Ganymede: this will keep the ESA operation team really busy.

Summary note on the mission

Juice, the JUpiter ICy moons Explorer, is the first European mission to study Jupiter and its icy Moons. It was launched on the 14^th of April 2023 from the European Spaceport in French Guyana by an Ariane 5 launcher. The spacecraft was built by a large industrial consortium led by Airbus Defence & Space and under the overall program management of the European Space Agency (ESA). The mission, which is part of the ESA Cosmic Vision 2015–2025 program, has two ambitious goals: improving our understanding of the solar system work (formation, evolution, stability, climate…) and exploring potentially habitable worlds with focus on three of the 95 moons of Jupiter: Ganymede, Europa, and Callisto. They are believed to be harbouring vast internal oceans. Identifying liquid water is crucial in the search for habitable worlds beyond Earth and to discover life as we know it. To achieve all these ambitious objectives the spacecraft carries a suite of ten instruments covering remote sensing, geophysics and in situ particles and fields. They will allow to study different aspects of Jupiter and Jupiter’s moons: geology, topography and gravity fields, cloud morphology and atmospheric chemistry, magnetic and plasma environments. A radiation monitor will provide information on electron, proton and heavy ions.

Postscript: Orbital Today would like to thank Giuseppe Sarri for taking the time and effort to show us more about the Juice fly-by of Earth and the Moon. Readers can find his description of the Juice programe, in two parts, here.