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We're off to Ganymede

A few weeks ago, a booming sound rumbled throughout the lush jungles of French Guiana as Ariane V took off for its penultimate launch, and successfully placed Juice (JUpiter ICy moons Explorer), the European Space Agency’s first dedicated spacecraft to the outer Solar System. Juice's mission, the culmination of more than a decade of planning, design, and testing by hundreds of scientists, engineers, and technicians across Europe, is to explore the Jupiter system, including the planet itself, its moons, and their interaction with each other.

Juice’s main focus though, will be Jupiter’s giant icy moons Ganymede, the biggest moon in our Solar System, and Callisto, the third biggest. Both moons hold particular interest as they have confirmed subsurface oceans under their thick ice shells. They are Ocean Worlds (with Titan, Europa and Enceladus).

While Ganymede and Callisto are both interesting, one can wonder why ESA didn’t focus all its attention on Europa instead as it is the most promising place for finding extraterrestrial life with Mars and Enceladus.

To answer this question, we need to go back more than fifteen years, when optimistic plans were being dreamed up by both NASA and ESA. In 2008, both space agencies envisioned a grand joint mission to design and launch a probe to investigate the Jovian icy moons through the Europa Jupiter System Mission, also known as the EJSM/Laplace mission. (A joint mission to Saturn was also proposed but later dropped).

The two agencies quickly decided for a two-spacecraft mission; NASA would focus on Io and Europa – the two “rocky” moons – with the Jupiter Europa Orbiter (JEO) spacecraft, while ESA would focus on Ganymede and Callisto – the two “icy” moons – with the Jupiter Ganymede Orbiter (JGO) spacecraft. As their names indicated, NASA’s spacecraft was planned to orbit Europa in its final mission stage, and therefore bear the brunt of Jupiter’s harsh radiation environment, while ESA’s spacecraft would do the same with Ganymede where radiation was far less of a problem.

Due to this, JEO would require additional shielding (such as aluminum and tantalum stack), custom made electronics, and the use of nuclear batteries (MMRTG). The budget quickly spiralled to $3.8 billion and, fearful of more budget overruns, the US congress pulled the plug.

This left ESA, who in the meantime had received the green light from the European governments, to go it alone. In the context of the ESJM/Laplace mission, the removal of NASA’s JEO led to a reduced science return in the study of Io, Europa, Jupiter’s atmosphere and magnetosphere.

Having considered this, ESA scientists concluded that JGO would still be relevant to the overarching science objectives put in place by the ESJM/Laplace mission as its chief target, the moon Ganymede, allowed the space agency to study a water-rich world as well as understand its interactions with the surrounding Jovian environment. However, given Europa’s importance, two Europa flybys were included in the mission to compensate for the loss of JEO. As these two flybys would add 25% of the total mission radiation dose, new shielding requirements were considered and found to be well within the acceptable limits of the mission, and thus the JGO mission was reformulated into JUICE, which was later rebranded as Juice. Thankfully, with NASA’s new Europa Clipper flagship mission, Europa will be studied in great detail as well.

As it stands, Juice will do various flybys of Callisto and Europa before settling into orbit around Ganymede (the first spacecraft to orbit another moon other than Luna) by September 2032, where it will study the moon in great detail for nine months.

The end of the mission will be an unusual one. The spacecraft will run out of fuel during its last orbit around Ganymede and make an uncontrolled crash on the surface of the moon. Ganymede, being a Planetary Protection Category II target, signifies that there is only ‘a remote chance that contamination by spacecraft could compromise future investigations.’ There is, therefore, no obligation from ESA to determine how or where the spacecraft will be deposited on the surface of the moon (as opposed to Europa or Enceladus, which are Planetary Protection Category III and IV targets). Nevertheless, if the spacecraft is still steerable – which might be doubtful after spending so much time in the harsh Jovian environment – the team might try to force it towards a specific location, such as a flat expanse instead of a region where cracks in the icy crust are apparent.

Exciting times ahead for this new mission to the Ocean Worlds! I can’t wait. (Quick addition: There is a problem with the deployment of the ice penetrating radar antenna at the moment, but ESA seems confident that they will be able to deal with it in the coming months. Crossing my fingers!)

As always, onwards and upwards Image Credit: ESA


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