Scientists launch JUICE mission to explore Jupiter's icy moons

Could we discover conditions necessary for life outside the Earth in the solar system? This is one of the mysteries that the space mission JUICE (for JUpiter ICy moons Explorer) will look to elucidate. Initially set to be launched from Kourou, French Guiana, in the early afternoon of Thursday 13 April 2023, the spacecraft will now be leaving on Friday due to bad weather.

To propel this mission to a planet located more than 600 million kilometres away, the European Space Agency (ESA) has brought together no fewer than 13 European countries, as well as the United States, Japan, and Israel.

Through this mission, the agency has also managed the feat of placing JUICE on the launch pad only 11 years after the project was greenlit. While the Covid pandemic slowed down the process, the delay was only nine months.

France's team, of which I am a part, also helped develop six of JUICE's ten state-of-the-art scientific instruments. The probe is expected to arrive in the Jovian system in 2031.

Stretching science's boundaries

Jupiter is both the largest planet in our solar system and the one with the most moons. To date, estimates of their number hover between 82 and 95, most of which have been discovered in the last two decades.

JUICE is the first mission to receive more than 1 billion euros of funding as part of the ESA's Cosmic Vision programme. It seeks to address four main questions:

• How do planets come to form and life to emerge?
• How does the Solar System work?
• What are the fundamental laws of physics in the universe?
• How did the present universe come into being and what is it made of?

JUICE was chosen ahead of other proposed missions because it was designed to address the first and last of these questions.

The Hubble Space Telescope and NASA's space probes Voyager, Galileo, Juno have already picked up some clues either by direct observation or deduction.

"Ocean moons" containing more water than the Earth NASA's Galileo was the first to discover water on the moons in 1995. Data captured by the space probe revealed gigantic liquid oceans not only under the crusts of its three icy moons, Callisto, Europa and Ganymede, but also on its volcanic moon, Io.

In 2014, the Hubble Space Telescope discovered geysers in Europa. Their bases appeared to be caked with salts, including carbonates. It is therefore likely Europa could meet the four criteria for habitability:

• The famous quartet of carbon, hydrogen, oxygen, nitrogen (CHON), symbols of the main chemical elements that constitute living beings.
• Liquid water that acts as a solvent.
• Energy to enable the development of life.
• A stable environment (orbits, rotation, average temperatures...)

The Galilean moons further enjoy the gravitational energy of Jupiter, creating significant tidal effects and allowing the last two conditions above to be met.

Why Ganymede is the main objective

Ganymede is set to studied in much more depth by JUICE than Callisto and Europa. This is not only because it is the largest moon in the Solar System, but also an ocean moon with its own magnetic field.

Similarly to the Earth's magnetosphere, Ganymede's has the potential to protect life by diverting the flow of cosmic rays and radiative particles from Jupiter's radiation belts.

Along the way, JUICE will have to contend with the Solar System's highest radiation levels.

This means that its electronic modules have to be housed in lead-shielded cavities and components have to be "hardened" to help them resist the harsh environment.

JUICE will also have to cope with extreme temperatures, ranging from +250°C as it flies by Venus to -230°C in the Jovian system. To maintain a stable internal temperature, the spacecraft has been coated with a multilayer thermal insulation made out of grey silicon aluminium alloy, earning the probe the nickname "silver beauty".

An energy problem

Around Jupiter, which is five times further from the Sun than Earth, the satellite will receive 25 times less solar energy than it would around Earth.

The spacecraft does not carry a radioactive battery because Europe is not yet able to produce them industrially, unlike the United States, Russia and China.

To enable the equipment and instruments to function with 1000W (the power of a small hairdryer), the craft will rely on huge solar panels - their surface area totals 85m2 - that have been tested to withstand the radiation and temperature variations.

Built by 80 European companies under the direction of EADS Toulouse, the JUICE probe has a wingspan of 28 metres (the length of a basketball court), a 2.5-metre long communications antenna (needed because of Jupiter's distance from the Earth).

It weighs nearly 6 tonnes at liftoff (most of which is propellant that will be consumed in manoeuvring the probe) and carries ten instruments (in all, less than 280 kg).

Ten scientific instruments on board

Of these instruments, France - with assistance from Italy - chiefly engineered the Moons and Jupiter Imaging Spectrometer (MAJIS).

It is the instrument that will allow the spacecraft to determine the physico-chemical compositions of the moons' surfaces as it flies over them and thus detect the CHON associated with potential habitability.

MAJIS will also study their ice sheets and liquid water. This will enable us to identify landing sites for future in situ exploration, and evaluate the structure and dynamics of Jupiter's atmosphere.

With an accuracy 10,000 times higher than the equivalent instrument on Galileo, the spatial resolution of MAJIS ranges between 100 metres and several kilometres depending on the probe's altitude at the time.

Finally, it should be noted that JUICE's plans may be revised based on the latest results from NASA's Juno mission. Juno is still orbiting Jupiter and has been flying over its poles since 2016.

Juno's nominal mission has been extended to fly past each of Jupiter's Galilean moons, starting with Ganymede in June 2021, and Europa in early 2023.

These observations and subsequent data analysis will allow JUICE scientists to better target the observations they make - 12 years after Juno and 30 years after Galileo.

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