JUICE: What to know about the ESA's mission to explore Jupiter's icy moons

This article was first published on 24 March 2023, and updated on 14 April 2023 and 8 June 2023.

The Italian astronomer Galileo Galilei was the first person to see Jupiter and its largest moons.

The four moons he first identified using his homemade telescope in 1610 are still referred to as the Galilean satellites. With the benefit of modern technology, astronomers have since identified more than 70 others in the orbit of the solar system's largest planet.

Now humankind is on the verge of observing Jupiter’s moons much more closely, if still from the comfort of Earth. The JUpiter ICy Moons Explorer, "JUICE", took off from Europe's Spaceport in French Guiana on 14 April.

The spacecraft will bear a plaque replicating pages of Galileo’s Sidereus Nuncius, where he describes his observations of the moons, to commemorate the astronomer.

JUICE is an international collaboration between the European Space Agency (ESA), which is leading the project, and several member states. A series of gravity-assist flybys of Earth and Venus will help propel it hundreds of millions of kilometres to the distant planet where it is due to arrive in July 2031.

If all goes to plan, JUICE will then spend the next four years orbiting Jupiter and conducting flybys of its moons, paying particular attention to the three large ocean-bearing ones – Callisto, Europa and Ganymede as potential habitats. It will hopefully then become the first spacecraft to orbit a moon in the outer solar system in December 2034.

To get JUICE safely there and back will require extreme navigation techniques, reliant on ESA's Estrack network of deep-space antennas in Spain, Argentina and Australia, controlled remotely from the European Space Operations Centre, based in Darmstadt, Germany.

“This is the biggest deep-space mission we've ever launched, and it needs to nimbly orbit the moons of the largest planet in the Solar System using no less than 35 flybys,” explains Andrea Accomazzo, Flight Operations Director for the mission. “JUICE's exploration of Jupiter and its moons will require us to perform a decade of operations we've never done before, and a lot could go wrong.”

Scientists believe that Jupiter’s largest moons hold vast quantities of water buried under their surfaces, potentially far more than is present in Earth’s ocean.

“These planet-sized moons offer tantalizing hints that conditions for life could exist beyond our home planet – perhaps on worlds orbiting giant planets instead of hot stars," says the ESA. Jupiter and its family of large moons represent an archetype for giant gas planet systems across the universe, and as such are some of the most compelling destinations in our solar system."

The Airbus-built JUICE spacecraft weighs 6.2 tonnes and will collect data from the moons using 10 state-of-the-art scientific instruments. These include telescopes, ice-penetrating radar and sensors to help detect whether Jupiter’s moons could host microbial life. Nine months of JUICE’s mission will be spent orbiting the largest moon in the solar system, Ganymede, in greater depth.

“The high-resolution visible telescope, which is called Janus, will take fantastic pictures very close to the moons because we will do flybys at just 400km altitude. They will be stunning shots," Cyril Cavel, JUICE Project Manager at Airbus Defence & Space, told BBC News.

While the JUICE mission is investigating three of Jupiter's moons, NASA's Europa Clipper mission will focus solely on Europa. This is the smallest of the planet's Galilean moons, but there is strong evidence that it has a large area of water under its icy crust that could provide a suitable environment for life.

"The existence of a vast ocean on a moon of Jupiter – which the Europa Clipper mission is equipped to decisively confirm and characterize – is what makes Europa such a promising place to better understand the astrobiological potential for habitable worlds beyond Earth," NASA says.

Europa Clipper is due to launch in October 2024 and to be orbiting Jupiter in 2030. The spacecraft will be engraved with a poem by the US Poet Laureate Ada Limón entitled In Praise of Mystery: A Poem for Europa. This is part of NASA's Message in a Bottle campaign, which seeks to express Earth's desire to "reach out and understand what makes a world habitable".

Anyone around the world can also put their name to the message in the interstellar bottle heading to Europa. Those who successfully submit their details to NASA will have their name etched onto microchips on the Europa Clipper. All names must be submitted by the end of this year.

The Europa Clipper and JUICE teams will collaborate to ensure they get as much evidence as possible from their missions, and will run joint meetings once a year.

The ESA, in partnership with the World Economic Forum and others, has helped create the world’s first sustainability rating for space missions. The Space Sustainability Rating aims to reduce space debris as well as ensure space exploration becomes more sustainable.

The Forum says: “The Earth’s orbit is cluttered with over a million objects larger than 1cm, 4,000 of which are satellites. Some 60,000 more satellites are planned for the next decade. It is vital that a strategy is implemented to reduce future debris.”