If the devil existed, how many astronomers would sell him all or part of their soul to go back four centuries and relive, alongside Galileo, one of the most fabulous moments of their science? At the very end of 1609, the Italian scientist, after having improved the manufacture of the telescope invented in Holland, raises his instrument towards the night sky. He watches the moon. Then stars. Finally, on January 7, 1610, here he is targeting Jupiter. In the story titled A starry Messenger (The Messenger of the Starsin his French translation) which he published a few weeks later, Galileo said that that evening he distinguished “three small stars, certainly cramped, but nevertheless very clear”, which surprise him, because they form a straight line with Jupiter, on either side of which they are arranged.
The next day, an even greater surprise: the luminous points have changed places and have all passed to the west of the planet. For several days, the mathematician – and now astronomer – follows the ballet of these three walkers who, like the three musketeers, become four from January 13.
Anyone who has just invented instrumental astronomy and discovered the first satellites of the Solar System (if we put aside the Moon) understands that these ” stars “, as he still calls them, revolve around Jupiter. This completely calls into question the geocentric system, where every star must revolve around the Earth. A position that does not lack courage at a time when accusations of heresy and the stake threaten those who support the heliocentric theory of Copernicus. But Galileo takes the precaution of placing his work under the patronage of the Grand Duke of Tuscany, Cosimo II de’ Medici, and goes so far as to call these four stars “Medicine planets”. Today, the scientist has been given back his due and this quartet – Io, Europa, Ganymede and Callisto – is known as the “Galilean satellites”.
Since Galileo’s small artisanal telescope, astronomy has progressed well, but, as far as the exploration of the Solar System is concerned, it has made a real leap with the space age which, by sending machines to other planets, has abolished distances better than telescopes do. Jupiter has been visited several times since the 1970s, whether during simple rapid flybys (Pioneer and Voyager probes, in particular) or when spacecraft entered orbit around the gas giant. This has been the case on two occasions, with the NASA Galileo missions (between 1995 and 2003) and Juno (since 2016). But, until now, no program had been dedicated to the big Jovian moons. This lack will be made up for with Juice (acronym of Jupiter Icy Moons Explorer), which must leave Earth on April 13 aboard an Ariane-5 rocket.
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