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The Galilean moons are named after Galileo Galilei, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610; [2] they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in 1892. [3]
In his book Mundus Iovialis ("The World of Jupiter"), published in 1614, Simon Marius, the court astronomer to the Margraves of Brandenburg-Ansbach, claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. [7] According to Marius, he began observing the Jupiter system in late November 1609. [13]
Jupiter III Galileo [9] [10] discovered the Galilean moons. These satellites were the first celestial objects that were confirmed to orbit an object other than the Sun or Earth. Galileo saw Io and Europa as a single point of light on 7 January 1610; they were seen as separate bodies the following night. [11] Callisto: Jupiter IV o: 8 January 1610
Although Galileo did indeed discover Jupiter's four moons before Marius, Io, Europa, Ganymede, and Callisto are now the names of Galileo's four moons. By 1626 knowledge of the telescope had spread to China when German Jesuit and astronomer Johann Adam Schall von Bell published Yuan jing shuo, (Explanation of the Telescope) in Chinese and Latin.
Io is the innermost of the four moons of Jupiter discovered by Galileo in January 1610. Rømer and Cassini refer to it as the "first satellite of Jupiter". It orbits Jupiter once every 42½ hours, and the plane of its orbit is very close to the plane of Jupiter's orbit around the sun.
A montage of Jupiter and its four largest moons (distance and sizes not to scale) There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024. [1] [note 1] This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. [4]
Io (/ ˈ aɪ. oʊ /), or Jupiter I, is the innermost and second-smallest of the four Galilean moons of the planet Jupiter.Slightly larger than Earth's moon, Io is the fourth-largest moon in the Solar System, has the highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System.
The European Space Agency selected the L1-class JUICE orbiter mission in 2012 as part of its Cosmic Vision programme [1] [2] to explore three of Jupiter's Galilean moons, with a possible Ganymede lander provided by Roscosmos. [3] JUICE was launched on April 14, 2023. [4] The Russian lander did not materialize in the end. [5]