Q:

# How do the moons of Jupiter stay in orbit around Jupiter?

A:

According to NASA, Jupiter has 50 officially recognized moons and 17 provisional moons, all of which are held in orbit around the gas giant by its immense gravity. The four largest of these moons are known as Galilean satellites, and the innermost three of them have fallen into a peculiar orbital resonance with each other.

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The four Galilean satellites are, in order from Jupiter, Io, Europa, Callisto and Ganymede. Each of these worlds orbits Jupiter in what is called a prograde motion, or aligned with the planet's rotation. The three inner moons tug at each other with every orbit and have settled into a stable tidal resonance with each other of 3:2. This resonance means that for every orbit Callisto completes, Europa circles Jupiter twice, and for every single orbit Europa completes, Io orbits Jupiter twice, according to NASA.

The four large moons are also tidally locked with Jupiter. As NASA explains, this means that each moon's orbital period is equal to its rotational period. Europa, lo, Callisto and Ganymede each have a day that lasts exactly as long as it takes to go around Jupiter a single time. This has the effect of ensuring that each of the moons always shows the same face to Jupiter, as the Moon does to the Earth.

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