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Jupiter has the most moons of any of the planets in our Solar System. So far, 63 moons have been discovered around the gas giant. The most well known, and largest, of these moons are known as the Galilean moons. These moons, Io, Europa, Ganymede, and Callisto, were discovered by Galileo Galilei and named in his honor. The names of the moons were prescribed by Simon Marius soon after Galileo discovered them in 1610.
There are different types of satellites around Jupiter. Two basic categories are regular and irregular satellites. The regular satellites are split into the inner and outer ones. The inner moons include Amalthea, Thebe, Metis, and Adrastea. Metis is the closest moon to Jupiter. It is about 40 km in diameter and approximately 128,000 km from the planet. The second closest moon is Adrastea, which is about 129,000 km from Jupiter and 20 km in diameter. Amalthea is the third moon of Jupiter and actually gives off more heat than it gets from the Sun. Thebe is the final inner moon and was discovered in 1980 by an astronomer, Stephen Synnott, analyzing Voyager photographs. Synnott also discovered moons of other planets in the Solar System.
The Galilean moons comprise the outer group of regular moons. These moons actually have radii larger than any of the dwarf planets. Io is best known for its volcanic activity. It is the most volcanically active planet or moon in the Solar System with more than 400 volcanoes that are currently active. Io’s diameter is about 3,636 kilometers and is close in size to Earth’s Moon. Europa is an icy moon 670,900 kilometers from Jupiter. With a diameter of 6,262 kilometers, Ganymede is actually the ninth largest object in the Solar System. With a diameter of 4,800 kilometers, Callisto is about the size of Mercury and the farthest out of the Galilean moons. Ganymede is the largest moon in the Solar System and the ninth largest object in our Solar System.
Irregular satellites are the satellites that are much smaller than the regular satellites are. They are also further from Jupiter and have more eccentric orbits than the regular moons do. Some of these moons include Himalia, Elara, and Sinope to name a few. The smallest of Jupiter’s moons have diameters of about one kilometer. Many of Jupiter’s smaller satellites have been discovered in the past decade due to better telescopes and photographs from probes.
Astronomy Cast has an article on Jupiter’s moons.