21 new satellites of Jupiter have been found this year. Many are thought to be due to an ancient collision of a larger moon with a comet. Jupiter has four large moons and dozens of smaller ones (there are 60 satellites known so far). Galileo first discovered the four largest moons of Jupiter, Io, Europa, Ganymede, and Callisto in 1610, using a 20-power telescope; these moons are known as the Galilean moons. The moons of Jupiter are (in order by their distance from Jupiter): Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede (the biggest), Callisto (the second biggest), Leda (the smallest), Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Sinope, and many newly-discovered moons that haven't been named yet. In the time it took to write this more might have been found. Jovian Factsheet.