The Galilean satellites are the four largest moons of Jupiter, discovered by the astronomer Galileo Galilei in 1610. They are named Io, Europa, Ganymede, and Callisto. These moons are some of the most heavily studied objects in the solar system due to their unique characteristics and potential for harboring life. Io is the closest and most volcanic of the Galilean moons, with hundreds of active volcanoes and a surface that is constantly changing due to volcanic activity. Europa is believed to have a subsurface ocean of liquid water beneath its icy crust, making it a prime target for astrobiological research. Ganymede is the largest moon in the solar system and has its own magnetic field, while Callisto is heavily cratered and is thought to have a subsurface ocean as well. Research on the Galilean satellites focuses on their geology, composition, and potential for habitability. Missions such as NASA's Galileo spacecraft and the upcoming Europa Clipper mission aim to further explore these moons and unlock the mysteries of their origins and evolution.