Short answers, pulled from the story.
Galileo Galilei pointed his telescope toward Jupiter in 1610 to observe four points of light that became known as the Galilean satellites. Simon Marius independently observed the same phenomenon at almost the exact moment and proposed the mythological name shortly after the discovery in 1610.
Callisto orbits Jupiter at an average distance of 1.883 million kilometers which is roughly five times farther than Earth's Moon orbits our planet. Its rotation period matches its orbital period exactly taking about 16.7 Earth days to complete one cycle.
The largest feature on the surface is Valhalla spanning up to 1,800 kilometers from its center while Asgard forms the second-largest basin at roughly 1,600 kilometers across. The cratered plains are estimated at around 4.5 billion years old making this the oldest and most heavily cratered terrain in the entire Solar System.
Data from the Galileo orbiter suggests a salty ocean potentially lies beneath the icy crust at depths greater than 100 kilometers with highly conductive fluid extending at least 10 kilometers thick. The Galileo spacecraft conducted eight close encounters between 1994 and 2003 providing measurements that indicate this layer exists.
ESA's Jupiter Icy Moons Explorer launched the 14th of April 2023 planning 21 flybys between 2031 and 2034. NASA's Europa Clipper launched the 14th of October 2024 scheduled for nine flybys beginning in 2030.