Short answers, pulled from the story.
Callisto was discovered independently by Simon Marius and Galileo Galilei in 1610, along with the three other large moons of Jupiter. The name was suggested by Marius, who credited the idea to Johannes Kepler.
Callisto has a diameter of 4,821 kilometers, making it the second-largest moon of Jupiter after Ganymede and the third-largest moon in the Solar System. It is nearly as large as the planet Mercury.
Callisto likely has a salty subsurface ocean 150 to 200 kilometers deep, beneath an icy lithosphere 80 to 150 kilometers thick. Magnetic field studies from the Galileo spacecraft indicate a layer of highly conductive fluid at least 10 kilometers thick within the moon.
In December 2003, NASA reported that a crewed mission to Callisto might be possible in the 2040s. Callisto's low radiation levels, due to its position outside Jupiter's main radiation belt, and its geological stability make it the most suitable base for human exploration of the Jovian system.
Callisto has a molecular oxygen atmosphere far larger than radiolysis of surface ice can explain, falling short by two to three orders of magnitude even in models where the entire surface is ice. The leading hypothesis is that oxygen stored in porous regolith or radiation-altered ice grains releases thermally into the exosphere over time.
Past missions include Pioneer 10 and 11 in the early 1970s, Voyager 1 and 2 in 1979, Galileo from 1994 to 2003, Cassini in 2000, and New Horizons in 2007. Future missions include ESA's JUICE, which launched on the 14th of April 2023 and will perform 21 close flybys between 2031 and 2034, NASA's Europa Clipper, which launched on the 14th of October 2024 and will conduct nine flybys from 2030, and China's Tianwen-4, planned for launch around 2030.