Voyager Program Page 1
Jupiter System Montage: Voyager 1
Jupiter and the four Galilean moons ("Galilean" because observed by Galileo in his telescope. Galileo correctly deduced that these four moons orbit Jupiter. Since these did not directly go around the earth, it supported the idea that all things in the solar system may not orbit the earth, but may orbit the sun instead.) Io, Europa, Ganymede, and Callisto. (Not to scale).
From Io, Voyager I saw the first active volcanoes from an object other than the Earth. Europa's surface has remarkably few craters - likely because large oceans of liquid water lie beneath the surface.
All four are very large, as moons go. (Ganymede is larger than the planet Mercury.) If they weren't hidden by Jupiter's glare, they would be visible by eye in the night sky.
This image © 2022 TALLabs using a 
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License
Original Image: NASA/JPL
Edited to be a black and white line drawing rather than a color photo.