IO

With a diameter of 3643km, Io is a little larger than Earth’s moon. It orbits far closer to its planet than the Moon does to us, speeding around Jupiter in just 42 hours. Just on the limit of naked-eye visibility from Earth, Io was discovered at the beginning of the telescopic era by Galileo Galilei.

Until recently Io and its companions remained little more than star-like points of light, their details beyond the reach of Earth-based telescopes. It was only when the first space probes flew past Jupiter in the early 1970s that they were seen as distinctive worlds.