This chapter reviews the history of Io observations up to and including the Voyager encounters. The material in this chapter is drawn primarily from Satellites of Jupiter (Morrison, 1982) and Satellites (Burns and Matthews, 1986), both published by the University of Arizona Press, and Time-Variable Phenomena in the Jovian System, NASA Special Publication 494 (Belton et al., 1989).

Io before Voyager

The study of Io dates from the very beginning of telescope-based astronomy. In his observation notes for January 7, 1610, Galileo Galilei wrote: “when I was viewing the heavenly bodies with a spyglass, Jupiter presented itself to me; and because I had prepared a very excellent instrument for myself I perceived that beside the planet there were three little stars, small indeed, but very bright.” Subsequent observations revealed a fourth “little star.”

Thus were discovered the Galilean satellites, named by Simon Marius (a contemporary of Galileo) Io, Europa, Ganymede, and Callisto. Subsequently, little attention was paid to the satellite system except as a means of measuring the speed of light (Roemer's method). Not until the nineteenth century did physical observations become important. In 1805, Laplace used the orbital resonant properties to estimate satellite masses. New refracting telescopes at Lick and Yerkes measured satellite sizes, and bulk densities were obtained.