A Historical Review
In the 2 March 1979 issue of Science 203 S. J. Peale, P. Cassen and R. T. Reynolds published their paper
“Melting of Io by tidal dissipation” indicating “the dissipation of tidal energy in Jupiter’s moon Io is likely to
have melted a major fraction of the mass.” The conclusion of their paper was that “consequences of a
largely molten interior may be evident in pictures of Io’s surface returned by Voyager 1.” Just three days
after that, the Voyager 1 spacecraft would pass within 0.3 Jupiter radii of Io. The Jet Propulsion Laboratory
navigation team’s orbit estimation program as well as the team members themselves performed flawlessly.
In regards to the optical navigation component image extraction of satellite centers in Voyager pictures
taken for optical navigation at Jupiter rms post fit residuals were less than 0.25 pixels. The cognizant
engineer of the Optical Navigation Image Processing system was astronomer Linda Morabito. Four days
after the Voyager 1 encounter with Jupiter, after performing image processing on a picture of Io taken by the
spacecraft the day before, something anomalous emerged off the limb of Io. This historical review written by
the discoverer recounts her minute-by-minute quest to identify what was a volcanic plume, the first evidence
of active volcanism seen beyond Earth. Many ingredients of this account reflect historic themes in the
process of scientific discovery.