Io, one of the moons of Jupiter, showing a volcanic eruption on the horizon, 1979.

Io, photographed by Voyager 1 from a distance of 490,000 kilometres. The two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 1 made its closest approach to Jupiter of 278,000 kilometres in March 1979 before flying on to Saturn. Io is the innermost and third largest of Jupiter's four main moons, which were first observed by Galileo. It is one of the most volcanically active bodies in the Solar System - Voyager 1 observed 8 volcanic plumes on Io between March and July 1979. The volcanic activity is believed to be due to immense tidal effects caused by Io's proximity to Jupiter and another moon, Europa. The ejecta from this volcano rise to a height of about 160 kilometres above the moon's surface.