On 6 August 2001, NASA’s Galileo spacecraft performed a close flyby of Jupiter’s volcanically active satellite, Io. Over the following months, as the data was slowly transmitted back to Earth, mission scientists realised that the spacecraft had passed through the top of a 500-km-high (310-mile) volcanic plume. This is the highest volcanic eruption plume ever witnessed in the Solar System.

Galileo flew by the moon at a distance of 194 km (120 miles) from the surface. The volcanic particles, believed to be composed of sulphur dioxide, were detected by Galileo’s plasma instrument. Images of the plume backlit by the Sun revealed its full extent. The plume originated at the Tvashtar volcano, near Io’s north pole.