Close Conjunction of the Galilean Moons of Jupiter
The last close conjunction was in 2009; these events are infrequent and difficult to predict. NASA’s Hubble Space Telescope captured the event in a string of snapshots. Jupiter’s moons, Europa, Callisto, and Io, raced across the sky and crossed in front of the planet.
According to NASA, “The ancient cratered surface of Callisto is brownish; the smooth icy surface of Europa is yellow-white; and the volcanic, sulfur-dioxide surface of Io is orange. ” There was a fourth moon, but it was outside Hubble’s field of view (Science NASA).