The images were taken with NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 on January 23, 2015.
Hubble captures these moons in great clarity they can also be seen here on Earth with a small telescope or even a decent pair of binoculars.
Jupiter’s four largest moons are Io, Europa, Ganymede, and Callisto, known as the Galiliean moons, after the 17th century scientist Galileo Galilei, who discovered them. Their complete orbits around Jupiter range from two to 17 days in duration. The moons can commonly be seen transiting the face of Jupiter and casting shadows onto its layers of cloud. But seeing three of them transiting the face of Jupiter at the same time is rare, occurring only once or twice a decade.
Missing from sequence of images is the Galilean moon Ganymede which was outside Hubble’s field of view.
The moons of Jupiter have very distinctive colors. The smooth icy surface of Europa is yellow-white, the volcanic sulphur surface of Io is orange and the surface of Callisto, which is one of the oldest and most cratered surfaces known in the solar system, is a brownish color.
The event is shown from start to finish in the video below.
Bottom line: New images taken by the NASA/ESA Hubble Space Telescope on January 23, 2015 capture a rare occurrence as three of Jupiter’s largest moons – Europa, Callisto and Io – parade across the giant planet’s banded face.