They are three of the four Galilean moons, named after the 17th-century scientist Galileo Galilei who discovered them with a small telescope. Their orbital periods around Jupiter range from 2 days to 17 days. The Galilean moons can often be seen transiting the face of Jupiter and casting shadows onto its cloud tops., but seeing three of them transiting the face of Jupiter at the same time is rare, occurring only once every five or ten years.
On the left, the moons Callisto and Io are above Jupiter’s cloud tops. The shadows from Europa, Callisto, and Io are strung out from left to right. Europa is not visible in this image. Approximately 42 minutes later (right-side image), Europa has entered the frame at lower left. Slower-moving Callisto is above and to the right of Europa. Fastest-moving Io is approaching the eastern limb of the planet; its shadow is no longer visible on Jupiter. Europa’s shadow is toward the left side of the image, and Callisto’s shadow to the right. The moons’ orbital velocities are proportionally slower with increasing distance from the planet. Ganymede, the other Galilean moon, was outside Hubble‘s field of view and too far from Jupiter to be part of this conjunction.
Each of these moons has a distinctive color. The cratered surface of Callisto is brown; the smooth icy surface of Europa is yellow-white; and the volcanic, sulfur-dioxide surface of Io is orange. The fuzziness of each depends on each moon’s distance from Jupiter. The farther away the moon, the softer the shadow because it is more spread out across the disk.
Image Credit: NASA