The Hubble Space Telescope recently took a string of images capturing a rare triple moon transit of Jupiter – Europa, Callisto and Io – in action. Hubble captured the images through its Wide Field Camera 3 in visible light.
Europa, Callisto and Io are known as Galilean moons as they were discovered by the scientist during the 17th century. With orbits ranging from two to 17 days, it’s common for at least one of the moons to be seen orbiting the Jupiter.
However, the convergence of the three is an occurrence so rare that NASA said it happens only once or twice per decade. The fourth Galilean moon, Ganymede, was outside the Hubble’s view and was not part of the celestial sight.
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 was one of the oldest and most cratered surfaces known in the solar system, is a brownish color.
The image on the left shows the beginning of the event, which took place on January 24, 2015. From left to right, 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.
Near the end of the event, 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.