Rare moon trifecta on Jupiter captured by hubble
NASA's Hubble Space Telescope captured images of three of Jupiter's largest moons -- Callisto, Io, and Europa -- crossing the planet's face in the same frame, an occurrence that only happens once or twice a decade.
In a rare occurrence, three of Jupiter's largest moons -- Callisto, Io, and Europa -- crossed the face of Jupiter at the same time, and images of this event were captured by NASA's Hubble Space Telescope.
These "Galilean" moons, named after Galileo Galilei who discovered them in the 17th century, generally take 2 to 17 days to complete their orbits around Jupiter.
While they are often seen crossing the planet, their appearance on the face of Jupiter at the same time only happens once or twice a decade.
The event began on January 24, 2015, when Callisto and Io were seen along with their shadows and only the shadow of Europa.
About 42 minutes later, Europa itself entered the frame, completing the trifecta.
The fourth and missing Galilean moon is Ganymede which was too far from Jupiter and Hubble's scope at the time.
Each of the moons in the images has its own distinctive color and can be seen with a telescope.
Callisto, considered one of the oldest and most cratered landscapes in the Solar System, appears brown.
Europa, with its icy smoothness, appears yellow-white; and Io, with its volcanic sulfur surface, looks orange.
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