Rare Jupiter triple-moon conjunction spotted by Hubble

Three of Jupiter's largest moons gather together to cross the planet's face for a rare astronomical show.

Jupiter moons in conjunction
This conjunction of Galilean moons is a rarity. NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

The last time I saw a major celestial conjunction, it involved three suns in the movie "The Dark Crystal." On January 24, a rare conjunction happened in real life, involving three moons and Jupiter. NASA's Hubble Space Telescope captured a series of beautiful images of moons Europa, Callisto and Io transiting the gas giant's face at the same time.

Jupiter has over 60 known moons. Astronomer Galileo Galilei discovered four moons around Jupiter in the early 1600s. Europa, Callisto, Io and Ganymede are now known as the Galilean moons. Three out of four of them transiting Jupiter together makes for quite a sight.

NASA notes that a conjunction such as this only occurs once or twice each decade. The three moons have very different orbiting times, ranging from 2 days to 17 days. The images capture both the moons and their shadows as cast onto Jupiter's clouds.

If you look closely, you can see the different colors for each moon. Io is a yellowish-golden color, Callisto is a dark brown and Europa is almost white. This has to do with the different makeup of each moon. Europa has a reflective, icy surface. Callisto is cratered. NASA describes Io as having a "volcanic, sulfur-dioxide surface."

Hubble has a history of capturing outstanding space images, many of which are views of nebulae and faraway galaxies. The rare triple-moon conjunction is a good example of what the space telescope can see within our own solar system.