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Jupiter Moons to Get Some Space Agency Love

It seems fitting that in a year being celebrated worldwide as the 400th anniversary of telescopic astronomy, NASA and ESA have chosen one of Galileo’s first loves, Jupiter, as their next top planet.

Cut-away images show the insides of Io, Ganymede, …


In January of 1610 the famed Italian Galileo Galilei pointed a homemade ‘scope at the heavens and witnessed something shocking: four “stars” moving in fixed paths around Jupiter.

What he saw were actually the gas giant’s four biggest moons, which became known as the Galilean satellites—Io, Europa, Ganymede, and Callisto.

… Europa, and Callisto


—Images courtesy NASA/JPL

The tiniest of these, Europa, is only slightly smaller than Earth’s moon. In fact, if it weren’t for big, fat Jupiter’s light drowning them out, people on Earth would be able to see all four of the satellites without the aid of a telescope.

Today NASA and ESA announced that Jupiter and its four famous moons will get priority as science targets for the next flagship mission, following in the footsteps of Cassini (Saturn) and New Horizons (Pluto).

The appropriately named Galileo orbiter, which operated from 1995 to 2003, was the first mission dedicated to studying Jupiter, and the planned Juno mission is slated to take an in-depth look under the massive planet’s gassy veil.

The newly proposed mission—a joint effort between the two space agencies—calls for two orbiters, one around Europa and the other circling Ganymede, although the goal is to study the whole of the Jovian system.

For the record, Jupiter is now known to have 62 moons, but the Galilean bodies are kinda special in more than just a historical context:



—Image courtesy NASA/JPL/DLR

  • 3,280 miles (5,262 kilometers) wide
  • named for a Trojan male beauty abducted by Zeus to be cupbearer to the gods
  • the largest moon in the solar system
  • the only moon known to have a magnetosphere
  • its mottled terrain indicates a complex geologic history that may have been created by tectonic forces


—Image courtesy NASA/JPL/DLR

  • 1,945 miles (3,130 kilometers) wide
  • named for a Phoenician princess abducted by Zeus in the form of a tame bull
  • has layering similar to Earth’s: an iron core, a rocky mantle, and a saltwater surface, albeit a frozen one
  • is warmed by tidal heating, so it could have a liquid ocean under the icy crust
  • has been touted as an even better possible location for simple life-forms than Mars


—Image courtesy NASA/JPL/University of Arizona

  • 2,274 miles (3,660 kilometers) wide
  • named for a priestess of Hera seduced by Zeus, who later turned her into a cow
  • the most volcanically active body in the solar system
  • thought to be the source of lightning and auroras on Jupiter
  • experiences “tides” of up to 330 feet (100 meters) in solid rock


—Image courtesy NASA/JPL/DLR

  • 2,986 miles (4,806 kilometers) wide
  • named for a nymph and follower of Artemis who was seduced by Zeus and turned into a bear
  • the most heavily cratered object in the solar system (with apologies to Edward James Olmos)
  • the oldest landscape in the solar system
  • both the darkest (in terms of reflectivity) and lightest (in terms of density) of the Galilean moons


  1. AnneMinard
    February 18, 2009, 8:26 pm

    WOW, great post! I’d never seen close-ups of the moons before. Io loooks bizarre, yes? – Anne