Credit: Cassini Imaging Team/Cassini Project/NASA
Before we move past Jupiter, let's take a look at some of its orbiting bodies. Jupiter has more than 60 moons, but I'll confine this post to the four Galilean satellites, so named because Galileo discovered that they were not stars, but moons orbiting Jupiter.Above we have Io casting its shadow against the Jovian clouds. The scale of Jupiter's size begins to become clear.
Credit: Cassini Imaging Team/Cassini Project/NASA
Here's Io again, in a breathtaking image that, like the one above, was taken by the Cassini spacecraft en route to Saturn. The clarity of this image is so startling that it conveys a sense of stillness. In fact, Io completes its orbit of Jupiter once every 42 hours. It's moving along at a very good clip. 
Credit: Galileo Project/JPL/NASA
Io has more volcanic activity than any other body in the solar system. This true-color close-up image of Io shows its yellow surface, the result of high levels of sulfur and silicate rock. 
Credit: Cassini Imaging Team/Cassini Project/NASA
Here is Jupiter and its largest moon, Ganymede, which is also the largest moon in the solar system. The Great Red Spot on Jupiter is visible to the right, almost as if Jupiter is looking at its moon.
Credit: Galileo Project/JPL/NASA
The surface of Ganymede is rocky, icy and cratered, characteristics it shares with its neighbor, Callisto (below).
Credit: Galileo Project/Voyager Project/JPL/NASA
Callisto is heavily cratered and its surface is thought to be four billion years old. Unlike Io, there is no geologic activity to drastically alter its surface, making it the oldest landscape in the solar system.
Credit: Galileo Project/JPL/NASA; reprocessed by Ted Stryk
Finally we come to Europa, with its strange surface that almost makes it look like a cosmic ball of twine. The bright areas are water ice, and this is the moon that scientists think may have oceans under its surface. The European Space Agency is planning to put a spacecraft into orbit around Europa to determine the thickness of the surface ice, with a long-range goal of using hydrobots to tunnel below the surface and search for life.Here are some helpful hints to find the moons of Jupiter yourself. Ordinary binoculars will do the trick. Stay warm!
