1.   A third is a large crescent moon, fashioned in wood, that hangs above our heads as a stand-in for the Muslim world.

2.   But, also like Jupiter and Saturn, they could have many large moons.

3.   One is orbiting Jupiter and its large moons right now, and another is on its way to orbit distant Saturn.

4.   That could mean that, despite its icy exterior, the large moon has a molten core generating a magnetic field, much as happens on Earth.

5.   The first fly-by of Ganymede, a moon as large as the planet Mercury, should come next summer.

6.   This innermost of the large Jovian moons is being pulled first one way and then the other in a gravitational tug-of-war between Jupiter and neighboring moons.