The news from the moons of Jupiter in 1997 was as follows: Ganymede definitely has a magnetic field; Callisto does not; Io has more volcanoes than ever; and Europa may have a liquid ocean surging underneath a thin crust of ice.
Still going strong eight years after its launch, the Galileo spacecraft had close encounters with the three outer Galilean moons this past year and snapped new photos of innermost Io from a distance. The probe’s trajectory—the repeated use of one moon’s gravity to change the orientation of the probe’s ellipse around Jupiter and to fling it toward the next moon—is one of its niftiest traits and a big improvement over the one-time flybys of the Pioneer and Voyager days. It gives Galileo’s handlers not only a closer look at its targets but also the novel luxury of a second look if they want one.
In 1996, for instance, Galileo ...
