Few space probes have had worse luck than Galileo. Built with late-1970s technology to explore Jupiter and its planet-size moons, it arrived some seven years late in the wake of the space shuttle Challenger explosion. Finally sent on its way in 1989, but with a low-power rocket, it was saddled with a convoluted 2.4-billion-mile slingshot trajectory that took more than six years to complete. Along the way the orbiter's giant primary antenna refused to deploy, and its antiquated tape-drive memory system jammed.
But Galileo is nothing if not plucky. Ground controllers have worked near miracles to keep the craft limping along, returning images so exotic they exceed imaginings. The crippled platform has succeeded in documenting a startling solar system within our solar system, where massive volcanoes blow their tops with astonishing regularity, lightning arcs hundreds of miles at a time, ice-covered oceans grind together to form breathtaking patterns, and all the right conditions may be in place to create life.
In a final burst of glory in November, Galileo will plunge into Io’s highly charged atmosphere, searching for close-ups of that molten moon’s surface. The flight is likely to be the end for Galileo but just the beginning of reasons to keep going back to these compelling worlds.
Callisto has one of the most battered and scarred surfaces in the solar system, the result of 4.6 billion years of abuse from incoming asteroids. Unlike its sibling moons, Callisto has no large-scale processes for erasing craters; some may fade as the underlying ice evaporates, but most simply sit on top of earlier scars. Still, something mysterious is going on beneath the dusty crust of this ball of ice and rock. During a recent flyby, Galileo detected a magnetic field that indicates a salty ocean.
The interiors of Io, Europa, Ganymede, and Callisto (clockwise from top left): While three of the moons have metallic cores similar to Earth’s, Callisto has a mixture of ice and rock at its heart. Europa, Ganymede, and Callisto may contain hidden oceans.
Images of a shattered landscape (right) led researchers to a remarkable conclusion: Europa’s surface is littered with icebergs frozen in place.