Galileo's on-time, on-target arrival at Jupiter Dec. 7 is a triumph for the tricky art of in-orbit refueling.
Like aircraft taking in energy in midflight, Galileo depends on taking in energy while speeding along its orbit. For aircraft, that energy comes in the form of liquid fuel. For spacecraft, it comes in the less tangible but no less substantial form of what physicists call the kinetic energy of motion: the energy associated with the speed of moving objects.
A refueling spacecraft like Galileo passes close by a planet or large moon. The moon or planet's gravity flicks the spacecraft into a new orbit. In doing so, it also transfers a little of its own kinetic energy to the spacecraft. It's an inconsequential loss for the planet, but a it's a big boost for the spacecraft - hence the term "gravity assist" for this kind of en-route "refueling."
Galileo would never have gotten to Jupiter without gravity assist. Now it will become the key to Galileo's forthcoming dance among the four largest of Jupiter's 16 moons: Callisto, Europa, Ganymede, and Io. These are the so-called Galilean satellites, first seen by Galileo Galilei in 1610. The spacecraft will pass close by Io on its way into the Jovian system. Then it will begin a 22-month odyssey involving 10 close encounters with the satellites.
Galileo will pass up to 350 times closer to these bodies than the Voyager spacecraft did when it took the first close-up images. It should view these moons in the kind of detail that Earth-mapping satellites see on our planet, according to Torrence V. Johnson, Galileo project scientist at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
JPL manages the mission for the National Aeronautics and Space Administration. None of this would be possible without the gravity-assist maneuvers by which each moon hands Galileo on to its next dance partner.
Here's how it works. Imagine you are looking down on the scene from a vantage point above the plane in which the planets generally orbit. You would see planets circling the sun and moons …