Gravity's Slingshot Effect

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 circling some of the planets. Navigators seeking gravity assist set their craft toward one of these bodies - say a planet - on a course that is fine-tuned for a specific amount of energy transfer. If the craft passes behind the planet as that planet moves along its sun-centered orbit, the craft picks up speed and changes direction in its own sun-centered orbit. On the other hand, if the craft passes in front of the planet, it can lose energy to the planet and slow down. That is the kind of maneuver used to send spacecraft in toward the sun by having them pass close by in front of Venus.

The rocket that launched Galileo from Earth orbit Oct. 18, 1989, was too puny to send the craft on a direct course to Jupiter - a journey that would have taken only 2-1/2 years. Instead, JPL navigators opted for the so-called VEEGA strategy - Venus-Earth-Earth-Gravity Assist. A Venus flyby Feb.10, 1990, added 2.2 km/sec (4,900 m.p.h.) to Galileo's speed. Passage within 960 km of Earth Dec. 8, 1990 added another 5.2 km/sec. A second Earth fly-by at 303-km on Dec. 8, 1992, added a final 3.7 km/sec to Galileo's speed to send it on to the giant planet. Now Galileo will use similar tactics to move among the Galilean moons as both satellites and the spacecraft orbit Jupiter.

Gravity assist has become a hallmark of 20th-century solar system exploration. Suggested by Derek E. Lawden in the Journal of the British Interplanetary Society in Sept. 1954, gravity assist was a theoretical curiosity until 1965. Then its practicality became obvious.

Analysis of about 1,000 trajectories by Gary A. Flandro - then a California Institute of Technology graduate student - showed that it could open up the outer solar system to exploration. That led to the Grand Tour of the outer planets by Voyager spacecraft in the 1980s. Before that, a flyby of Venus at a distance of 5,750 km on Feb. 5, 1974 sent Mariner 10 on to make the first - and so far only - close-up survey of the innermost planet Mercury between March 1974 and March 1975.

Now gravity assist will do for Jovian exploration what it has done for planetary research.