Almost as important as getting to Jupiter was the way Galileo got to Jupiter. By saving fuel using a long winding approach to the planet, Galileo made a longer stay in Jupiter's neighborhood possible, with more flybys of the moons and observations of the great planet itself. This also enabled Galileo to get unique pictures of the Earth and moon in the same photo frame, which had never been done before, as well as special observations of Venus and a few asteroids, one of which was found to have its own moon, a real surprise! Could all this happen on the way to Jupiter? Yes, by a plan called VEEGA, which stands for Venus-Earth-Earth gravity assist.
Galileo didn't just get blasted into space on a direct course to Jupiter, although at one point that was the plan. A straight path would mean using a lot of fuel or mean Galileo would need a boost from a powerful rocket when leaving Earth. Both these plans were possible but eventually one was judged impractical and the other just plain dangerous. So Galileo team members borrowed a plan used since the days of the Pioneer spacecraft, picking up speed using "gravity assists" from nearby planets and from the Earth itself. Pioneer had used only one gravity assist, though, while Galileo, just to *get* to Jupiter, would need three: a fly by Venus first, then back by Earth, and then Galileo would circle around the sun past Mars' orbit and back to Earth again for the final assist, which would swing it at last fast enough to reach all the way to Jupiter. How exactly does that work, you might ask?
Well, first you need to understand a little about gravity. You probably already know that large bodies like planets and moons and, of course, the sun have gravity which pulls objects toward them, when objects gets close enough. But actually every body has a "gravitational force" and exerts a pull on other bodies around it. A house, a mountain, another person, even a flower has this power. It's just that the force of gravity is usually extremely weak unless the body is very massive. Gravity is the weakest of the forces of nature. But nearness also increases the influence of gravity. A baby held in its mother's arms, for example, is more affected by that mother's gravitational pull than by any other body in the solar system (except Earth, that is!), because the baby is right up against Mom.
Now to understand a gravity assist like Galileo used, let's use some examples which are easy to imagine. Parents and their children are good examples of the force of gravity, since a parent and child often seem like a planet/moon system: you may have noticed, for example, how a parent will constantly watch the child to make sure it doesn't stray outside of his or her "power," and you might even have seen a mother or father hold a child's hands to swing her around in a circle in play, until the child's feet lift off the ground, as if that child were "orbiting" the parent.
For our gravity assist demonstration, let's say that a little boy of about 7 years old wants a "gravity assist" to go faster than he can go by himself. He sees his father jogging down the street, and he knows his father can run faster than he can. As his father comes back down the street and passes the house, he runs out and comes up behind his father. Now Dad reaches back and grabs his son's hand, and with his arm's force, he swings him forward, giving him energy to go faster. The effort might slow down the father a little, but the son, once his father lets go, gets a boost on his running.
With just a few technical differences, this is what Galileo did when it flew by Venus and then twice by Earth. Each planet's speed around the sun is a well-known fact, and the scientists who planned Galileo's path knew that if Galileo allowed itself to be captured or "grabbed" by the gravity of Venus and the Earth, by flying close enough, it would appear (technically from the sun's point of view only) to speed up. Since each time the speed would only increase a little, three gravity assists were needed to get Galileo all the way to Jupiter. And even more amazing, just as the father would sacrifice some of his speed by helping the little boy, Venus and the Earth actually slowed down a teeny little bit when they gave the gravity assists to Galileo.
Now, that shows how a gravity assist increases speed. But at the same time, it also alters direction. Let's imagine that boy and his dad again, now in an open park. This time the father is angry at the boy and wants him to stop running around. Unfortunately he can't chase the boy because his feet are both stuck in cement and he can't move. The boy, on the other hand, is running around his father in a circle, and sometime comes so close that dad can reach out and try to grab the boy and hold on to him. But the boy is going so fast and never comes quite close enough to get "captured" by the father.
What happens, though, when the boy comes so close that his father can get a hold of him for a short time? Let's say the son runs past the father on the right side of him, and when he's nearest his dad, he's about two feet away. His dad reaches out and can only grab his shirt for a few seconds. But he tries to hold on, so when the boy breaks away, he's no longer going straight anymore. His father has pulled him a little off course to the left. The boy was deflected by his dad's hold, but only slightly. On the next pass by dad, however, the boy gets a little *closer*. This time he passes his father on the left so close that his dad reaches out and grabs his arm! Now he's got a better grip, but the boy is just going too fast again and he breaks free. But his dad held on longer this time, so the boy *really* got swung off his straight line past his father, and this time, he was thrown off to the right. So whichever side of a planet a spacecraft flies by, it ends up having its path bent a little in the direction of the planet, since that's the direction the planet is pulling on the spacecraft, using the power of its gravity.
This is the basic principle of gravity assists. Scientists know how fast planets like Venus, the Earth, and Mars travel around the sun, how strong their gravity is, and how close a spacecraft must fly to get the desired speed and change in direction. With that information, they can plan for spacecraft to use these nearby neighbors along with Earth to swing spacecraft out to the farther reaches of the solar system, saving fuel and, as a bonus, often gaining information on the planet giving the assist at the same time.