How It Works

From Jupiter's point of view, the situation is similar to a bicyclist speeding up going downhill into a valley, then slowing down again on the uphill part of the road.

Gravity Assist Figure 1

In the diagram on the right, the situation is described in two dimensions. One can see the magnitude and direction of the spacecraft's velocity on its way toward Jupiter in the lower right corner of the diagram. In the upper left corner, the accelerating force of Jupiter's gravitation has made a significant change in the direction of the spacecraft's velocity, but not in its magnitude. (These represent velocity at "infinity," from Jupiter, that is, before and after being noticeably changed by Jupiter's presence.) Near the middle of the diagram, the long arrow shows that there's a significant, but temporary, increase in the magnitude (speed).

To look at the same phenomenon in terms of a cyclist, "V-in" shows the cyclist approaching a downhill grade into a canyon. "V-out" shows that the cyclist slowed down again at the top of the ensuing uphill grade. Indeed, after negotiating the canyon, the cyclist's direction has changed, but in the end the cyclist has not increased or decreased the speed of the bicycle.

Gravity Assist Figure 2

The planet's motion is one key. A gravity assist with Jupiter involves not a stationary planet as illustrated above, but a planet with enormous angular momentum as it revolves around the Sun. In the diagram on the left, Jupiter's motion along its solar orbit has been simply illustrated with a red-colored vector. The spacecraft acquires this vector, or a significant portion of it, during its interaction with Jupiter. The red vector is added to "V-in" and "V-out." The result shows how the spacecraft's velocity, relative to the Sun, gets an adequate boost from Jupiter.

A concept called "trajectory bending" is the other key. Notice how rotation of the vector from "V-in" to "V-out" (the bending of the spacecraft's path by the planet's gravity) helps increase the result. The spacecraft is a physical mass, so it has its own gravitation. That's how the spacecraft can tug on Jupiter and actually decrease the planet's orbital momentum by a tiny amount. In exchange, the spacecraft acquires a significant amount of momentum from Jupiter, compared to the momentum the spacecraft already had.

In the analogy of tossing a ping-pong ball into an electric fan, the ball would take energy from the fan blade, and we can presume it would bounce off at a speed greater than it had coming in. In this case, the ball interacts with the fan blade directly rather than gravitationally, and it slows the fan slightly as it hits. (If the fan's motor were on, the blade's lost momentum would instantly be replaced, which of course is not true in the case of a freely orbiting planet.)

The Slingshot Myth

Individuals have often compared gravity assist with the effects of a slingshot. But in reality, gravity assist is, in terms of physics, a different example. Using a slingshot, a person would sling a projectile around a few times, stronger and better aimed each time, before letting it go. At that point, the projectile's centrifugal force becomes the same as its propulsion force. On the other hand, using a gravity assist, a spacecraft comes up and steals some angular momentum during a single flyby of a planet in motion, removing momentum from that planet. Gravity assist is really much more like a ping-pong ball hitting the revolving blade of a ceiling fan than it is like a slingshot.