Gravity assist space travel

In order to reach their destinations spacecraft use gravity assist to gain speed and momentum. The Cassini spacecraft now orbiting Saturn used assists from the Earth and Venus.

The concept seems straightforward, as the spacecraft approaches the planet its gravity causes the craft to accelerate. The craft then makes a near miss and is flung out into space with greater speed and momentum.

But I’m confused, wouldn’t the same gravity that caused the craft to accelerate on approach cause it to decelerate as it departed? It seems like it should be a zero sum gain.

IANARS, but I think the advantage is that it picks up some of the velocity of the planet’s orbit around the sun.

Likewise, the planet slows down a little with each passing spacecraft, but thankfully not enough to cause a drastic change in orbit.

I had the same question in my head a while ago.

Here is a great answer:

Another great explanation of how this works at USS Clueless.

Some fairly basic things to keep in mind when reading the above links:

Gravitational fields act as refractive objects rather than reflective targets. I.e., we are all familiar with baseball bats and how a baseball striking a bat can be sped up (gaining momentum from the bat) or lose speed as in a bunt. A bat reflects a baseball. A gravitational field also “bounces” objects but inward like a lens, it’s a refractive bouncer.

The change of angle of the object is key. Without a change in angle, nothing of use can happen. Depending on the angle and where the object “strikes” the gravitational field, you get different effects as to gaining or losing momentum.

The rest is basic Physics.

Earl had it essentially right. It’s true that in the center-of-mass frame of the Cassini/Venus system, Cassini exited with the same velocity it entered with. But the Cassini/Venus system is moving with respect to the Sun, so when you add on that extra velocity vector to the “before” and “after” vectors, the magnitude of the “after” velocity after ends up much greater than the “before” velocity (if you’ve done it right.)

You can also arrange things (if you’re so inclined) arrange things so that the gravity of the planet slows you down. I think this technique has been used in the past as well, but I’m not sure by which probe.

Mercury MESSENGER will be flown slightly ahead of Venus and maybe Mercury, too, on its numerous flybys to slow it down enough to orbit Mercury.

Here’s an analogy:

Imagine two people floating in a spaceship (weightless), throwing identical baseballs at each other. The balls hit mid-air, and bounce away at an angle. The balls change direction, but don’t gain or lose any energy in the collision. If you attempt a gravity assist on a free-floating planet (i.e. not orbiting a star), the same will happen.

Now imagine two people standing on earth doing the same. The balls hit midair, and one is deflected upwards while the other goes down. They may have the same speed immediately after the collision, but with one important distinction: one is moving up. That ball would go higher than each person can throw by himself/herself. This is akin to a gravity assist using a planet in a solar system; the planet goes down (towards the sun), the probe goes up.