Gravitational slingshots

At this moment, the Voyager 1 space probe is the human-made object that’s farthest from Earth. It’s traveling at a whopping 62,140 kilometers per hour away from the sun. How did it get there?

Force and speed

Objects accelerate and decelerate relatively to other objects because of forces applied to those objects. This is probably something you learned in physics 101, but it bears to look at this a little before we continue.

When you floor the gas pedal on your car, the engine revs up and starts applying a forward force to the car — well, actually, controlled explosions in the cylinders in the motor are converted to force which is converted to rotation, which is conveyed to the wheels using the transmission, which in turn results in a forward force on the car.

Anyway, the car starts to accelerate. If you let go of the gas pedal, you slow down. This is because the car wheels generate friction with the road, and the car itself causes friction with the air. Both of which produce a force to the back of the car. Without the force forward, the force backward ‘wins’ and the car decelerates.

In space, there is barely any friction. Space is a near perfect vacuum and the few particles floating around hardly slow you down — if they hit you, they can cause massive damage because of high speed, but a gradual slowdown not so much. In space, a force applied to an object will accelerate it, but the object won’t slow down unless caught by a gravity well.

One way to apply force to it, is by controlled explosions in the form of rocket fuel. The fuel expands, sending gas out of the rocket in one direction, applying a large force in the other direction. However, that is not what you want in all cases.

Enter the slingshot

Let’s assume we’re sending a probe from Earth to Mercury. Let’s also say you want to put it in orbit around Mercury. As the probe travels from Earth to Mercury, the sun will apply gravity to it. Since it’s moved out of Earth’s stable orbit, that gravitational force will be greater than the centripetal force, meaning it will start to accelerate toward the sun.

When the probe reaches Mercury it will need to decelerate hard to be able to enter orbit.You could use fuel to do that, but depending on the weight of your probe, that’s going to take a lot of fuel. Worse, you first need to lift all that fuel from down on Earth into space, which costs even more fuel.

There’s another way: use the gravity of the planets or moons as a slingshot to accelerate or decelerate.

How does it work?

If an object slingshots past a planet, that planet will start to exert gravity on it. That force will cause it to accelerate as it gets closer to the planet, then decelerate as it moves away again. The acceleration and deceleration roughly balance each other out. In the frame of reference of the planet anyway.

In the frame of reference of the planet, only the trajectory of the object changes. The planet’s gravity is a centripetal force, which turns the object into a new direction. From other frames of reference, like the sun, something else happens. That’s because the planet itself isn’t stationary. The planet moves around the sun, meaning that from the point of view of the sun, movement energy is transferred from the planet to the object.

We like to picture the solar system as a large ball — the sun — with eight other balls — the planets — circling that large ball slowly. Slowly is a relative term, though. The Earth moves around the sun in one year. That orbit is 940 million kilometers long, meaning the Earth moves at 940 million kilometers per year, which equates to 108,000 km/h. Yep, that’s nearly twice as fast as Voyager 1. So, there’s a lot of movement energy to transfer. And remember: a planet is vastly heavier than a space probe, meaning transferring only a fraction of the planet’s energy has huge consequences.

So, if an object slingshots around a planet, it will accelerate or decelerate relative to the sun, depending on the direction it slingshots around the planet.

The same happens when an object slingshots around a moon orbiting a planet. In other words, you can use the moon to accelerate or decelerate relative to Earth.


Back to our Mercury probe. That hypothetical probe actually exists and it did use slingshotting. It pulled a slingshots around Earth, Venus twice, and Mercury three times. That’s right, six-fold slingshot braking FTW. The MESSENGER probe did it, and achieved an orbit around Mercury.

Voyager 1 did a double slingshot: once past Jupiter, and once past Saturn. In those two flybys, it took some amazing pictures, and picked up a lot of speed relative to the sun, ending up hurtling out of the solar system at 62,140 km/hour. Voyager entered interstellar space in August of 2012, becoming the first man-made object outside of the solar system.

Author: Martin Stellinga

I'm a science fiction and fantasy writer from the Netherlands

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