For any of you who have seen the movie adaption of, “A Wrinkle In Time” may remember the scene where two of the protagonists hide in a tree stump in order to be thrown over a wall.
Meg, one of these protagonists, has parents who work in quantum theory, so she has picked up knowledge in physics throughout the years at home. She was the one who directed her friend to hide in the tree stump because she knew that the tornado was giving speed to objects that fell into it, calling it, “The Slingshot Maneuver”.
From the very first time I watched this film and learned of this trick, it immediately seemed like a physical conundrum: how something can escape an attractive entity with more speed than it entered with. It actually wasn’t until tonight that I fully grasped the concept with the help of some articles and Neil DeGrasse Tyson.
I found the graphics online of the gravitational slingshot to be rather ambiguous and misleading. The following one, however, is an exception. So as I explain the mechanics operating behind the curtains of the “slingshot maneuver “or “gravity assists” as I’ll be referring to them from this point foward, reference the diagram I’ve embedded below:
So gravity assists have actually been somewhat of a popular yet unsung hero of space travel and research. In this article from NASA, you can read about how space probes have historically made use of multiple during their treks to the exterior solar system for their practically free acceleration.
But how does a body falling into a planet gain speed? Even if said object is later able to escape the heart of the planet’s gravitational pull, all of that energy gained while advancing toward it will be stripped from the body upon departure.
The conservation of energy does still hold during this process, so it is very correct to assert that there is no net change in speed of the probe. But you’re forgetting that this is not the only motion at play in these conditions— planets revolve around the sun.
In order for the planet to have its gravitational influence, the probe has to catch up to its speed, relative to the sun (we use the Sun as a rest frame, since compared it doesn’t have motion like the satellites of our solar system). If the probe weren’t able to match the planet’s speed, it would gain too much distance from the planet for a gravity assist to take place.
So in approaching the planet from behind (remember this detail) the probe gains the velocity of the planet. And through doing this, it saps some of the planet’s angular momentum, although the effects of this are negligible since the planet is immense in mass.
And while the planet will continue in its elliptical path of orbit, the probe will not; it will continue in the same direction it was moving with the planet when it matched its speed. If you look at the diagram above, the path of the prove moves in an overall hyperbolic fashion with the eccentricity increasing or decreasing contingent on the angle in which the probe enters in reference to the planet’s direction of motion.
And it is through this minor purloin of momentum that our exploratory satellites reach speeds and distances in space that were never even thinkable prior.
