What is Gravitational Slingshot Effect?

**Effects of Gravity in space-time #4**

Gravitational Slingshot of objects in space  

Image by SpaceX-Imagery from Pixabay

Despite being the weakest force among all of the four fundamental forces of nature, gravity has been a real game-changer since its modification by Einstein in 1915, through his General Theory of Relativity. It's not surprising that when it comes to macroscopic physics, gravity shows its effects in astonishing yet mind-blowing ways. since it's a long-range force. Gravitational lensing, time warp in black holes, and tidal effects are all induced by gravity. One such effect is what we call the gravitational slingshot effect or Gravity Assist, which is an interesting concept used in orbital mechanics and aerodynamics.

How does gravitational slingshot work? 

Animation of slingshot effect of Voyager 1- Wikipedia 
The purple colour represents the path of Voyager 1

Gravitational slingshots effects are mainly used to deflect the courses or to accelerate or slow down the satellites or spacecrafts, with the help of a planet's or a moon's gravitational pull. The principle is that the energy needed by the spacecraft to accelerate or deflect its path is provided by the orbital energy of the planet or moon in motion. 

This happens because the spacecraft, when it's near to the planet (which is following its own orbital path), the spacecraft gets dragged by the planet's gravity, just enough to change the spacecraft's trajectory. The spacecraft's velocity is designed beforehand so that at a precise time interval, it's known that the spacecraft will reach near to the planet, which will then, drag or accelerate the spacecraft under that planet's gravity. 

The slingshot effect can be used for both accelerating or decelerating the spacecraft, depending on its position relative to the planet. If the spacecraft is slightly behind that planet, the spacecraft will accelerate, and if its position is after the planet, it will decelerate. 

Energy transfer and Conservation of Energy  

One important point to note in this mechanism is that there's a transfer of energy taking place between the spacecraft and the planet, and with energy - we're talking about their orbital energies. The spacecraft is made to accelerate by the planet's gravity, the required energy for its acceleration is provided by the planet's orbital energy. 

But according to the law of conservation of energy, if there's a specific amount of energy loss in a system, there should be an equal amount of energy gain by another system through energy transfer. 

In this case, a part of orbital energy is transferred from the planet to the spacecraft, meaning the planet loses some of its energy required for orbiting around the sun. But even though the planet loses some of its energy, it'll be a very negligible difference, since the planet is enormously large in size compared to the tiny spacecraft, to which, the amount of gained energy seems to be large enough to note the difference.

Gravity assist is very helpful when it comes to accelerating a spacecraft or a satellite or changing its trajectory because it doesn't require extra energy to trigger it since the spacecraft manages to 'steal' the energy off the planet or the moon. 

Jupiter's slingshot effect is the reason why there's an asteroid belt between Mars and Jupiter, by attracting asteroids and comets from outer space and trapping it within Jupiter's gravity. It's been also said that Jupiter protects the Earth from being hit by deadly asteroids and comets by deflecting them further away.