Gravity Assist or Gravitational Slingshot Explained

May 18, 2020

Gravity Assist or Gravitational Slingshot Explained

Gravity Assist:
Without getting into the complexities of mathematics and physics, I have attempted to give a simplified brief account of the technique by which humans have managed to send spacecraft deep into space almost without fuel. This technique is called ‘Gravity Assist’. As the name suggests, a planet’s gravitational force is used to accelerate or decelerate an object/spacecraft’s velocity. So, how does it work? A spacecraft destined to reach a distant point in space (planet, asteroid, interstellar space, etc.) is made to get pulled by a planet’s gravity and fly so close in a particular orbit, thereby gaining incremental momentum from the planet’s orbital velocity, and at the precisely calculated point of the orbit, the spacecraft is made to deflect from the planet’s gravity, and with the newly acquired increased speed, spacecraft resumes its odyssey to destiny. If the spacecraft needs to accelerate, it orbits alongside the planet’s orbit and to decelerate, it orbits in the opposite direction.

Using ‘Gravity Assist’ or ‘Gravitational Slingshot’, humans accomplished in sending Voyager 1 spacecraft into interstellar space – the space between stars, and Voyager 2 is on its path to exit the solar system. The rationale for talking about ‘Gravity Assist’ today is, NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer) made a close flyby of 17,000 KM above our planet Earth’s surface, on 22nd September, 2017 so as to reach its destination, asteroid ‘Bennu’.

#HelloOSIRISREX #GravityAssist

Image: An artist's illustration of spacecraft OSIRIS-Rex making gravitational slingshot around Earth. (Credit: NASA)
For further reading on OSIRIS - Rex mission:
https://www.nasa.gov/feature/goddard/2017/ground-based-telescopes-to-photograph-osiris-rex-during-flyby

Why does it take more than a month for "Chandraayan-3" to complete it's 384,000 kms journey to the moon? Reportedly, Chandraayan3's lander would be soft landing on the moon by 23rd August 2023. It is because, ISRO (Indian Space Research Organization) will be adopting a fuel and cost efficient, tried tested and proven method so as to reach the moon - "Gravitational Slingshot".

What is Gravitational Slingshot?

As the name suggests, an object orbiting a planetary body acquires momentum and deflects from the planet's gravity at a certain calculated point. With the newly acquired momentum and positioning of the object in a mathematically calculated orbit, the object will be able to reach it's destination with almost no fuel. This method has been used by space agencies for decades, especially NASA's Voyager missions in sending Voyager 1 and Voyager 2 to orbit every planet of the solar system and eventually exit solar system itself. However, ISRO is yet to build a rocket powerful enough to put a spacecraft on it's direct path to moon and they would soon.

Know How Does Chandrayaan 3 is travelling to the moon with no fuel from Sivan?

Yesterday, Bharath's space agency Indian Space Research Organization (ISRO) successfully burned the thrusters of the spacecraft (only now fuel is used after insertion into earth's orbit) "Chandrayaan_3", and made it deflect from the Earth's gravitational field. With the velocity acquired from orbiting planet Earth, the spacecraft is on its path to the moon with no fuel, that's the catch!

Now let's recall to the basics of rocket science. Boosters are used to lift a rocket from ground zero to putting the spacecraft at a particular orbit above a planet (otherwise known as Insertion Orbit). Then the engines or the boosters are shutdown or the fuel gets exhausted in some cases. It's time go back to inertia - an object obtaining certain momentum will continue to be in motion even after there is no force exerted or applied to move the object.

In the case of space, there is almost no resistance, therefore an object acquiring certain velocity is theoretically expected to move at the same or higher velocity forever. The intriguing aspect of rockets is, while it's is orbiting at an incredible speed without fuel, it is held in an orbit by a planet's gravity - essentially the planet's gravity is not letting the rocket run away like holding the spacecraft with its hands and swinging around.

Now let's go back to the burning of thrusters or fuel usage, the spacecraft is made to deflect or making the planet release it's hands, which are holding the spacecraft together in an orbit. This is what is a simple illustration of "Gravitational Slingshot".

Now that because of gravitational slingshot, Chandrayaan 3, has successfully left Earth's orbit and headed towards Moon. The same gravitational slingshot will be used to enter the Moon's orbit. Conversely, the orbit will be decreased sequentially and the spacecraft will eventually descend into Moon. A few kilometers from Moon's surface the spacecraft will remain in an orbit, but the Lander will be detached and will descend to attempt to physically land on the moon. If this part of the mission goes well, it is what will be called as soft landing. Most importantly, Bharath will become the fourth nation for having landed a spacecraft on another planet, apart from US, Russia and China.

If everything goes well until the very last minute as per the plan because anything can go wrong any moment as we are talking about attempting of landing an object on a foreign surface not of Planet Earth.

Bharath's lunar mission Chandrayaan_3's landing vehicle Vikram would appear like this if the landing was successful 2 days from now, 23rd August 2023.

The space agency ISRO - Indian Space Research Organisation has done an exceptional work in achieving this mission at a reasonable cost relative to other agencies.

Irrespective of the outcome, you should remember that you already are heroes.

May the force be with you.

What was the possible reason for the failure of Russia's lunar mission named "Luna 25"?
What I'm going to try to explain is how difficult and how delicate landing an object on another planet is?
In a matter of few seconds, decades of hardwork of an entire mission is termed a failure. Why? Let's explore:
Firstly, why do space agencies keep saying "burn"? What is burning in the space?
In fact, the engine is switched on. If you recall my earlier discussions, I had tried to explain how does a spacecraft travel in the vaccum of space without fuel with the help of a planetary dynamics called " Gravitational Slingshot ".
After reaching the destination, in most of the cases, it this the orbit of the planet. Now the engine or the thruster is turned on or in the language of the space agencies the fuel is burnt for a few seconds to change the trajectory or the orbit of the spacecraft so that it gradually descends. Again the engine is switched off. This when engineers say "the burn was successful and it was for 20 seconds".
On completing the required descend or lowering the altitude of the spacecraft, the engine needs to be precisely burnt for a mathematically calculated duration - a matter of seconds so that the spacecraft doesn't collide with the surface and actually lands gently without damaging it's equipments or incurring fatal damage. This is what is called a soft landing.
In the case of Russian Space Agency Roscosmos "Luna 25" mission, reportedly the burn exceeded the rduration - a few seconds more than the required. Consequently, the spacecraft collided or crashed into the surface of Moon.
Now you may appreciate, why did I say earlier that nothing is certain until the very last moment in the space exploration missions.
Image source:
https://twitter.com/katlinegrey/status/1693638624678412639?t=PaOtRow4lFSeCeoYZ-GSRw&s=08

India is on the moon!

And, India that is Bharath (as per the constitution), now has become the 4th nation after US, Russia and China to land an object on a foreign surface, which is not of Planet Earth on 23rd August 2023 at 18:02 hours Indian Standard Time (IST)

Jai Hind

The landing site of Bharath's historic lunar mission is named "SivaSakthi". Jai Hind!