High-five for ISRO's 'slingshot technique' as Aditya-L1 leaves Earth's orbit for its destination

Chennai: 19 September 2023 (Agencies) After 17 days of circling Earth, India's solar observatory Aditya-L1 craft has been slingshot on a long journey towards its destination, 1.5 million kilometres away from Earth. In the wee hours of Tuesday (Indian Standard Time), ISRO issued commands to fire the on-board engines of the Aditya-L1 craft, thereby granting it adequate departure energy to travel on the trajectory that will take it to the Sun-Earth Lagrangian Point 1.

This point is at 1 percentage distance of the 150 million kilometre Earth-Sun distance. 

"The spacecraft is now on a trajectory that will take it to the Sun-Earth L1 point. It will be injected into an orbit around L1 through a maneuver after about 110 days," ISRO said. 

Notably, Aditya-L1 is the fifth consecutive Indian spacecraft to be slingshot from Earth's orbit towards its destination in interplanetary space or towards a celestial body.

As Indian rockets do not possess adequate lifting power to directly offer adequate departure velocity to spacecraft and hurl them to faraway destinations, India uses what is commonly known as the 'slingshot technique'.

This method has been deployed successfully in missions such as Chandrayaan-1 (2008), Mangalyaan (2013), Chandrayaan-2 (2019), Chandrayaan-3 (2023) and Aditya-L1. 

In technical terms, such a method makes use of what is known as the 'Oberth Effect', which states that the most efficient place for a spacecraft to change velocity is at the lowest point of its orbit.

Aditya-L1 and other spacecraft listed above were placed in an elliptical orbit around the Earth. Typically, satellites that are meant to remain in orbit around the Earth are placed in a circular orbit. So, at all times the craft is equidistant from earth. 

However, when a craft has to be transferred from one orbit to another or has to be slingshot on a path towards another celestial body, then it is placed in an elliptical or egg-shaped orbit.

In an egg-shaped orbit, the craft's distance from the earth's surface keeps varying. The closest approach that the craft makes to the earth is known as perigee and the farthest approach is known as apogee. 

The velocity of the craft is highest when it is at perigee (the closest approach to Earth).

Therefore, when the craft's engines are fired at perigee, the craft will gain more kinetic energy (departure energy), than it would during an engine firing at apogee.

This is why all engine firings for raising orbits are done at perigee. It is fuel-efficient and low-cost (as opposed to using a larger rocket). 

By mastering such complex calculations and executing multiple orbit-raising manoeuvres and injection manoeuvres, ISRO has proven its mettle in carrying out such complex missions to the Moon and Mars and now to study the Sun from afar. 

According to ISRO, placing the spacecraft at the L1 point provides a greater advantage of observing solar activities and their effect on space weather in real-time.