More than five decades ago, the Americans reached the Moon in four days, but India’s Chandrayaan missions take a month to complete the same journey. Besides, the Chandrayaan-3, like its predecessor, the Chandrayaan-2, will take a rather intriguing route to the Moon. It will circle the Earth five or six times, then sling off to the Moon, and then once again circling the Moon 5-6 times before the lander began its descent on to the Moon’s surface.  

What is the reason behind this intriguing routing? Well, you can shoot off a rocket straight to the Moon, but it will have to be a very big and expensive rocket. To travel the distance of 384,400 km, the rocket will have to carry enormous amounts of fuel. The fuel adds to the weight of rocket, so it would need to be more powerful. The Saturn V rocket that took Apollo 11 to the Moon in 1969 stood 363-feet tall. The LVM-3 is 142 feet. Besides, there is no urgency, there is no need for the Chandrayaan-3 to reach the Moon fast.  

But why circle the Earth so many times before beginning the moonward journey? The answer is simple: to make use of the Earth’s gravity to impart enough velocity to the spacecraft — Chandrayaan-3 — to shoot off to the Moon. In other words, the gravity of the Earth does the same function as rocket fuels.   

Kepler’s second law says the line connecting a satellite and the parent body sweeps equal areas in equal intervals of time. This means that as the satellite gets closer to the parent body, it acquires more velocity. And the farther the object comes from, the higher the velocity it acquires when it turns around the parent body.  

ISRO will make use of this property to give Chandrayaan-3 enough power to hurtle to the Moon’s vicinity.  

After the LVM-3 puts Chandrayaan-3 above the Earth, the spacecraft will start circling the Earth on its own, in an elliptical orbit. When it reaches the farthest point, engineers on the ground will nudge it slightly to change the direction a little so that its next loop is bigger than the first. So, when the spacecraft approaches the Earth on its second loop, it will acquire a higher velocity. Again, when it reaches the farthest point, called apogee, the engineers will once again change the direction a little, so that on the third loop, the spacecraft acquires an even higher velocity. It will go on thus for 5-6 loops, at which point the spacecraft will have acquired enough velocity to sling itself towards the moon.  

Once it reaches the Moon, the reverse will happen. Loop-by-loop the spacecraft will get closer to the moon. When it is about 100 km from the moon’s surface, the lander will detach itself and begin its descent on to the moon.  

No easy task

Of course, the manoeuvres are not easy. You have to know, in extremely precise terms, how much of a nudge you should give the spacecraft and at which point. ISRO will be taking the help of the Jet Propulsion Lab in Pasadena, California, to do these manoeuvres.