ISRO’s rocket science behind Chandrayaan-2 explained in simple terms

Chandrayaan 2 orbiter at launch centre.

Chennai: Placing an object in space is not an easy task and things become all the more difficult when one tries to make a soft landing of a rover on a celestial body.

To explain it in simple terms, it is like trying to throw a ball from a moving train in Delhi to reach a particular seat in another moving train in Chennai through the window gap. And, all of this should be done blindfolded. This is exactly what ISRO is trying to do in the second lunar mission Chandrayaan-2 15 July. Here the ball is Chandrayaan 2, the train to which it is going to be thrown is the moon and the train from which it is thrown is earth.

There are several challenges involved in a moon landing. First and foremost is accuracy in trajectory as the distance to the moon is approximately 3.844 lakh km. Ensuring trajectory accuracy while navigating such a large distance poses many challenges as trajectory is influenced by the non-uniform gravity of the earth and the moon, gravitational pull of other astronomical bodies, solar radiation pressure and the moon’s true orbital motion, ISRO said in its website. Here are some of the challenges in the mission:

Deep Space Communication
Owing to the large distance from earth and limited on-board power, radio signals used for communication are weak with heavy background noise, which need to be picked up by large antennas. Trans Lunar Injection (TLI) and Lunar Capture Chandrayaan-2 will perform a series of TLI burns for raising its apogee successfully to reach the vicinity of the moon’s orbit. As the moon’s location is continually changing due to orbital motion, the intersection of Chandrayaan-2 and the moon’s path has to be predicted sufficiently in advance with a high level of accuracy. As the moon approaches the apogee of Chandrayaan-2 on-board thrusters fire precisely to reduce its velocity for lunar capture. The margin of error in these calculations and manoeuvres is very narrow.

Orbiting around the moon
Lunar gravity is ‘lumpy’ due to uneven mass distribution under its surface. This influences the orbit of the spacecraft. Also, precise knowledge of the thermal environment at orbital altitude is essential for keeping on-board electronics safe.

Soft landing
This is the most challenging part of the mission and is divided into ‘rough braking’ and ‘fine braking’. Variation in local gravity has to be factored into the lunar descent trajectory. The on-board NGC and propulsion system has to work in unison, autonomously and automatically for a successful landing. Further the landing site, landscape features should not result in a communication shadow area.

Lunar dust
The lunar surface is covered with craters, rocks and dust. Firing of on-board engines close to the lunar surface results in backward flow of hot gases along with dust. Lunar dust is miniscule and is hard, barbed and jagged. Its negative charge makes it stick to most surfaces, causing a disruption in deployment mechanisms, solar panel performance and NGC sensor performance.

Extreme temperatures and vacuum
A lunar day or night lasts 14 earth days. This results in extreme surface temperature variations. Moreover, the ambient pressure of lunar surface is a hard vacuum. This makes the lunar surface an extremely hostile environment for lander and rover operations.

Balasubramani Muniyandi