How India learnt its lessons from Chandrayaan-2 debacle

On the evening of August 23, when the clock crept four minutes past six, a billion hearts heaved in joy of a kind most had never experienced before. India was on the moon, with Chandrayaan-3 landing on its unexplored south pole. Softly and safely. With that, India became the fourth country to soft-land on the moon and the first to land on its south pole. “This sounds the bugle for a developed India,” said Prime Minister Narendra Modi, who witnessed the landing virtually from Johannesburg where he was attending the BRICS summit.

There has been palpable nervous excitement about the Rs615-crore mission since Chandrayaan-3’s launch from the Satish Dhawan Space Centre in Sriharikota on July 14. But India has been moony about the moon since 2008 when it launched Chandrayaan-1. That was a wet October day, with the northeast monsoon winds having a field day. There were apprehensions that the launch would have to be postponed. But everything went on schedule. And, Chandrayaan-1 successfully orbited the moon. Through its observations, it uncovered evidence of water both on and beneath the lunar surface, sparking fresh geological inquiries. But in the tenth month of its two-year mission, it went radio silent, abruptly ending its exploration. Nonetheless, India’s inaugural lunar mission was a success. A decade later, in 2019, India decided to soft-land on the moon with Chandrayaan-2. It ended in disappointment, as the lander, Vikram, crash-landed due to a last-minute glitch. The lander was trying to make a controlled landing near the moon’s south pole, where scientists expect to find water ice. It started descending at 1:38am on September 7, 2019, and took 10 minutes to slow down from a speed of 1,640m per second to 140m per second. It ceased communication when it was merely 2.1km from the moon’s surface.

Hence the nervous excitement around Chandrayaan-3. It was not misplaced. Recent moon landing missions had been met with mixed success. While China successfully landed a rover on the moon in 2013, other missions, including those by India, two private companies and recently by Russia had failed. But Indian Space Research Organisation Chairman S. Somanath had been confident of Chandrayaan-3’s soft-landing abilities. That was not misguided optimism, but belief in ISRO’s capabilities and learnings from Chandrayaan-2. “The learning that we got from the previous mission really helped us,” said Somanath. “The most difficult phase was the launch itself and it was perfectly done. Then the lunar insertion, the separation of the lander module and, of course, the soft-landing were all crucial moments and had to go perfectly. All the people associated with the mission did a lot of hard work and meticulously studied the previous Chandrayaan-2 mission.”

The goal was not only to rectify the issues that led to the 2019 failure but also to enhance its technologies to significantly increase the chances of a successful soft landing with Chandrayaan-3. “One critical aspect that received considerable attention was the software systems,” said Srimathy Kesan, founder and CEO of Space Kidz India, which is into design, fabrication and launch of small satellites, spacecraft and ground systems. “Advanced algorithms and simulations were developed to optimise the onboard software, enabling precise navigation, control and coordination. These enhancements allowed the team to simulate numerous landing scenarios, ensuring preparedness for various contingencies and enhancing the mission’s overall resilience.”

Learning shot: Students build models of Chandrayaan-3 at Nehru Science Centre in Mumbai | Amey Mansabdar

Software improvements were just the beginning. “The lander itself underwent substantial modifications to augment its capabilities and increase the chances of a successful touchdown,” said Kesan. “Engineers conducted meticulous studies on the lander’s legs, fortifying them to withstand the harsh lunar terrain and minimising the risk of damage upon landing. Extensive upgrades were also made to the lander’s sensors, equipping them with state-of-the-art technology to ensure safe and accurate navigation during descent.”

The power system was also upgraded, added Kesan. Chandrayaan-3’s lander, also named Vikram, and robotic rover―Pragyan―now relied on advanced solar power technology, featuring larger and more efficient solar panels. This upgrade guaranteed a stable power supply through the mission, enabling extended exploration and seamless data collection on the lunar surface (it is suspected that Chandrayaan-2 ceased communication as it had run out of power).The Vikram lander of Chandrayaan-3 weighed 1.75 tonne and carried the Pragyan rover, weighing 26kg. The six-wheeled rover was designed to explore the moon for around 14 earth days. The Vikram lander was optimised for a safe landing, employing a sophisticated propulsion system. This propulsion setup, with four throttleable engines that generate 800 Newton of thrust, enabled precise adjustments to the lander’s velocity and trajectory during the descent phase, ensuring a controlled and gentle landing. The lander’s advanced sensors enabled precise navigation, altitude control and hazard avoidance.

Earth erupts in joy: People celebrate Chandrayaan-3’s landing at India Gate, New Delhi | Sanjay Ahlawat

“It included ‘lander hazard detection and avoidance cameras’ that help coordinate with the orbiter and mission control while descending on the moon’s surface,” explained aerospace expert Girish Linganna. “Unlike its predecessor, Chandrayaan-3 carried two of these cameras instead of just one. ISRO developed better sequences for a safe landing, and the lander now has four thruster engines instead of five.” Also, extra fuel was added to the lander to effectively deal with potential disruptions. The landing site was picked with careful consideration. ISRO, after factoring in crucial aspects such as surface topography, illumination conditions and potential hazards, chose a 4kmx2.4km rectangular area on the moon’s south pole. “The selection of the landing place is done based on the data collected from previous missions, including photographs, as we have not physically gone to that place on the moon,” said Mylswamy Annadurai, former programme director, Chandrayaan-1, Chandrayaan-2 and Mangalyaan. “Even if the lander had hit the ground with a little bit of a thud, the legs of the lander could withstand it as they are stronger than before.”

India is only the fourth country after Russia, the US and China to have a successful lunar soft landing. But many countries have tried to join this exclusive club. Recently, Japan’s ispace, a private firm, and the UAE had joined forces for HAKUTO-R Mission 1, named after the moon-dwelling white rabbit of Japanese folklore. The UAE’s Rashid rover was one of the payloads on the HAKUTO-R lunar lander. The rover, which weighed just 10kg, was designed to roam the lunar surface for two weeks and send back images. However, the lander crashed in April. On August 10, Russia launched its Luna-25 mission, which many termed as a race with Chandrayaan-3. Luna-25, however, spun into an uncontrolled orbit and crashed on to the moon’s surface on August 19. From its launch to landing, Chandrayaan-3 ticked all the boxes, making the right manoeuvres on its way to the moon. It was launched atop the powerful GSLV-Mark III (LVM3), ISRO’s largest rocket. Chandrayaan-3 had a propulsion module and a lander module with the rover inside it. Both the Vikram lander and Pragyan rover carried payloads. The lander payload was responsible for the safe landing. The rover once deployed would explore and study the lunar surface.

Girish Linganna

THE LAUNCH WAS a sight to behold―orange plumes rose in the air as the rocket fired off with help from two solid strap-on boosters flanking it. Shortly after, the boosters were discarded and the liquid-fuelled core took over. The payload’s protective covering―put in place to safeguard it during the ascent and to maintain aerodynamics―was dropped once the rocket reached space. Subsequently, the liquid core stage was also detached. Chandrayaan-3 was then propelled by India’s largest cryogenic engine into a significantly elliptical earth parking orbit, sort of a standby area where the spacecraft coasts for a while before firing off to the final destination. And, it was here that the propulsion module fired on all cylinders to elevate its orbit. As the module revolved around earth in an elliptical orbit, it reached its maximum velocity when crossing the point nearest the planet within that orbit. This specific point is referred to as the ‘perigee’. “Imagine the satellite going around earth in a huge circle. When it is on one side of the circle, it moves fastest and closest to the earth. This point is called perigee,” explained Linganna. “But, on the opposite side, it is farthest and moves the slowest. That farthest point is called apogee. The reason it goes faster when it is closer to earth is because earth’s pull is stronger there. As it moves away, earth’s pull is weaker, so it slows down. This difference in speed happens because of earth’s gravity pull on the satellite as it goes round.” Whenever the Chandrayaan-3 module reached the perigee, the onboard engine ignited. This action further boosted its speed, propelling the module into a more elongated orbit. As the module kept moving, it got faster and faster. Once it reached the speed needed to escape the earth’s pull, it broke free and headed towards the moon. ISRO had to fire the engines five times to get Chandrayaan-3 on the right path, called the lunar transfer trajectory (LTT). This process led to a very stretched-out orbit, bringing the module near the path that the moon takes around earth. Chandrayaan-3’s module entered the LTT at a specific time to match where the moon was in its orbit. This timing is crucial as the moon’s position matters when the module gets close to its orbit. Once the module was close to the moon’s orbit, it used its onboard engine to perform what is called lunar orbit insertion (LOI). This move slowed down the module, making it easier for the moon’s gravity to pull it into a steady orbit around it. Chandrayaan-3 performed the LOI on August 5. Chandrayaan-3’s course utilised a sequence of actions that slowly expanded its range by taking advantage of the gravitational forces of both the earth and the moon. This strategy helped ISRO to save fuel, while progressively aligning the module with the moon’s orbit. After getting into the moon’s orbit, the module started going around the moon in a stretched-out circle.

AROUND 100KM from the moon’s surface, another set of manoeuvres were employed to gradually bring down the module’s height and put it into a circular orbit around the moon. After finishing the lunar-bound manoeuvres on August 17, the propulsion module and the lander module parted ways. From July 14 to August 17, the propulsion module was steering the spacecraft’s journey. After separating from the lander module, the propulsion module is circling the moon and using its payload to gather information about earth’s spectrum, which would help scientists understand the earth’s climate and atmosphere better and identify potential habitable planets beyond our own. It will keep circling the moon, just like Chandrayaan-2’s orbiter.

Detached from the propulsion module, the lander started its descent with two tactical ‘deboosting manoeuvres’―intricate braking actions―to gently land on the moon’s surface. As it approached the surface, all sensors swung into action to ensure precision landing. Based on the information from the sensors, the lander independently calculated the necessary trajectory to reach the designated landing site on the south pole. And, just like that, India got its moon pie.