Why return of Chandrayaan-3’s Propulsion Module to earth orbit is a milestone for ISRO

The Indian Space Research Organisation (ISRO) achieved a unique milestone when it relocated the Propulsion Module (PM) of Chandrayaan-3 from a lunar orbit to an orbit around the earth, after it achieved its mission objectives, demonstrating the technology and capability to return from the lunar surface. This is the first time that any country has retrieved a spacecraft from the moon and brought it back to earth orbit.

“This remarkable feat involved meticulously orchestrated manoeuvres – an orbit-raising manoeuvre and a Trans-Earth injection manoeuvre – strategically placing the PM in an earth-bound orbit inclined at 27 degrees,” said Srimathy Kesan, founder and CEO of Space Kidz India, which is into design, fabrication and launch of small satellites, spacecraft and ground systems.

“Throughout its trajectory, the perigee and apogee altitudes have exhibited variability, with a predicted minimum perigee altitude of 1.15 lakh kilometres. Encouragingly, the current orbit prediction assures no imminent threats of close approach with operational earth-orbiting satellites. This mission plan was worked out considering the collision avoidance such as preventing the PM from crashing on to the moon’s surface or entering into the earth’s GEO belt at 36,000km and orbits below that,” Kesan said.

This space expert further pointed out that the unprecedented return of the Chandrayaan-3 PM to earth’s orbit holds pivotal implications for future missions. The noteworthy outcomes include the meticulous planning and execution of trajectories and manoeuvre for the return from the moon to earth, accompanied by the development and preliminary validation of a software module specifically designed for such a manoeuvre. Moreover, these manoeuvres have paved the way for planning and executing gravity-assisted flybys across planets and celestial bodies, further expanding the repertoire of space exploration methodologies.

“Crucially, these manoeuvres have successfully steered clear of the uncontrolled crashing of the PM on to the moon’s surface at the end of its operational life,” said Kesan. “This crucial aspect aligns with the stringent requirement of avoiding debris creation, underscoring the meticulous planning and execution strategies inherent in ISRO’s space missions. The successful return of the Chandrayaan-3 PM to earth’s orbit heralds a new chapter in India’s space exploration narrative, marked by technological prowess, meticulous planning, and groundbreaking achievements. This milestone not only amplifies India’s presence in the global space community but also paves the way for innovative advancements in future space missions.”

The PM had carried the lander and the rover from the launch injection to the lunar orbit. It entered the lunar orbit on August 5, and released the lander on August 20. The lander touched down near the lunar south pole at 6.03pm on August 23, making India the fourth country to successfully land on the moon and the first nation to do so in the lunar south polar region.

The PM performed four lunar flybys between October 13 and November 10 using its thrusters to alter its orbit. The PM came as close as 100km (62 miles) to the lunar surface during these flybys and captured high-resolution images of the landing site and other regions of interest. The PM also carried a scientific payload called Spectro-polarimetry of HAbitable Planet Earth (SHAPE), which measured the polarisation of the sunlight reflected by earth. SHAPE is a scientific instrument on Chandrayaan-3 that helped scientists understand the habitability of earth and other exoplanets by analysing atmospheric and surface characteristics. By studying the polarisation of sunlight, SHAPE provides insights into the atmosphere’s composition, cloud presence and other factors important for habitability. This data is vital for assessing the habitability of exoplanets and improving our knowledge of earth’s environment. SHAPE’s observations contributed to Chandrayaan-3’s goals and advance our understanding of habitability conditions on earth and beyond.

“The original plan was to use this payload for approximately three months while the PM mission was ongoing,” said space expert Girish Linganna. “However, due to the precise orbit injection by LVM3 and efficient earth and lunar burn manoeuvres, the PM ended up having more than 100kg of fuel left even after conducting operations in lunar orbit for more than a month. The team decided to make use of the remaining fuel in the PM to gather more data for upcoming lunar missions and showcase the operational strategies for a sample return mission. They also decided to reposition the PM to a suitable earth orbit so that the SHAPE payload could continue observing earth.”

ISRO stated that they carefully considered both the estimated fuel availability and the safety of the GEO spacecraft. They subsequently designed an optimal earth return trajectory for October 2023.

“The PM underwent manoeuvres to adjust its orbit, transitioned from the moon’s influence and is currently in an earth orbit with specific characteristics, ensuring safety regarding potential collisions with other satellites,” Linganna said. “Additionally, the return manoeuvres conducted on Chandrayaan-3’s PM are manifold for future missions such as plotting and performing the path and manoeuvres needed to travel from the moon back to earth, creating a software component to plan such a manoeuvre and testing it initially, planning and executing a gravity-assisted flyby around other planets or celestial bodies and preventing the PM from crashing uncontrollably on to the moon’s surface when it reaches the end of its lifespan, thereby fulfilling the requirement of not creating any debris. These pivotal points in the Chandrayaan-3 mission not only illustrate the technical achievements of adjusting orbits and executing complex manoeuvres, but also lay the groundwork for future missions.”