In a distinctive experiment reminiscent of the hop maneuver executed by the Vikram Lander, Chandrayaan-3’s Propulsion Module (PM) underwent a transition from orbiting the Moon to orbiting Earth.
Chandrayaan-3’s primary aim was to exhibit a soft landing near the lunar south polar region and conduct experiments using the instruments on the Vikram and Pragyaan. Launched on July 14, 2023, via the LVM3-M4 vehicle from SDSC, SHAR, the spacecraft achieved a historic touchdown on the Moon with the Vikram Lander on August 23. Subsequently, the Pragyan rover was deployed, operating scientific instruments in both the Lander and Rover continuously for one lunar day, fulfilling Chandrayaan-3’s mission objectives.
Regarding the Propulsion Module, its core objective was to transport the Lander module from GTO to the final lunar polar circular orbit and then separate from the Lander. Following the separation, the SHAPE payload in the PM was operated for data collection. Originally planned for around three months during the PM’s mission life, the precise orbit injection and efficient burn maneuvers left over 100 kg of fuel in the PM after a month in lunar orbit. Consequently, this surplus fuel was utilized to gather additional information for future lunar missions and demonstrate operational strategies for a sample return mission.
To continue observing Earth using the SHAPE payload, a decision was made to reposition the PM into a suitable Earth orbit. This mission plan meticulously considered collision avoidance measures, ensuring the PM did not crash into the Moon’s surface or interfere with Earth’s GEO belt at 36,000 km and orbits below it. Factoring in estimated fuel availability and safety concerns for GEO spacecraft, an optimal trajectory for Earth return was plotted for October 2023.
The initial maneuver on October 9, 2023, elevated the apolune altitude from 150 km to 5112 km, extending the orbit period from 2.1 to 7.2 hours. Subsequently, based on estimated propellant availability, the second maneuver aimed for an Earth orbit ranging from 180,000 x 380,000 km. The Trans-Earth injection (TEI) maneuver occurred on October 13, 2023. Following the post-TEI maneuver, the propulsion module conducted four Moon fly-bys before departing the Moon’s Sphere of Influence (SOI) on November 10. Presently, the propulsion module orbits Earth and passed its first perigee on November 22, reaching an altitude of 154,000 km. With an orbit period of nearly 13 days and a 27-degree inclination, the perigee and apogee altitudes vary along its trajectory, with the predicted minimum perigee altitude at 115,000 km. Consequently, the current orbit prediction suggests no imminent threats of close approaches to operational Earth-orbiting satellites.
As planned, the SHAPE payload operates whenever Earth is within its field of view. Additionally, a specialized operation of the SHAPE payload took place during a Solar Eclipse on October 28, 2023, with ongoing operations continuing further.
ISRO’s UR Rao Satellite Centre flight dynamics team developed an analysis tool from fundamental principles for this operation, currently undergoing validation through the return maneuvers executed for the Chandrayaan-3 PM.
Key outcomes from the return maneuvers of CH3 PM pertaining to future missions encompass:
- Planning and executing trajectories and maneuvers for returning from the Moon to Earth.
- Developing a software module to plan such maneuvers, including preliminary validation.
- Strategizing and executing gravity-assisted flybys across celestial bodies.
- Preventing uncontrolled crashing of the PM onto the Moon’s surface at the end of its life, meeting the requirement of minimizing debris creation.