Chandrayaan-3: How Propulsion Module’s Return From Lunar To Earth Orbit Paves Way For Future ISRO Moon Missions

With the Chandrayaan-3 mission exceeding its objectives, the Indian Space Research Organisation (ISRO) has been finding ample opportunities to conduct unique experiments that build some preliminary foundation for more ambitious future lunar missions — such as the return of a piece of Moon back to Earth.

In the latest such experiment, the space agency said Monday (4 December) that Chandrayaan-3’s propulsion module, which was in orbit around the Moon, conducting the lone experiment that it was tasked to carry out from there, was moved back successfully to an orbit around Earth.

This feat counts as a significant showcase of ISRO’s technical ability to not only send a spacecraft to the Moon, but also bring it back near Earth through precise, stepwise manoeuvres.

Through this return operation, the Indian space agency demonstrated foremost the planning and execution of trajectory and manoeuvres to bring a spacecraft back from Moon to Earth.

The journey back necessitated the development of a software module for planning and validating such a crucial manoeuvre, which ISRO has counted as among the main outcomes of the experiment.

The agency also successfully demonstrated the planning and execution of a gravity-assisted flyby around another celestial body — a key operation for space missions further out into the solar system.

Further, ISRO cleaned up after itself well by preventing the creation of debris through avoiding the uncontrolled crashing of the propulsion module on to the Moon’s surface.

This remarkable return operation involving the propulsion module, thus, provides a glimpse into some of ISRO’s future mission strategies and operations.

Propulsion Module And Its SHAPE

The propulsion module’s main objective was to ferry the lander module from geostationary transfer orbit to the final polar circular orbit around the Moon and then part ways with the lander so that it can remain in lunar orbit while the lander goes on to set foot on the lunar surface. ISRO accomplished this operation precisely.

While in orbit, the propulsion module’s scientific work began. It was equipped with a scientific instrument called the Spectro-polarimetry of Habitable Planet Earth — in short, the SHAPE payload — for the study of the spectral and polarimetric measurements of Earth from lunar orbit.

Specifically, it was meant to do two things:

– Perform spectroscopic study of the Earth’s atmosphere, and

– Measure the variations in polarisation from the clouds on Earth — to accumulate signatures of exoplanets that would qualify for habitability

SHAPE was switched on for work on 20 August, three days after the propulsion module’s separation from the lander module.

Going Beyond

ISRO initially planned to operate SHAPE for about three months. But so precise were the operations of ISRO that there was plenty of fuel left over on board to try and test out possible future mission operations and strategies.

“The precise orbit injection by LVM3 and optimal earth / lunar burn maneuvers,” ISRO said, “resulted in the availability of over 100 kg of fuel in the PM (propulsion module) after over one month of operations in the lunar orbit.”

“It was decided to use the available fuel in the PM to derive additional information for future lunar missions and demonstrate the mission operation strategiesfor a sample return mission,” the space agency said.

ISRO made the decision to change the propulsion module’s orbit to a “suitable Earth orbit” to continue SHAPE’s Earth observation activity. This return journey was planned for execution in the month of October 2023.

In its statement, ISRO noted that they drew up a mission plan ‘considering collision avoidance’, as they were wary of the propulsion module crashing on to the surface of the Moon or entering various Earth orbits, including the geosynchronous equatorial orbit at 36,000 kilometres from the Earth’s surface, where, for instance, communication satellites operate.

The Return Mission

The space agency made its first move on 9 October, raising the “apolune” altitude from 150 km to 5,112 km and lifting the orbit period from 2.1 hours to 7.2 hours. (Apolune refers to the point in a lunar orbit when a spacecraft is at its greatest distance from the Moon.)

Subsequently, estimating the available propellant on board, the second move — the Trans-Earth Injection (TEI) manoeuvre — kicked in on 13 October. The target Earth orbit was 1.8 lakh km x 3.8 lakh km.

Thereafter, the propulsion module made four lunar flybys before exiting the Moon’s sphere of influence on 10 November, ISRO said.

Presently, the module is orbiting Earth. It crossed its first perigee in Earth orbit on 22 November, with an altitude of 1.54 lakh km. The orbit period is nearly 13 days with 27 degree inclination.

The perigee and apogee altitude vary during its trajectory, and ISRO has predicted the minimum perigee altitude to be 1.15 lakh km. (The perigee and apogee tell us the closest and farthest points from Earth in an orbit, respectively.)

“As per current orbit prediction,” as per ISRO, “there is no threats of close approach with any operational Earth orbiting satellites.”

The SHAPE instrument, meanwhile, carries out its Earth study whenever the blue planet emerges in its field of view. Notably, a special SHAPE operation was carried out during the solar eclipse on 28 October. Its regular operations are set to continue in Earth orbit.

Not For The First Time

Even earlier, ISRO was able to take the Chandrayaan-3 mission well beyond its objectives — thanks to the precise execution of operations.

With all three objectives of the Chandrayaan-3 mission met, and the lunar night fast descending, the Vikram lander went ahead and re-did objective one, sort of: demonstration of safe and soft landing on the lunar surface.

The Vikram lander remarkably underwent a "hop experiment" on 3 September with implications, once again, for future sample return and human missions.

The lander fired its engines after it was commanded to do so, lifted off the lunar surface by about 40 cm, and then landed safely at a distance of about 30-40 cm away.

On a tiny but definitive scale, Vikram achieved a 'hop' or 'kick-start', which can be seen as equivalent to a lunar launch and landing.

Whether to bring lunar samples back to Earth or ferry humans to the Moon and back, a lunar launch and landing is essential.

The lander deployed its onboard ramp once again after its hop, and subsequently redeployed its payloads for more scientific results.

Also Read: Vikram Lander Went Hippity-Hop On The Moon — Here's Why That's Significant