India's ambitious Chandrayaan-3 mission hit another mark on Wednesday when its spacecraft successfully underwent a fifth and final lunar-bound orbit manoeuvre, which brings it even closer to the surface of the Moon.
With this, the spacecraft has completed all of its Moon-bound manoeuvres, and it will now prepare for separation of the lander module -- comprising the lander and rover -- from the propulsion module.
"Today's successful firing, needed for a short duration, has put Chandrayaan-3 into an orbit of 153 km x 163 km, as intended. With this, the lunar bound maneuvers are completed. It's time for preparations as the Propulsion Module and the Lander Module gear up for their separate journeys," ISRO said in a post on X (formerly Twitter).
Separation of the lander module from the propulsion module is planned for August 17, it said.
Following its launch on July 14, the Chandrayaan-3 spacecraft entered into lunar orbit on August 5, following which orbit reduction manoeuvres were carried out on August 6, 9 and 14.
As the mission progressed, a series of manoeuvres were being conducted by ISRO to gradually reduce Chandrayaan-3's orbit and position it over the lunar poles.
After separation, the lander is expected to undergo a "deboost" (the process of slowing down) to place it in an orbit where the Perilune (closest point to the Moon) is 30 kilometres and Apolune (farthest point from the Moon) is 100 km.
From this orbit, the soft landing on the south polar region of the Moon will be attempted on August 23, ISRO has said.
The space agency's chairman S Somanath recently said that the most critical part of the landing is the process of bringing down the velocity of the lander when it begins its descent from a height of 30 km to the final landing (position), and that the ability to transfer the spacecraft from horizontal to vertical direction is the "trick we have to play" here.
"The velocity at the starting of the landing process is almost 1.68 km per second, but this speed is horizontal to the surface of the moon. The Chandrayaan-3 here is tilted almost 90 degrees, it has to become vertical. So, this whole process of turning from horizontal to vertical is a very interesting calculation mathematically. We have done a lot of simulations. It is here where we had the problem last time (Chandrayaan-2)," Somanath explained.
Further, it has to be ensured that fuel consumption is less, the distance calculation is correct, and all the algorithms are working properly, he added.
"Extensive simulations have gone, guidance design have been changed, and a lot of algorithms have been put in place to make sure that in all these phases required dispersions are handled...to attempt to make a proper landing," he said.
Over five moves in the three weeks since the July 14 launch, ISRO had lifted the Chandrayaan-3 spacecraft into orbits farther and farther away from the Earth.
Then, on August 1 in a key manoeuvre -- a slingshot move -- the spacecraft was sent successfully towards the Moon from Earth's orbit. Following this trans-lunar injection, Chandrayaan-3 escaped from orbiting the Earth and began following a path that would take it to the vicinity of the Moon.
Chandrayaan-3 is a follow-on mission to Chandrayaan-2 (2019) to demonstrate end-to-end capability in safe landing and roving on the Moon's surface.
It comprises an indigenous propulsion module, lander module, and a rover with an objective of developing and demonstrating new technologies required for inter-planetary missions.
The propulsion module other than carrying the lander and rover configuration till about 100 km lunar orbit, has Spectro-polarimetry of Habitable Planet Earth payload to study the spectral and polarimetric measurements of the Earth from the lunar orbit.
Expressing happiness over Chandrayaan-3 mission going successfully so far, former ISRO chairman K Sivan, who was heading the space agency during the second lunar mission, said the lander's touch down on August 23 "is a great moment we are looking forward to."
He said that Chandrayaan 2 also had gone through all these phases successfully, and during the landing's second phase, an "issue" came up, and the mission couldn't be achieved as intended.
"Now definitely more anxiety will be there on the landing manoeuvre. Last time it couldn't be successful. This time everyone is waiting for the great moment. I'm sure it will be successful because we have understood the failures we had during Chandrayaan 2. We have corrected it and, in addition to that, wherever margins were less, extra margins are added. Wherever redundancy was not there, that also now has been provided. This time we are expecting the mission will be successful. We are confident in that," he added.
Further commenting on tomorrow's lander and propulsion module separation, Sivan said, "Tomorrow's manoeuvre is very critical as any activity in space is a critical activity. Tomorrow's activity happening in space remotely separates Chandrayaan-3 into two parts, one is propulsion and lander. It is very important and we are expecting that it would be normal and will go successfully without any problem."
Dr M Annadurai, who was Project Director of India's first Lunar Mission Chandrayaan-1, said that after the propulsion module says goodbye to the lander, the lander will have its own initial checkups.
"The four main thrusters, which will enable the lander to have a smooth landing on the lunar surface, need to be tested, as well as other sensors. Then it (lander) will take its own course to go to the near 100kmx30km orbit, and from there on August 23 early morning manoeuvres will start," he said.
Noting that the last leg (landing) will be autonomous, he said all the required commands and sequences -- failure mode identification in case of any failure or faults, and selecting a redundant system for ensuring secure and soft landing -- will be uplinked.
The mission objectives of Chandrayaan-3 are to demonstrate safe and soft landing on the lunar surface, to demonstrate rover roving on the Moon, and to conduct in-situ scientific experiments.
The lander will have the capability to soft land at a specified lunar site and deploy the rover that will carry out in-situ chemical analysis of the Moon's surface during the course of its mobility.
The lander and the rover have scientific payloads to carry out experiments on the lunar surface.