In 2008, India made its first big leap into space as Chandrayaan-1, our first lunar mission, set out for the moon.
A small cube about five feet on each side, it orbited the moon and beamed back evidence of water on the lunar surface and tens of metres below it, and, in the process, gave life to new geological curiosities.
But 10 months into its two-year-long mission, it fell silent, bringing the exploration to an abrupt end.
Ten years later, in 2019, India returned to the moon with Chandrayaan-2 to find answers to those curiosities and many more that continue to elude explanation — this time on the lunar surface.
However, the mission to soft land on the cratered sphere failed as the lander, Vikram, made a crash-landing on the lunar surface due to a last-minute glitch.
Four years later, Chandrayaan-3 will trace the same path to the moon and go where no human or machine has ever soft-landed — the moon’s apparently water-rich but freezing-cold south pole.
Chandrayaan-3’s journey to the moon will begin atop the Indian Space Research Organisation’s (ISRO’s) heaviest rocket, geosynchronous satellite launch vehicle (GSLV)-Mark III.
The lander and the rover (lander module) will be mechanically interfaced with the propulsion module and accommodated inside this launch vehicle.
When the launch is set into motion, the first to ignite will be the two solid strap-on boosters on both sides of the rocket. Breathing out orange plume, the boosters give the rocket the thrust required for lift-off and guide it through its brief but majestic vertical ascent.
A few seconds later, the liquid-fuelled core, or the second stage, powered by a pair of Vikas engines, will fire, and the strap-on boosters, having served their purpose, will be dumped.
This will be followed by the jettisoning of the protective payload fairing, or the covering over the payload.
The covering is designed to protect the payload during the rocket’s ascent through the Earth’s atmosphere and maintain the rocket’s aerodynamic structure during the flight. When the rocket is in space, the fairing is no longer needed.
After this, the liquid core stage will shut down and get detached. Then, the cryogenic upper stage, powered by CE-20, India's largest cryogenic engine, will ignite, driving the module to a highly elliptical Earth parking orbit.
At this stage, the onboard propulsion system will raise the module’s orbit around the Earth through a number of burn.
While orbiting the Earth in an elliptical orbit, the module will be at its highest speed when it passes through the point in that orbit closest to the planet. This point is called the perigee.
Exactly opposite to this point in the orbit is the apogee, where the module will be the furthest from the Earth and at its slowest speed. The speed varies across different points in the orbit due to the variation in the Earth’s gravitational pull.
The closer the module is to the Earth, the more the gravitational pull, and the greater the speed.
Every time the module reaches the perigee, or the point of highest speed, the onboard engine fires, increasing its speed even more, pushing it into a higher, more elongated orbit as a result.
With every burn of the onboard propulsion system, the module will keep spiralling outwards in increasingly elongated ellipses.
Eventually, as the module continues its journey, its speed will steadily increase until it reaches the escape velocity necessary to break free from Earth's gravity. At this point, the module's orbit will elongate, allowing it to set a course towards the moon.
According to some accounts, achieving this trajectory requires the module's engine to undergo five to six burns, resulting in a highly elliptical orbit that brings it close to the moon's path around the Earth.
In the case of Chandrayaan-1, the smaller spacecraft, ISRO had to perform five engine firings to propel it into the Lunar Transfer Trajectory (LTT), the specific orbit necessary for the mission.
The entry of the Chandrayaan-3 module into the LTT is carefully timed to align with the moon's position in its own orbit. This strategic timing ensures that the module reaches proximity to the moon's orbit precisely when the moon is located in that region.
Once the module reaches this point, a precise manoeuvre is executed using the onboard propulsion system. This manoeuvre, known as lunar orbit insertion, is designed to significantly reduce the module's velocity.
As a result, the gravitational field of the moon can then pull the module into a stable lunar orbit.
If all goes according to the plan, the lunar orbit insertion will be achieved by 5 August. Over 20 days after it set out for the moon, Chandrayaan-3 will be orbiting it.
This is largely how ISRO’s Mangalyaan, or the Mars Orbiter Mission, and Israel-based SpaceIL’s Beresheet lander, travelled from their orbits to the Mars and the moon, respectively.
However, the path taken by NASA's Apollo 11 mission, a manned mission launched in 1969, differed significantly.
The spacecraft was directed to the LTT through a single six-minute-long burn of the Saturn rocket's third stage, akin to a slingshot effect. This unique approach allowed the Apollo 11 spacecraft to swiftly reach the lunar surface in just over three days.
ISRO's Chandrayaan-3 is being launched atop the GSLV Mk III. Its thrust output is relatively lower compared to the Saturn V rocket used by NASA during the Apollo program.
Consequently, Chandrayaan-3's trajectory involves a series of manoeuvres that gradually extends its reach by leveraging the gravity of Earth and the moon. This approach allows ISRO to conserve fuel and gradually align the module with the moon's orbit.
Having escaped Earth’s gravity and entered the lunar orbit, the module will start revolving around the moon in an elliptical orbit.
A series of manoeuvres will be used to progressively lower the altitude of the module and place it in a 100 km circular orbit around the moon. Visually, these manoeuvres will be the opposite of those performed around the Earth.
Now comes the trickiest part of the mission.
After the module arrives in the 100 km orbit, the propulsion module will separate from the lander module and continue to revolve around the moon, like the orbiter part of the Chandrayaan-2 mission.
A separate entity now, the lander module will de-boost with the firing of its breaking engines. With this manoeuvre, the lander will begin its short but tricky journey towards the lunar surface.
When it reaches close to the surface, cameras and hazard avoidance sensors will study the landing site for accuracy. Using the data obtained, the lander will autonomously determine the trajectory it will have to take to get to the landing site.
It was in this phase of the mission that the Chandrayaan-2 lander, Vikram, suffered a glitch and crash-landed on the surface of the mon.
The chosen landing site for the upcoming Chandrayaan-3 mission is more or less the same as Chandrayaan-2. It is situated near the lunar south pole at approximately 70 degrees latitude.
If all proceeds as planned, Chandrayaan-3 will achieve a groundbreaking feat as the first mission ever to successfully soft-land in the vicinity of the lunar south pole.
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