Chandrayaan-3’s success will be determined during the final 15 minutes on August 23 (Wednesday) when the Lander will perform the critical technical manoeuvre to make a soft landing.
In the final manoeuvre, the Lander has to change its position from horizontal position to a vertical one and minimise the speed of descent before the touchdown at 6:04 pm today.
The final 15 minutes on Wednesday evening will determine the success of the mission, which former ISRO chairman K Sivan has described as “15 minutes of terror”.
What Happens in Final 15 Minutes?
Chandrayaan-3’s landing is scheduled at around 6.04 pm India time on August 23. The lunar craft has completed its second and final deboosting where its orbit was reduced to 25X134 km around the Moon.
When the craft is 30 km from lunar surface, it will undergo Rough Braking Phase, where the lander’s horizontal velocity will be significantly reduced from 1.68 km/sec (or 6,000 km/h) to almost zero for a soft landing.
There are sensors fitted to the lander which will carry out calculations to reduce the velocity as it descends from the height of 30 km to a height of 7.42 km in the first 10 minutes of the landing process.
The final landing process begins at 5.47 pm on August 23. The entire 15-minute process of descent will be guided by computer logic that has already been fed into the lander’s computers, guidance and control navigation systems.
Chandrayaan-3 lander can touch down at a maximum speed of 10.8 km per hour without endangering the instruments on board. However, the optimal speed is around 7.2 km per hour.
Chandrayaan-3 has to also tilt almost 90 degrees from the horizontal position to the vertical position for a landing. The lander can also have a tilt up to 12 degrees and still land safely.
Chandrayaan-2 Failed Mission
It was in this final “15 minutes of terror” during which Chandrayaan 2 lander Vikram had crash landed on the surface of the moon on September 7, 2019.
According to the ISRO scientists, Chandrayaan-2 failed after the lander did not switch from the horizontal to the vertical position appropriately and hurtled on to the surface of the Moon while entering the “fine braking phase” which the scientists now call as “15 minutes of terror”.
According to a report in The Indian Express, the landing was on track until around 3 minutes before the final “terminal descent phase”, when the lander ended up spinning more than 410 degrees, deviating from its calibrated spin of 55 degrees to crash on the lunar surface.
Four Phases of Chandrayaan’s Landing
Chandrayaan-3 was launched from the Satish Dhawan Space Centre in Andhra Pradesh from the Geosynchronous Satellite Launch Vehicle Mark III heavy-lift rocket on July 14, 2023.
Post launch, the lunar craft conducted a series of orbit-raising manoeuvres to gradually move the spacecraft into higher and higher orbits before beginning to move directly towards the moon. The earth-bound manoeuvres went on till July 31 after which the craft moved into translunar orbit.
On August 1, Chandrayaan-3 performed the Trans Lunar Injection, a propulsive manoeuvre to slingshot the spacecraft towards the Moon from Earth’s orbit. The manoeuvre sent the craft on a trajectory towards the Moon.
After staying in the translunar orbit, Chandrayaan-3 finally entered into the lunar orbit on August 5, following which orbit reduction manoeuvres were carried out on the satellite on August 6, 9, 14 and 16.
After the fifth and final orbit reduction manoeuvre, the Lander Module of Chandrayaan-3, comprising the Vikram lander and the Pragyan rover, successfully separated from the Propulsion Module on August 17.
The lander then underwent a “deboost” (the process of slowing down) that reduced its orbit to 113 km x 157 km. After a second and final deboosting operation on August 20, the Landing Module was placed in a 25 km x 134 km orbit around the Moon. Finally, the touchdown of the Lander will be attempted on August 23 at 6:04 pm.
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