This last leg of the mission carries a heavy burden, with vivid memories of Chandrayaan-2’s 2019 failure when the Vikram lander failed to execute the necessary transition, leading to a lunar surface collision during the “fine braking phase,” just 7.42 km from the target.
Recently, the Chandrayaan-3 lander underwent a crucial deboosting process to adjust its orbit to 25X134 km around the Moon, preparing for the upcoming landing attempt.
The landing process involves a simultaneous reduction of the lander’s horizontal velocity from a staggering 1.68 km/sec (over 6,000 km/h) at 30 km above the lunar surface to nearly zero, ensuring a gentle touch down at the designated site, approximately 70 degrees South latitude. The anticipated landing is scheduled for 6:04 pm India time on August 23.
As ISRO Chairman S Somanath explained, the challenge lies in turning the lander from its nearly 90-degree tilt during the start of the landing process at 5:47 pm on August 23 to a vertical position, a critical maneuver that led to Chandrayaan-2’s mishap in 2019. This transition demands precise fuel management, accurate distance calculations, and flawless algorithm execution.
During Chandrayaan-2’s ill-fated landing, the lander lost control around three minutes before the final “terminal descent phase,” spinning over 410 degrees away from its calibrated spin of 55 degrees, ultimately resulting in a lunar crash. To control its speed and direction, the lander employs 12 onboard engines, including four for velocity reduction and eight for directional control, allowing it to hover against the Moon’s gravity.The rough braking phase of the landing aims to reduce the initial horizontal velocity of 1.68 km/sec to 358 m/sec horizontally and 61 m/sec vertically, all within a 690-second span, covering a distance of 713.5 km across the lunar surface from 30 km to 7.42 km altitude.At 7.42 km altitude, during an “attitude hold phase” lasting approximately 10 seconds, the lander will transition from horizontal to vertical orientation, reducing altitude to 6.8 km, and velocity to 336 m/sec horizontally and 59 m/sec vertically.
The final phase, the “fine braking phase,” extends for about 175 seconds, fully transitioning the lander to a vertical position, covering the remaining 28.52 km to the landing site, reducing altitude to 800-1,000 m, and reaching a nominal speed of 0 m/sec.
Crucially, between the “attitude hold phase” and the “fine braking phase,” Chandrayaan-2 lost control and crashed. Lessons from this failure have informed Chandrayaan-3’s improvements, including the use of a second-order guidance system in the rough braking phase and the introduction of instantaneous thrust regulation.
With extensive simulations and revised guidance designs, ISRO aims to achieve a vertical landing even in the face of sensor failures or propulsion system glitches, thanks to a meticulously designed lander.
The Chandrayaan-3 lander can safely touch down at speeds of up to 3 m/sec (10.8 km/h), with the optimal speed being around 2 m/sec (7.2 km/hr). It can tolerate a tilt of up to 12 degrees and still achieve a secure landing.
Upon successful touchdown, the lander will deploy a rover to capture images of the lunar surface and conduct experiments using two onboard instruments.
ISRO is confident that the errors encountered during Chandrayaan-2 have been addressed, ensuring a smoother and more successful mission for Chandrayaan-3.