The Chandrayaan-2 mission failed primarily because of three key reasons related to the Vikram lander's descent and landing phase:
- The five engines of the Vikram lander, which were used to reduce its velocity (retardation), generated higher thrust than expected. This caused accumulated errors during the descent.
- The lander's software was limited in its ability to detect and correct these accumulated errors and cope with the unexpectedly high rates of rotation during the descent. The software's constraints on the lander's turning rate led to further loss of control.
- The landing site was very small (500m x 500m), which limited the lander's ability to adjust its trajectory and land safely. As the lander tried to reach the small targeted landing area while descending too fast, it lost control and crashed.
Additional factors included:
- A software glitch caused navigation algorithm errors during the descent.
- The lander's engines' thrust control algorithm applied corrections too late, allowing large navigation errors to accumulate.
- A rigid requirement to land within the small planned site despite non-nominal flight status limited corrective options.
- The lander ended up increasing its horizontal velocity to reach the target, resulting in a hard landing.
Overall, it was a combination of propulsion, software, and landing site constraints that led to the crash landing of Vikram on the lunar surface in September 2019. ISRO learned from these issues and made significant corrections in the follow-up Chandrayaan-3 mission, which successfully landed later.
