Chandrayaan-3 landing on the moon is scheduled between 5:30 pm and 8 pm on Wednesday. One can bet on success because the reasons for the last-minute failure of Chandrayaan-2 have been pinpointed and corrective steps taken. 

Chandrayaan-2 failed to soft-land because of a chain of events triggered by a valve’s failure to reduce flow into the engines. As more than ordered fuel went to the engines, a higher thrust was generated, which disturbed the attitude (orientation) of the lander, a software glitch failing to correct it. The slow rate at which the thruster was designed to turn around itself and the not-so-big landing area. 

During the landing, there are three phases—rough breaking, camera coasting and fine breaking phases. 

The story begins when the lander (Vikram) begins the rough breaking phase, at 30 km above the moon’s surface, above a point 574 km away from the chosen landing site. From here, the lander was supposed to move on an inclined trajectory, moving sideways as well as losing height, up to a point about 400 meters above the landing area, from where it would descend vertically. 

After a smooth, 10-minute ‘rough breaking phase’ it entered the ‘camera coasting phase’, when the camera would be pointing to the landing area, basically telling the lander where to land. 

Now the thrusters were supposed to slow down, didn’t--apparently because the valve that was supposed to reduce fuel flow into the engines malfunctioned. The lander began to tilt, meaning the camera began to point elsewhere. As the navigation system tried to correct the tilt, the lander somersaulted, rotating 410 degrees. This meant that at one point, the thrusters were pointing towards the sky, pushing the craft fast downwards. The rotation was too slow, as the lander was designed to rotate not more than 10 degrees a second. There was insufficient time to get the craft back in the correct position to land on the smallish landing area. The lander crashed 459 meters away from where it was supposed to land. 


ISRO has taken corrective measures, mainly on the thrust and the speed of rotation of the lander. According to ISRO, the “engine thrust controller throttle value slewing rate has been reduced”, and the “instantaneous thrust regulation scheme has been implemented for the entire powered descent phase.”  

The tolerance limit for deviation in thrust in each of the four individuals has been increased (to 48N). This means that if an engine develops higher or lower thrust within the limit, it won’t matter. 

An indigenously developed “laser doppler velocimeter” has been brought in. The LDV measures the rate of flow of liquid (fuel) to an engine. 

The lander’s rotate rate (when necessary) has been enhanced to 25 degrees a second (10 degrees in Ch-2). This means the craft can turn around itself quicker. 

ISRO also speaks of “multiple mission paths to handle contingencies” and “enhanced touchdown limits for soft landing”—meaning that if the lander deviates from its trajectory, it can still soft-land somewhere rather than crash.