Chandrayaan-3 landing on the moon is scheduled to take place 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 the failure of a valve to reduce the flow of fuel into the engines. As more than ordered fuel went into the engines, higher thrust was generated which disturbed the orientation of the lander; a software glitch failing to correct it.

Essentially, during the landing, there are three phases—rough breaking, camera coasting and fine-breaking.

The story begins when the lander (Vikram) starts the rough breaking phase, at 30 km above the moon’s surface. It was 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 altitude, 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, telling the lander where to land.

At this point, the thrusters were supposed to slow down, but it didn’t; It was found that the valve that was supposed to reduce the flow of fuel into the engines had malfunctioned. The lander began to tilt, and the camera also 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 just not enough time to get the craft back in the right position to land on the small-ish landing area. So, the lander crashed 459 metres away from where it was supposed to land.

With Chandrayaan-3, ISRO has taken corrective measures, mainly on the thrust and the speed of rotation of the lander. According to ISRO, the “engine thrust controller and throttle value slewing rate has been reduced” and “instantaneous thrust regulation scheme has been implemented for the entire powered descent phase.”

Tolerance limit for deviation in thrust 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 velocimetre” has been brought in. The LDV measures the rate of flow of liquid (fuel) to an engine. The rate of rotation of the lander, (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.