At 1.38 a.m. on September 7, Vikram, recently separated from the orbiting Chandrayaan-2 parent craft, began its much awaited descent to the moon as planned. It was to reach the lunar surface 30 km below in 15 minutes.
The speed (“velocity” is the official word) of 6,048 metres per second was being gradually reduced by its five throttleable engines.
There was applause in the ISTRAC control centre in Bengaluru as everything went as planned. First there was a “rough braking” period of 23 km lasting 10 minutes, when all engines were on. This phase was smooth until around 1.48 a.m. and the mission control engineers clapped. It was just 7 km more to the moon.
Then began the next stage of “fine braking”, when only the central engine was on. The speed had fallen to 86 metres per second.
By 1.51 a.m., Vikram, gliding in a parabolic path, was 2.1 km from moon’s surface, slowing down to about 50 metres per second.
Suddenly there were bated breaths all around. The tense faces and body language indicated that something was wrong even as the big screen in the media centre went blank.
Some 15 minutes later, at 2.15 a.m., ISRO Chairman K. Sivan declared that they had lost contact with the lander when it was just 2.1 km from its landing spot, during the fine-braking phase.
It is surmised that the lander, prima facie , got physically disoriented. It deviated from its path as it lost communication with the control rooms and deep space antennas of ISRO in Bengaluru and of NASA in California and Canberra.
Was it because of the newly used throttleable thrusters? That is the answer ISRO is piecing together from the last numbers and data on the lander’s final journey collated by all the tracking teams.