(The Gist of Science Reporter) Why Vikram fell Silent?

[NOVEMBER-2019]

Why Vikram fell Silent?

Data showed that after 11 minutes and 28 seconds, the descent vertical velocity of Vikram was 42.9 metres per second. A minute and a half later, the speed dramatically increased to 58.9 metres per second. At that time Vikram had horizontal velocity of 48.1 m/sec and it was around 1.09 km from its designated landing spot on the Moon.
As per the plan, Vikram should have lost most of its velocity by the time it reached 400 metres altitude and thereafter it should have been hovering above the intended landing site, set to make a soft vertical lending. The frozen screens at mission control showed that communication was lost when the lander was barely 335 meters above the surface of the Moon. The green dot representing Vikram started deviating when its altitude was just above 2 km, and continued to deviate before stopping.
Analysis of data shows that touchdown of Vikram on the Moon occurred at a much higher velocity than intended. The data suggests a deviation during the Fine Braking Phase. It is not the purpose here to undertake any detailed failure assessment.

However, based on the assessment of the limited data available so far, the following possible reasons can be arrived for Vikram’s silence:

Deficiency in the throttling of Vikram’s propulsion system. After rough braking, the orientation of Vikram was to be changed from horizontal to vertical. ISRO has located Vikram about 500 meters away from the designated landing spot. The deviation in Vikram’s descent trajectory (green line in Figure 3) may be due to over performance/glitch associated with imbalance in the thrust of engine triggering rotation and affecting the stability of Vikram.
Vikram on-board computer suffered communications data overload. As the Vikram descends toward the moon at a very high speed, its computer rapidly processes large streams of data simultaneously. On-board computer has to be autonomous as a large number of real-time commands from multiple systems have to be executed in milliseconds. If there is time lag, i.e., there is no sufficient time for executing the commands, the data loss may result in malfunction.
Inadequacy in the design of the autonomous system. Broadly, it could be inferred that the system was not adequately designed for the foreseen faults to take corrective measures. The strength of autonomy of any system would depend on the basic design factors. The design would be mainly based on the anticipated anomalies. When an autonomous system operates in complex and open-ended environments, it is difficult, but it has to be designed ruggedly and tested to identify the unforeseen changes in time and respond accordingly.
Sensors on-board Vikram could not provide the precise measurement on the acceleration, orientation, and trajectory of the Vikram during landing. The problem could be associated with either the measure accuracy, component failure or malfunction or working on the data from different sensors in tandem.
Power systems and/or electronic component(s) malfunction. It may be the case of power failure as observed in some of our earlier missions, like Chandrayaan-1, INSAT-3D and more recently GSAT-6A. The power failure may be triggered due to degradation of electronic components due to exposure to excessive Sun radiation or otherwise.These are some of the most probable reasons. ISRO is already conducting the detailed failure analysis. It is always important to learn lessons from failures.

Current Status:

The Chandrayaan-2 Orbiter is currently placed in a 100 km polar lunar orbit. All the eight payloads of Orbiter are functioning normally, providing valuable scientific data of the Moon’s surface by remote sensing.
The Orbiter was originally designed for a mission life of one year. However, due to injection of spacecraft into a better orbit by GSLV MkIII-M1 (higher apogee by 5075 km) and thereafter fuel saving by optimization of mission manoeuvring towards its journey to the Moon, its life has been extended.
The orbiter at launch had 1697 kg of fuel after insertion to the designated Moon orbit, the balance fuel is around 500 kg, which is sufficient for a 7.5-year life. The orbiter module of the mission with eight scientific instruments remains operational and will continue remote sensing observations of the Moon’s surface.