Chandrayaan 2: What could have gone wrong with the Vikram lander?

These include use of five engines in the lander rather than one, change in satellite carrier from GSLV MK II to heavier carrier GSLV Mk III, and finally the 30 km distance from where the final descent began.

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Let us look at the final descent first.

Close to 50 days after the launch of Chandryaan 2, the lander and rover composite reached an orbital distance of 30 km from the Moon's south pole, when final descent to soft landing began.

The soft landing follows four phases. From 30 km, the lander will reach 7.4 km and then to 5 km. From the distance of 5 km, it will reach the 400 metres from where it will soft-land on the lunar south pole. Sivan had described it as the terrifying 15 minutes.

However, ISRO lost connection with the lander at 2.1 km and was unable to make the connection till the end. The source explained that this 30 km distance could itself have been the cause of landing failure.

"Clearly, the landing programme failed. One of the challenges the source described was lack of study of the Moon's gravitational pull, especially when it is the very first time a spacecraft is reaching the Southern Pole," the source said.

Considering the last 15 minutes is completely autonomous, orbital measurements should take into account the Moon’s gravitational pull for the landing to be precise.

"If you take NASA, it had a lot of failures before they soft-landed on the moon. ISRO should have run programmes to measure gravitational pull," he added.

For the same reason, in Vikram lander's case distance of 30 km was not the right orbit measurement. "In fact, probability of errors is lesser larger the distance," the source added.

For instance, if you are falling from the larger distance, there is more room for manoeuvres that the short distance will not provide given that Moon’s gravity is only 1/6th of the earth's gravitational pull.

So, for a mission of this stature, larger distance translates to lesser errors.

"Falling from 100 km distance will have fewer errors and 200 km will have even lesser errors," he said.

Engines

Another issue was the number of engines. According to the source and other reports, having a single-engine would have ensured more chances of success.

Consider this. One of your legs is a little weaker than the other. So when you run, you unconsciously put the weight on the stronger one making you unstable and might result in you stumbling.

Consider the same situation with the lander. Vikram lander had five engines, four at the corner and one at the centre. In case one or two of the four engines did not operate as expected or operating at a different velocity, it creates an imbalance. Without the balance, the weight is not evenly distributed resulting in toppling.

"This could have been what happened to the lander. This will not happen with a single-engine at the centre. With accelerator on board, you can adjust the speed better," explained the source.

What are the other factors?

Some of the reports have mentioned the change from the original plan when it comes to satellite carrier. The carrier was changed from GSLV Mk II to heavier GSLV Mk III at a later stage. There have also been reports of not enough simulation being done, taking into account every possible situation that the spacecraft could run into.

The source added that many of the key scientists, who were part of Chandrayaan-1, were sidelined.