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On September 7th 2019, ISRO attempted to land a lunar lander ,Vikram, with rover , Pragyan, on the moon. It was very exciting to watch until the lander lost communication with ground station during its final descent, just 2.1 kms above the lunar surface!

I have screenshot the profile of the landing trajectory from the live stream and it is clear that it went off its profile at the very end.

enter image description here

I wanted to know what went wrong for such a failure to occur, and if it's not known for sure, what the most likely scenario is.

My rough guess is that something went wrong with the attitude control system of the lander. Maybe the lander had some level of autonomy in choosing a landing ground; as it landed, the sensors identified the landing area not suitable for landing and it decided to change its route leading to high momentum build up that was hard to recover from. Or it was just a matter of some external disturbance that was not accounted for.

The latest news that has come out 2 days ago about the investigation of what happened but nothing solid has been stated. Link: https://www.hindustantimes.com/india-news/keep-it-simple-former-isro-scientist-explains-what-lead-to-chandrayaan-2-s-vikram-crash/story-M6DDnfOFJl3OkZ12F2fJIM.html

I think this would be a great case study to learn lessons from.

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    $\begingroup$ According to this article it's an issue with the braking thrusters. Beyond that, you can tell that it was over-firing (not under-firing because the braking was more rapid than expected) which could be caused by many different issues from a stuck throttle to an invalid calculation. As of right now I'm not sure they've released data (or even have enough for public release) if it impacted at full speed but I'm checking for more. $\endgroup$ – Magic Octopus Urn Jan 4 at 2:39
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    $\begingroup$ There's one vote to close as "primarily opinion-based" which seems unnecessary. I'll just delete the word "might". $\endgroup$ – uhoh Jan 4 at 11:13
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    $\begingroup$ Until more information does come out, which eventually will happen, we without adequate information are just guessing. So a vote to close as opinion-based is entirely appropriate at this point in time. On the other hand, there might be some who post at this site who have inside information that has not yet been divulged. For those few, a vote to close as "inviting one to commit a crime" would be a valid reason to close. Since we don't have that close option, a vote to close as opinion-based still works.$$\ $$I'm voting to close, and no, I don't have insider information. $\endgroup$ – David Hammen Jan 4 at 13:19
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    $\begingroup$ @DavidHammen The questioner just asks for facts, not opinions, in this edited form. He is not to blame if you are just guessing, that 's your responsibility ! And it's the responsibility for those who want to give an answer that's opinion-based. $\endgroup$ – Cornelisinspace Jan 4 at 15:15
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    $\begingroup$ @DavidHammen if you think this site should start closing questions that you feel may not be answerable any time soon, you can leave an answer to What's best to do about questions that might not be answerable for a few months?. But I think that unless you're currently at ISRO and working on the mission, you can't really be completely, 100% sure that nobody in the world can post an answer to this question. Closing prevents others from answering, so I think there needs to be a high bar, otherwise it's a kind-of a self-fulfilling prophesy. $\endgroup$ – uhoh Jan 4 at 15:33
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Now that's it's been a few months... according to Lok Sabha (a representative of the Indian Department of Space):

During the second phase of descent, the reduction in velocity was more than the designated value. Due to this deviation, the initial conditions at the start of the fine braking phase were beyond the designated parameters. As a result, Vikram hard landed withing 500 m of the designated landing site.

So it seems that the problem was something unexpected with the landing thrusters. I wish I could be more specific, but ISRO hasn't been any more specific (at least, that I know of in the English web...)

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  • $\begingroup$ I don’t think it’s clear that the thrusters were the problem; a software error or an altitude measurement error could have caused the excess reduction in velocity as easily. $\endgroup$ – Russell Borogove Jan 31 at 4:30

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