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From what I hear about the Chandrayaan-2 failure analysis, the spacecraft overshot its landing ellipse and, while struggling to backtrack, crashed. I have been wondering why they had specified a specific patch for landing. I found a couple of articles about site selection, this seems to be the best one. Based on engineering and science constraints for a south pole landing, they picked a landing rectangle 32 x 11 km. On the other hand, the Hindustan Times wrote "The Chandrayaan-2 lander was to land within a 500mx500m area. That meant the lander had no flexibility . In the case of Chandrayaan-3 the landing area has been increased to 4km x 2.5 km." 500x500 m seems awfully tight, even 4x2.5 km. Do you have any insight into why they picked such tight landing specs?

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While I don't know why they chose such tight landing target but we know that while the landing box for Chandrayaan-2 Vikram lander was 500×500 meters the exact spot they were aiming for was even tighter at almost 60×60 meters! Tracing this location helped locate Vikram's debris eventually.

The source of that Hindustan Times report is this lecture and this interview by ISRO Chairman S Somanath.

Chandrayaan-2 lander didn't overshoot the target it just fell short by about 500 meters.

One reason to have such precise landing target could be that they eventually want to land at polar regions where near constant solar illumination is available. And indeed they have a future mission called LUPEX in collaboration with JAXA that will target such landing site and ISRO is supposed to provide lander for it.

Here are few papers on rationale behind Chandrayaan-3 landing site.

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  • $\begingroup$ It did overshoot, sort of. The problem started when the fuel valve stayed open wider than it was supposed to right before the probe entered its "camera coast" phase where it couldn't make any changes to its orientation and throttle. When it exited that phase, it was too high (overshooting) and rotated into a head-down orientation to try to get back to its landing track, which it then couldn't recover from because of the rotation rate limits they'd imposed. It did end up short, but it got there because it was initially overshooting. $\endgroup$ Aug 14, 2023 at 21:08
  • $\begingroup$ @DarthPseudonym During second phase of descent, the reduction in velocity was more than planned due to higher thrust from engines. Being high doesn't mean it was overshooting the target. $\endgroup$
    – Ohsin
    Aug 15, 2023 at 14:28
  • $\begingroup$ I may be mistaken but my understanding of the situation was that the position they were in during the camera coast was reducing their vertical velocity more than their horizontal, which due to the overperformance was making the target point push out long. $\endgroup$ Aug 15, 2023 at 14:33

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