Fate of VIKRAM lander ; Chandrayaan II

A grab of display panel of Vikram landing @ Bylalu, ISTRAC ; Courtesy: DD

It is just around 60-hrs that the contact of Vikram lander had lost. The support, ISRO got starting from the Prime minister and other ministers is commendable; not to forget the foreign media, NASA and of course millions of Indians who kept themselves awake post mid night and were not able to sleep after the episode. A poem went on to pour the feelings of Vikram mentioning that how hard it was as it had to face the aspirations of 1.2 billion people. While ISRO has been working very hard to get to the bottom of the facts, a 1st statement from Chairman ISRO, was that they have identified the VIKRAM by IR-imager. There are dozens of anonymous opinions by ISRO scientists/engineers coming out to the public domain by media; few 10s by experts all around the globe and innumerable by every other soul; explaining if it had not happened this way, then could it not have happened the other way.

I was the team leader of CHACE payload in Moon Impact Probe of Chandrayaan I; spent 4+ years in VSSC-ISRO and have gone through the grill/drill of ISRO in transforming a commercial instrument into a smart, most sophisticated, highly rugged which sucks a very small amount of power from the main power bus with every nut and washer which has been used was argued quite a few times in its selection of weight, dimension. Having left ISRO post Chandrayaan-I, right now I head the central instruments facility here at the University of Hyderabad. The following discussion is purely based on what has been floating around in the media along with my own personal comments and has nothing to do with ISRO. As I have been doing; I will treat this to be a highly popular level; without compromising the value of message I intend to put across.

Vikram lander showing 4-engines firing; Courtesy : Digit

Vikram took a "power descend" from its intended 30X0 km orbit; basically starting from Perigee point of its earlier orbit and heading towards the intended 71-degree southwards journey. Power descend meaning, Vikram is using its forward orbital velocity of around 1.66 km/s (a whopping ~ 6,000 kmph) for its southward journey while the 4 - (throttelable) retro-engines will be keep on firing against the orbital velocity and hence the net effect of "breaking". As per the reports appearing in the media [WITHOUT ANY AUTHENTICATION]... these were major milestones along its 15-minutes of terror covering 30-km journey in a parabolic trajectory.

1. De-orbit operation : 30-18 km

2. Rough breaking : 18-7.4 km

3. Fine breaking : 7.4km - 400 m  [ Activate Central Retro-engine; Shut off 4-Engines]

4. Hover for 12-s : @ 400-m : [Activate Hazard Detection & Avoidance (HDA)]

5. Hover for 25-s : @ 100-m : Select the target A or B

6. Prepares for touch down: 10m-2m in 13-s

7. Shut down engine : @ 2-m

Free fall from 2-m: [Four wheels are equipped to take the shock of this order]

As the nation and entire world watched; Vikram's path veered during soft breaking [as can been seen on the display panel above] and at 2.1-km altitude the green signal on the display panel disappeared; the gloom prevailed all over the faces was very evident and after span of 20-minutes or so, Chairman, ISRO declared that the data link to Vikram has been lost. It was not only the ISTRAC control center of ISRO at Bylalu, outside Bangalore which dipped in ocean of sorrow, every other Indian who was waiting for that successful moment were completely devastated. On my personal behalf, I kept on reviving the possible scenarios which were possible; also got involved in discussions in my 3-4 Whattsapp groups; in a way taking away the gloom prevailing around. Waited for the following day to head to the TV discussion.

The following day; 2-leading news papers published completely opposite views (quoting anonymous ISRO scientists) on the probable causes of the Vikram incident. While one said that it is the over breaking during the soft landing which could have contributed to skidding effect [aka car] on the Vikram. While the other attributes it to under performance of retro-engines. Many other experts gave their views as widespread as possible.

By noon: ANI news broke the story for the first time, quoting chairman, ISRO that " Vikram has been spotted by IR imager of the Orbiter; later to CNN News18 that the health of the Vikram can not be confirmed."

On the 2nd day of incident; many more stories [speculative/anonymous] appeared....
1. Vikram might have crash landed [either on its side or upside down]
2. Vikram is in one-piece [suggesting no greater damage]
3. Orbiter ORHC camera can detect the damage
4. ISRO would persist contacting Vikram for all the 14-days
5. The chances of revival would fade faster as the time passes by


POSSIBILITIES:

Let's be clear that; when the contact was lost; Vikram, at 2.1-km above was guzzling with the speed of around 200-kmph. Again, as per the newspaper report; the soft breaking operation would involve switching from 4-corner based engines to a single central engine.

Scene-1 [status-1]: Could only be a tumble, due to malfunction of throttelable engine; or the 4-engines. However, the close loop guidance system [CLGS] working properly running through the remaining milestones of 4 - 7 as explained above. Vikram has survived the incident and has landed safely. Also supported by one of the reports that Vikram is in one piece

Scene-2 [status:0] : Lets face it, the 200-kmph is enormous speed. Signal loss means the Vikram has tumbled; so much so that it has not allowed the close loop guidance system [CLGS]  to tackle the situation. Let me just remind you all that CLGS will work only when there are aroun 20+ sensors feeding the input to the computer and then on the decisions were to be followed. Which means, a meager failure of one retro-engine, gyroscope could lead to failure in breaking system. Hence the CLGS not at all getting a chance to run thru the remaining milestones of 4 to 7.

Scene-3: Neither 1 nor 0: There is a very finite possibility that a combination both the above scenerios could lead to system surviving but with a BIG question mark... will it perform the oprations of letting the Pragyan slide out.

My take:
Since communication link snapped before event; believing that the Artificial Intelligence has done its job to its best trained scenario; finding the hardware in one piece (unconfirmed report)....
SCENE-3 APPEARS TO BE MOST PROBABLE....

Lets hope best out of the above three cases and we get to see some ray of hope... Tomorrow...

ALL THE BEST ISRO...... you have done all the humanly possible actions, to quote chairman ISRO.