Monday, September 30, 2019

The Elusive Moon

ISRO’s recent mission to the moon, Chandrayaan-2, aroused interest amongst masses in space science. By itself, this is no mean achievement and probably the biggest contribution of the mission, besides its the scheduled objectives; for, it is rare that in our country, everyone at nook and corner talks about science! This article, though, is neither about reporting on the mission nor about the events that surrounded it but more about sharing my own journey through the couple of weeks that followed early hours of Sep 7, 2019. There are many questions, but fewer answers, at least yet.
When many of us were glued to the TV at around 1:50 AM on Sep 7, and it was obvious that something had gone wrong, the world waited with bated breath to hear the official word from ISRO. All that was told was that the communication with the lander was lost at 2.1 km above the lunar surface. My curiosity needed more information, but unfortunately, none was forthcoming.
That is when I started getting in touch with a few radio astronomers around the world on Twitter. I casually started interacting, specifically, with Dr Cees Bassa, Scott Tilley, Edgar Kaiser and Richard Stephenson. Dr Cees Bassa is based out of Netherlands and is an astronomer at The Netherlands Institute for Radio Astronomy (ASTRON_NL) working with LOFAR, the low frequency radio telescope. Scott Tilley, in his own words, is an amateur astronomer with his own dishes and antennas at home that he uses to track radio signals from deep space. He is based out of Roberts Creek in British Columbia in Canada. Scott Tilley is a citizen scientist, who became famous for an accidental discovery he made of NASA’s lost spacecraft, IMAGE. Edgar Kaiser is based out of Kiel in Germany and has varied interests in Physics, computer modeling, maritime life besides radio astronomy and amateur radio. Richard Stephenson is responsible for the operations at the Canberra Deep Space Network complex and is based out of Canberra in Australia.
For most part, these “conversations” were monologue, for I was the student and I was learning from each of them. Each had a theory based on their own radio measurements on what exactly happened to the Chandrayaan-2 lander and each was convincing. This article is an account of those theories, and a few more believable conspiracy theories. I am capturing below each of those episodes that contributed to my better understanding of radio astronomy.
Episode 1: Dr Cees Bassa had published a plot of the Doppler data from the NASA JPL Horizons of chandrayaan-2 lander vs the line-of-sight velocity of the lander. He also had Doppler data from the Dwingeloo observatory to corroborate those findings. In fact, Dr Bassa has published his Python code for this and a few of independently executed to generate the same plot. A couple others contributed to his original version, to make it more impactful. The Doppler data output plot is as shown in Fig 1. While it may be difficult to read the plot, there are few salient points that stand out.
  1. Orange curve shows the planned orbit of the Chandrayaan-2 lander from the NASA JPL Horizons. That curve shows that the scheduled landing was close to 20:21 UTC (1:51 AM IST). Do recall that ISRO had always mentioned that the scheduled landing was at 20:23 UTC.
  2. The black curve shows the approximate Doppler data of the lander from the Dwingeloo radio telescope. This curve closely matches the orange curve in the rough braking phase when everything was going on fine.
  3. As can be seen from the figure, things started going wrong 15 seconds into the fine braking phase.
  4. The black curve suddenly stops around 20:20 UTC (1:50 AM IST) indicating loss of signal. From what was known till now, this was supposed to have coincided with the ISRO proclamation that the contact was lost at 2.1km above the lunar surface.

Figure 1: Dr Cees Bassa’s Doppler predictions
  1. If the orange curve coincided with the black curve (during rough braking phase it did), then that Doppler data is authentic. What is also authentic is that the orange curve coincided with the times of rough braking phase. With these two in mind, one can conclude that the landing time was around 20:21 UTC (as per orange curve) rather than 20:23 UTC as suggested by ISRO.
The above points only pointed to one plausible explanation that the contact was not lost at 2.1 kms above lunar surface but upon impact on the lunar surface. That meant it was a hard landing and it was logical to conclude the loss of communication was due to equipment damage from the hard landing.
Episode 2: ISRO announced on Sep 10, 2019 that the Chandrayaan-2 orbiter had taken a thermal image of the area near the proposed landing site of the lander and that the thermal image proved that the lander was lying on the lunar surface. Many unfounded, unaccounted statements quoting sources within ISRO started doing rounds and one of the suggestions was that the lander was lying on side (not on its 4 legs) and that scientists could see the elongated shadow of its two legs! ISRO unfortunately, never release this thermal image – so all speculations and conclusions are within the realm of fantasy.
Episode 3: There was another statement attributed to ISRO during these times that suggested that to take a better image of the lander, ISRO was considering lowering the altitude of the Chandrayaan-2 orbiter from 100 kms to 50 kms! Thankfully, that was never attempted. For, the orbiter was already conducting scientific data as per schedule and there was no point in putting that part of the mission in jeopardy just to take a better picture of the lander, which was by now, presumed crashed. Scott Tilley and Edgar Kaiser would tune their radio antennas towards the moon and start collecting Doppler data of the orbiter over 24 days duration. For days together, each day, they would independently measure and report the altitude of the Chandrayaan-2 orbiter to be around 101-105 kms. That was a reassurance that the orbit wasn’t lowered, and they would put in more hours next day to reassure themselves and res of us that orbiter wasn’t touched. During these measurements, both would share their assumptions and publish the data and would adjust their setup and run other controlled experiments – each time verifying that the dreaded decision wasn’t implemented. In the process, I learnt many news things about practical radio astronomy. Now, I know from where to source the Doppler data, how to analyze it and how to understand Azimuth and altitude of orbiter, radio telescope etc.
Episode 4: In the meanwhile, Richard Stephenson would help us understand plans of the Deep Space Network (DSN). The DSN is NASA’s international array of giant radio telescopes that support interplanetary spacecraft missions. The DSN also provides radar and radio astronomy observations. The DSN is operated by NASA JPL and consists of three facilities spaced equidistant from each other – approximately 120 degrees apart in longitude around the world. These sites are Goldstone in California, near Madrid in Spain and near Canberra in Australia. Stephenson works with the Canberra observatory. The strategic placement of these sites permits constant communication with spacecraft as our planet rotates – before a distant spacecraft sinks below the horizon at one DSN site, another site can pick up the signal and carry on communicating. Richard would tell us when one of the giant antennas at one of the three sites would start sending a probe signal towards the moon – in hope of reviving the communication onboard Chandrayaan-2 lander. These were powerful signals (approx. 11KW power) that were transmitted towards the moon and round-trip time of these signals is about 2.5 seconds after hitting the surface of the moon. If the lander had received these signals and was able to activate its communication, it would have. One of the 4 antennas at each of the 3 sites would always transmit these “Hello” signals to the lander each day round the clock! Sadly, no response. Richard Stephenson summed up thus: “If at the end of the 14-day recovery window, nothing is heard. You have to accept that @isro has attempted everything humanly possible from earth to recover their Lander.”
Episode 5: Finally, the word came that NASA’s LRO (Lunar Reconnaissance Orbiter) which also orbits the moon at about 100kms above the surface would be passing by the landing site of the lander and would try to take picture. The time wasn’t on our side. The LRO was to have crossed the area on Sep 19, which was close to the window when lunar day would be over, and 14 earth-days of lunar night would set in. As it turned out, on the day of the fly-over, the long shadows had started creeping in and LRO couldn’t take a picture of Vikram lander. I created this image (Fig 2) which was appreciated by many on Twitter which showed the lunar shadows and suspected landing site of the lander.

Figure 2: Blue dot is where the lander is and LRO was in the shadow region on Sep 19
Episode 6: There are many other theories doing the rounds. Some of them are preposterous, some are very believable. Here is a summary.
  1. It was clear from Dr Bassa’s chart, that 15 seconds into fine braking phase, something had gone wrong. That time, the lander was still nowhere near the lunar surface. So, what was it? Was a command wrongly issued? Was the config data wrongly fed in? Wasn’t the testing fully done? Were some failed tests waived? Worse still, was there a bug in the code? At this time, till ISRO answers, any of this is possible.
  2. The 4-legs of the lander were supposed to be horizontally aligned during rough braking phase and were to be turned by 90 degrees to orient them towards the lunar surface during fine braking phase. Is it possible that instead of -90, a +90 was entered that caused the lander to flip upside down? Then the automatic lander program (ALP) spent its time trying to recover to a stable configuration – but lost precious time and crashed. Again? Possible.
  3. If indeed the orientation was wrongly given, then its possible that the thrusters meant to put brakes to the lander accelerated it further. This explains change in line of sight velocity around 15 seconds into the fine braking phase.
In conclusion, this is not our domain. ISRO has access to all the data and will make their own investigations and hopefully publish the findings. The moon has remained elusive. Not just to Chandrayaan-2 lander on Sep 7, but to Israel’s Beresheet on Apr 11 as well. It is quite ironical that despite 40 years after Neil Armstrong set foot on the lunar surface, it is not easy to land there.
But for me, personally, these 2 weeks of learning was almost real-time and it has got me sufficiently excited to a stage where Scott Tilley has shown interest to help me design dishes and antennas that I can put up on my home terrace to start my own radio astronomy in future. In words of Scott and Edgar, they don’t have someone in this part of the world who regularly collects this data – so it could be me! All I was told that I needede are 1m dishes for S- and X-band, cross yagis for VHF and UHF, an LPDA for L- and S-band and an LPDA for VHF/UHF. Moon and other celestial folks, here I come 😊