FEATURE ARTICLE Chandrayaan-2 – What Went Wrong With The Lander? Anand Kumar Sharma
Had Vikram landed softly on the surface of the Moon it would have been the first soft landing on the South Pole of the Moon. What went wrong? Here’s an insight from someone who has been associated with the project for long.
missions with success rate of 65.6%, Chandrayaan-2. Composite spacecraft Orbiter and Vikram in stacked configuration (Image credit: ISRO) the success rate of landing missions on Moon is very low (47.8 %); out of 46 missions undertaken so far only 22 were successful. Chandrayaan-2 has been described as the most complex mission ever undertaken by the Indian Space Research Organisation (ISRO). This is because the Chandrayann-2 Vikram had to land on the South Pole and Pragyan had to carry out in-situ advanced scientific experiments on the lunar soil. Spacecraft descending toward the lunar surface face a number of challenges. There are high levels of radiation to contend with, and as a spacecraft begins its descent, solar reflections can saturate the instruments and backscattered communication signals may interfere with the primary communication system. But the last few meters of the descent can be especially challenging. As the spacecraft gets closer to the surface, it kicks up dust, FTER the launch of proceeding as planned until just 2.1 km which can jam the navigational sensors. Chandrayaan-2 on 22 July above the surface of the Moon when Chandrayaan-2 was aiming at soft- A 2019 followed by successful telemetry was lost. Vikram had most landing Vikram lander on the South Lunar Orbit Insertion and separation likely hard crashed on the Moon. Pole of the Moon, which would have of Vikram lander from the Orbiter, been a first for any country. But there Why was Chandrayaan-2 mission ISRO was waiting for soft landing of are challenges in landing on the Moon’s so challenging? Unlike orbiter-only Vikram on 7 September 2019. All was South Pole. All the earlier missions
20 | Science Reporter | November 2019 Figure 1. Phases of Chandrayaan-2 craft farther away from the Earth and mission manoeuvring closer to the Moon. These burns took place in over 23 days, with the final burn serving as the Trans-Lunar Injection (TLI) manoeuvre on 13 August 2019. On 20 August 2019, the Orbiter’s engine was fired again (first lunar bound manoeuvre) to conduct the Lunar Orbit Insertion (LOI) and place the Chandrayaan-2 composite module in an initial lunar parking orbit of 114 x 18,072 km. After four more lunar bound manoeuvres (Selenocentric phase) spread over 13 days, on 1 September 2019 the composite craft reached the Moon’s orbit of 119 x 127 km. Next have opted for landing closer to the System. Ice found inside these craters day on 2 September 2019 Vikram was equator because it takes less energy and within the lunar soil could provide a successfully separated from the Orbiter. (less propulsion). The trajectory to reach vital source of water as humans venture the Moon’s equator is much simpler back to the lunar surface. Subsequently, two de-orbit burns requiring a few simple manoeuvres. This region is also considered were executed by using Vikram’s main Further, the South Pole region has very difficult to explore as it is home liquid engine during 3-4 September craters that are cold traps (largely to some harsh conditions, including 2019 to achieve the 101 x 30 km orbit. remain shadowed), hence, it is tougher temperatures as low as -233°C. to establish communication. Chandrayaan-2 Vikram was to soft Time Event The lunar South Pole was of special land in a high plain between two craters, (IST) interest as the Chandrayaan-1 found Manzinus-C and Simpelius-N, at a 01:38 Rough braking signatures of water ice deposits in the latitude of about 70° south. Starts (30 km) craters of the Moon’s poles. The South Pole areas of the Moon that remain in Lunar Injection, Vikram Separation 01:48 Fine braking Starts shadow are much larger than that the & De-orbiting (7.4 km) North Pole. There is a possibility of the After the successful launch, ISRO 01:50 Local Navigation presence of icy walls in the permanently had performed 6-apogee raising burns Starts shadowed areas. These cold trap craters (Geocentric phase) with the Orbiter’s in the South Pole region may also 01:52 First Image of main engine to propel the composite Moon Surface sent contain a fossil record of the early Solar to Earth 01:53 Vikram Touch Down 03:53 Pragyan Ramp Deployment 04:23 Pragyan ON
05:03 Pragyan Solar Panel Deployment 05:19 Pragyan Roll Down
05:29 Pragyan Touch Down 05:45 Vikram Imaging
Table 1. Chandrayaan-2 Vikram & Pragyan Timeline (07 Sept-2019) Landing and Rover Roll Down Events Figure 2. Chandrayaan-2 Vikram soft landing sequence (image credit: ISRO)
November 2019 | Science Reporter | 21 Figure 3. Vikram Descent Trajectory (green showing deviation from the intended path)
Just a few seconds before landing, hovering – the most complex phase–starts. At this stage the camera on-board Vikram takes a photograph of the lunar surface and matches it with the already stored images of landing site that were captured earlier by the high-resolution camera (OHRC) of the orbiter. This had to be carried out to ensure the safe landing of Vikram to the predefined place. After confirming this Vikram descends slowly by reducing vertical thrust, the central engine is switched on and the two diagonal engines are switched off. It was apprehended that the two corner Vikram Landing of precise measurement of Vikram’s thrusters, if active would blow the moon velocity and height from on-board Critical landing manoeuvring of Vikram dust and it would create a centre jet instruments, including cameras was scheduled from 1:38 IST on 7 upwards, covering the lander with dust. photographing the lunar terrain. September 2019. Vikram was to softly Hence, the central engine was added to Variations in the velocity, altitude or touch down about 350 km away from reduce this upward jet. inclination of the spacecraft were to be the South Pole – Aitken Basin rim on corrected by the autonomous control At 4 m height the thrust is cut off, the Moon at around 1:53 IST. The first systems, which arrive at their own Vikram is at free fall to the impact point 15 minutes of operation are very crucial logical decisions on the adjustments that with touchdown velocity less than 2 m/ for the mission. need to be made. sec. Vikram was horizontal at 30 km Through the rough braking burn altitude at a speed of 1681 m/sec. of Vikram carried out at the perilune While landing on the lunar surface, it The Heartbreak (nearest point to Moon) to reduce the should have been at maximum vertical horizontal velocity (horizontal braking) All operations at ISTRAC (ISRO’s touchdown velocity of 2 m/sec. It the horizontal velocity was to be reduced Telemetry, Tracking and Command follows a parabolic curved track during from 1681 m/sec (6052 km/hr) to146 m/ Network) were going as planned. Rough this operation. Everyone inside Mission sec (526 km/ hour). Vikram’s altitude braking completed with applause. Fine Operations Complex at Bengaluru was was to be lowered from 30 km to 7.4 braking started amidst clapping. There highly optimistic towards the mission km. The four corner liquid engines were was jubilation all over. Then all of success. ISRO scientists were glued to be fired for this operation and the a sudden the computer screens went to the computer screens for updates. central engine had to be switched off. blank, the communication with Vikram Vikram began its descent at 1:38 IST. Next, the orientation of Vikram had to was lost. There was pindrop silence, There were two phases of lunar be changed from horizontal to vertical. disappointment and despair on the faces descent; rough braking and fine braking This operation completes in 620 seconds of ISRO scientists. & hovering. The entire landing operation (10.33 minutes). While going through the data was planned to be accomplished within In fine braking, Vikram vertically (Figure 3), one can see that Vikram 15 minutes. Vikram has five 800 N liquid descends from 7.4 km to 100 metres and started deviating from its intended engines in clustered configuration for its velocity is gradually reduced to less trajectory during the Fine Braking phase braking and hovering – four engines at than 2 m/sec. Fine braking starts with at ~2.1 kilometres altitude, and had the corners (one at each corner) and one simultaneously switching on the two lost communication during its attempt to at the centre. diagonal liquid engines and switching land on the Moon. After a few minutes, The descent and soft-landing were off the other two. At 100-meter altitude, ISRO officially issued a statement of this fully autonomous mission operations, Vikram is three axes stabilized with deviation of telemetry loss. without manual intervention from free floating. The Moon’s gravity is the ground. These operations were compensated with upward thrust by carried out by a series of time tagged firing the two diagonal liquid engines. Why Vikram Fell Silent? commands, loaded in the Vikram’s on- The horizontal movement (sidewise Data showed that after 11 minutes and board computer, a few hours before move) of Vikram is now controlled by 28 seconds, the descent vertical velocity from the ground. These commands firing the small 50 N thrusters. At this of Vikram was 42.9 metres per second. were generated based on the checking phase, local navigation takes over. A minute and a half later, the speed
22 | Science Reporter | November 2019 dramatically increased to 58.9 metres the resultant vertical force vector 5. Power systems and/or electronic per second. At that time Vikram had will not pass through the centre of component(s) malfunction. It may horizontal velocity of 48.1 m/sec and it gravity and will trigger spinning be the case of power failure as was around 1.09 km from its designated in the vertical plane. If spinning observed in some of our earlier landing spot on the Moon. in two orthogonal planes goes out missions, like Chandrayaan-1, As per the plan, Vikram should of control, it will tumble down the INSAT-3D and more recently have lost most of its velocity by the Vikram. Tumbling of Vikram with GSAT-6A. The power failure may time it reached 400 metres altitude and thrusters on will result in zig zag be triggered due to degradation thereafter it should have been hovering random motion, beyond the control of electronic components due to above the intended landing site, set to of on-board control system. exposure to excessive Sun radiation make a soft vertical lending. or otherwise. The frozen screens at mission 2. Vikram on-board computer suffered control showed that communication was communications data overload. These are some of the most probable lost when the lander was barely 335 As the Vikram descends toward reasons. ISRO is already conducting the meters above the surface of the Moon. the moon at a very high speed, its detailed failure analysis. It is always The green dot representing Vikram computer rapidly processes large important to learn lessons from failures. started deviating when its altitude was streams of data simultaneously. just above 2 km, and continued to deviate On-board computer has to be before stopping. autonomous as a large number of Current Status real-time commands from multiple Analysis of data shows that The Chandrayaan-2 Orbiter is currently systems have to be executed in touchdown of Vikram on the Moon placed in a 100 km polar lunar orbit. milliseconds. If there is time lag, occurred at a much higher velocity than All the eight payloads of Orbiter are i.e., there is no sufficient time for intended. The data suggests a deviation functioning normally, providing valuable executing the commands, the data during the Fine Braking Phase. scientific data of the Moon’s surface by loss may result in malfunction. It is not the purpose here to remote sensing. undertake any detailed failure assessment. The Orbiter was originally designed However, based on the assessment of 3. Inadequacy in the design of the for a mission life of one year. However, the limited data available so far, the autonomous system. Broadly, it due to injection of spacecraft into a following possible reasons can be arrived could be inferred that the system for Vikram’s silence: better orbit by GSLV MkIII-M1 (higher was not adequately designed apogee by 5075 km) and thereafter for the foreseen faults to take fuel saving by optimization of mission corrective measures. The strength 1. Deficiency in the throttling of manoeuvring towards its journey to the of autonomy of any system Vikram’s propulsion system. After Moon, its life has been extended. The would depend on the basic design rough braking, the orientation orbiter at launch had 1697 kg of fuel factors. The design would be of Vikram was to be changed after insertion to the designated Moon mainly based on the anticipated from horizontal to vertical. ISRO orbit, the balance fuel is around 500 kg, anomalies. When an autonomous has located Vikram about 500 which is sufficient for a 7.5-year life. system operates in complex and meters away from the designated The orbiter module of the mission landing spot. The deviation in open-ended environments, it is with eight scientific instruments remains Vikram’s decent trajectory (green difficult, but it has to be designed operational and will continue remote- line in Figure 3) may be due to over ruggedly and tested to identify the sensing observations of the Moon’s performance/glitch associated with unforeseen changes in time and surface. imbalance in the thrust of engine respond accordingly. triggering rotation and affecting the stability of Vikram. Dr Anand Kumar Sharma (anukampa2003@ Sensors on-board Vikram could not 4. yahoo.com) superannuated from the Indian Let us assume simultaneous provide the precise measurement Space Research Organisation (ISRO) as operation of four corner engines. on the acceleration, orientation, Distinguished Scientist on 31 August 2019. If one or more of them are not and trajectory of the Vikram during At ISRO, he was involved in the design, operating simultaneously or there landing. The problem could be testing and realization of Thermal Control/ is imbalance in thrust output among associated with either the measure Mechanical Systems of all spacecraft them, the resultant uncompensated accuracy, component failure or including Chandrayaan-2. His research focus horizontal force will spin the lander malfunction or working on the data is on the development of thermal control in the horizontal plane. In that case, from different sensors in tandem. coatings for space applications.
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