Why did Chandrayaan-2 fail? Corrections may help ISRO's next moonshot, says report

On July 22, 2019, India embarked on its first-ever science lander mission with the launch of a GSLV (Geosynchronous Satellite Launch Vehicle) carrying Chandrayaan-2. This mission aimed to achieve the first-ever landing in the Lunar South Pole Region.

But communication was lost prior to the landing when the lander was approximately 2.1 km above the lunar surface. The communication issue occurred during the rough braking phase of the landing at 01:53 am on September 6, 2019.

Unfortunately, the lander, named Vikram, experienced a hard landing on September 7, 2019, dashing India’s hopes of becoming the first nation to achieve a successful lunar landing in its inaugural attempt.

A study titled “Report On The Loss Of Vikram Lander Of Chandrayaan 2 Mission” tries to unravel the story behind the cause of failure and offers suggestions to fix these issues.

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How it happened

The Chandrayaan-2 mission began on 22 July 2019 at 2:51 am when the GSLV was launched from SHAR. On 20 August 2019, the orbiter and lander Vikram approached the Moon and successfully entered the Moon’s orbit after the first Lunar bound maneuver. After achieving successful orbital insertion, the spacecraft underwent five more lunar bound maneuvers to prepare for lander separation and lunar landing, says the report.

The Vikram lander separated from the main bus orbiter on 6 September 2019 at 1:38 am and began its descent towards the lunar surface. Initially, all mission parameters were normal as the lander reached an altitude of over 2.1 km. However, at 1:53 am (15 minutes into the landing phase), communication with the lander was unexpectedly lost, resulting in the failure of the lunar landing, according to the report.

Following the search for the impact site, Chennai-based engineer Shanmuga Subramanian was the first to report changes on the Lunar surface, leading to NASA’s identification of the lander and its impact site. The lander crashed at coordinates 70.8810°S, 22.7840°E, with an elevation of 834 m.

Trajectory deviation: Analysing the trajectory map of the Vikram lander’s operation, it is evident that the lander followed a normal path until reaching an altitude of 2.2 km. However, at 2.1 km, the lander’s trajectory deviated from its expected path, resulting in the loss of communication and uncertainty about the mission status. The observed trajectory deviation occurred between 2.1 km and 0.2 km above the lunar surface.

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Hypotheses on possible causes for Lander failure:

1. Vibrations during the rough braking and powered descent phases might have dislodged or disconnected the power cord of the communication system or antenna, leading to the loss of communication from the ground.
2. At the point of deviation in the trajectory graph, the thrusters might have been shut down or improperly fired, causing the spacecraft to lose control of its orientation and resulting in the lander crashing onto the lunar surface.
3. No indication of spacecraft power was observed during the descent phase. Therefore, power depletion from either the battery or solar panels could have caused the spacecraft computer to shut down, subsequently cutting off thruster operation and communication. The lander is programmed to perform an automatic landing in the event of communication issues, which would have resulted in a safe and soft landing. However, given the crash of the lander, power depletion during the descent phase is suspected.
4. The depletion of spacecraft power could be attributed to the limited availability or minimal presence of solar irradiance in the south polar region of the Moon.

The report recommends that space organisations consider using Nuclear Thermoelectric Generators (NTG) or Radioisotope Thermoelectric Generators (RTG) as secondary power sources and backup options for spacecraft destined for locations like the South Lunar Region or beyond Mars’ orbit. This is to mitigate the risks associated with power loss and battery depletion caused by limited availability of sunlight or solar irradiance. Additionally, it suggests implementing a direct communication system for both lander and rover missions to interplanetary destinations. This would enable confirmation of the mission status even if one of the communication systems becomes impaired.

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Despite the failure of the first lander mission, ISRO has put forth a proposal for another landing demonstration mission called Chandrayaan-3. This upcoming mission will involve a lander and rover and will mark India’s third polar lunar exploration mission. Lessons learned from the previous lander mission, Vikram, have been taken into consideration, leading to several improvements in Chandrayaan-3. The primary objective of this mission is to showcase the landing capabilities required for exploring the polar region of the Moon.

In collaboration with the Japanese Aerospace Exploration Agency, ISRO will provide the lander while Japan will contribute the launcher and rover for the mission. Chandrayaan-3 will feature additional equipment, including a Laser Doppler Velocimeter (LDV) and night survival technologies, to facilitate site sampling exploration.

The report notes that the results and findings presented are based on experiences and studies conducted on previous lunar and Mars probe failures. They do not provide a definitive explanation for the specific cause of the Vikram lander mishap. As ISRO has not released the official investigation report on the failure of the lander, this study was conducted to explore the potential factors that could have contributed to the failure of the Indian lander, Vikram.

Landing site selection is a crucial factor in the success of a soft landing mission. It depends on the capabilities of onboard engineering instruments and landing sensors, such as temperature, landing instrument accuracies, and terrain slope shadows. During the Chandrayaan-2 mission, potential landing sites were identified, and a prime landing site covering an area of 500m x 500m in the south polar region of the lunar surface was targeted. Unfortunately, the Vikram lander hard-landed near the identified site, resulting in debris being spread in and around the landing site, stated the report.

Importance of landing sites

The report further adds that to ensure a safe landing for future missions like Chandrayaan-3, it is necessary to identify new landing sites. The criteria for landing site selection include factors such as local and global slope, sun illumination, radio communication with Earth, and sizes of craters and boulders. Several datasets from missions like Chandrayaan-1, Clementine, Lunar Reconnaissance Orbiter (LRO), and Chandrayaan-2 were analysed to identify suitable landing sites.

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Initially, three shortlisted sites from the Chandrayaan-2 mission within the 70-80 degree latitude range were revisited for Chandrayaan-3 landing site selection. However, these sites did not meet the landing area requirements of 4km x 2.4km. Therefore, the study focused on the area between 60 to 70 degree latitude for identifying potential landing sites. Topography, slope data derived from LOLA DEM, and shadows detected from Selene images were used to create a hazard map. A moving window technique was applied to search for hazard-free areas within the desired landing site dimensions.

Twenty sites were selected for further characterisation using medium-resolution datasets, and eight sites satisfying various conditions were chosen for terrain analysis using high-resolution camera data from the Chandrayaan-2 orbiter. Stereo images acquired by the Orbiter High-Resolution Camera (OHRC) provided detailed information about terrain undulations, slope, aspect, and illumination constraints. Criteria such as slope, boulder size, crater and boulder distribution, sunlit duration, visibility to Earth for communication, and shadowing were considered to finalise the landing site selection.

Based on these criteria, three sites were selected, and LS-2 was identified as the prime landing site for Chandrayaan-3. It offers better distribution of safe grids within the 4km x 2.4km landing area, providing flexibility for the lander to choose a landing spot within a distance of 100m from the hovering point. LS-2 is located between Manzius U and Boguslawsky M craters.

Chandrayaan-3 mission 

Chandrayaan 3 is a continuation of the previous mission, Chandrayaan 2. Chandrayaan 3 will be launched into space by the country’s most powerful rocket, GSLV-MKIII (also known as LVM-3).

The mission will feature an indigenous lander module, propulsion module, and a rover. Its primary objective is to develop and demonstrate new technologies essential for inter-planetary missions.

By incorporating advanced technologies, Chandrayaan-3 aims to further enhance India’s capabilities in space exploration. According to ISRO, the estimated cost of the Chandrayaan 3 mission is over ₹600 crores. In comparison, the previous mission, Chandrayaan-2, had a total cost of around ₹960 crores.

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The reduced cost for Chandrayaan-3 indicates a more focused mission with a specific goal of showcasing the ability to land a rover on the lunar surface and explore its environment. The optimisation of resources and lessons learned from the Chandrayaan-2 mission have likely contributed to the cost reduction for Chandrayaan-3.

Piggybacking on a GSLV MARK III launcher, Chandrayaan 3 is tentatively scheduled to lift off from the Satish Dhawan Space Centre in Sriharikota on July 12, 2023. It is set go into an elliptical orbit around the Moon, before attempting to soft-land on the celestial body.