Chandrayaan-3: Why is soft-landing on the Moon so challenging?

India is waiting with bated breath for Moon mission Chandrayaan-3’s successful landing on the lunar surface. The spacecraft’s lander is set to attempt a soft landing on the Moon’s south pole on Wednesday (23 August) evening. If all goes well, India will become the first country to land on this region of the Moon, strengthening its position as a major space power. Russia’s spacecraft Luna-25, which was to make a soft landing on the lunar south pole before India, crashed on the Moon on Sunday. Only three countries, including the United States, the former Soviet Union and China, have managed to soft-land on the moon. But why is a safe landing on the lunar surface so difficult? Let’s take a closer look. What is a soft landing? A soft landing entails a successful landing by a spacecraft without sustaining any significant damage to itself or its payloads, noted Deccan Herald (DH). Whereas, in a hard landing, the probe suffers damage which may result in the failure of the mission. Why is landing on Moon challenging? The Moon has much less gravity than the Earth. Its thin and tenuous atmosphere, along with lunar dust, also makes it tough for spacecraft to land. Citing NASA, The Hindu reported, “The presence of dust, even in very small quantities, can have a significant effect on temperature control and optical performance of hardware on the lunar surface.” According to Indian Space Research Organisation (ISRO), deep space communication also poses a challenge as the “large distance from the Earth and the limited on-board and radio signals are weak with heavy background noises that need to be picked up by large antennas”. Why is Moon’s south pole particularly tricky? Before Russia, countries such as Japan, Israel and the United Arab Emirates (UAE) have tried and failed to land on the Moon’s south pole. The region has a difficult terrain, full of craters and deep trenches. It is also far from the equatorial region explored by previous lunar missions. As per a DH report, some areas on the south pole are shrouded in darkness and have never received sunlight. Temperatures are so cold there that they can plummet to as low as -230 degree Celsius. This rocky terrain, complete darkness and extremely cold weather make it more difficult for electronic instruments to function properly, the report added. Chandrayaan-3’s ‘15 minutes of terror’ Chandrayaan-3’s lander module, which comprises the Vikram lander and the Pragyaan rover, will kickstart its attempt to soft-land on the Moon on 23 August at 5.45 pm.

Chandrayaan-3 Mission:
The mission is on schedule.
Systems are undergoing regular checks.
Smooth sailing is continuing.

The Mission Operations Complex (MOX) is buzzed with energy & excitement!

The live telecast of the landing operations at MOX/ISTRAC begins at 17:20 Hrs. IST… pic.twitter.com/Ucfg9HAvrY

— ISRO (@isro) August 22, 2023

This landing phase was described as “15 minutes of terror” by ISRO’s former chairman K Sivan after Chandrayaan-2 mission had failed and crashed on the lunar surface in 2019. Chandrayaan-3’s lander module is currently in the 25 km x 134 km orbit from where the powered descent to the Moon’s south pole will commence. In this ‘powered braking phase’, the Vikram lander will use its thrusters for a touchdown on the lunar surface. For a successful soft landing, the lander will have to change from a horizontal position to a vertical one. “One of the biggest things is that it has got to get the spacecraft from a horizontal position to a vertical position. That is difficult. All these aspects have to be looked into,” space strategist PK Ghosh told news agency ANI earlier. “At around 100 m altitude, the lander would scan the surface for obstacles. If there are no obstacles, it would begin a slow descent, firing its thrusters until touchdown,” as per The Hindu. In the ‘rough braking phase’, the lander’s horizontal velocity has to be brought down from a range of 6,000 km/h to almost zero for a soft landing. “Chandrayaan-3 is tilted almost 90 degrees at this time (when the landing process begins at 5.45 pm) but it needs to be vertical (for a landing). This process of turning the lander…is a very interesting calculation mathematically. We have done a lot of simulations. This is where we had a problem the last time (which led to the crash of Chandrayaan-2 on September 7, 2019),” ISRO Chairman S Somanath was quoted as saying by Indian Express. [caption id=“attachment_13026092” align=“alignnone” width=“640”] An illustration showing Chandrayaan-3’s lander in a 25 km x 134 km orbit. PTI[/caption] When the lander descends to the 7.42 km altitude, the ‘attitude hold phase’ will begin in which it will shift from a horizontal to a vertical position covering a further distance of 3.48 km, as per the newspaper. The Vikram lander will move completely to a vertical position in the ‘fine braking phase’, covering the last 28.52 km distance to the landing site. With this, the altitude will further reduce to 800-1,000 metres. At this altitude, the Vikram lander will carry out a verification of the sensors. Then at a height of 150 metres, it will do a “hazard verification and decide whether it should land vertically there itself or move laterally to a maximum extent of 150 metres to avoid any boulders or craters. Such choices and decisions will be taken at the appropriate time (by the lander)”, Somanath said, as per Indian Express.  If anything goes wrong in these 15 minutes, ISRO scientists will not be able to intervene as the final touchdown is controlled by computer logic of the lander’s computers, guidance and control navigation systems, reported Indian Express. “This is nothing but computer logic residing in Chandrayaan-3. This is what is steering it to the right position for landing. Whenever the lander has to take decisions on how to go it is the guidance and control system that will take the lander on the path for landing,” the ISRO chief said previously. “The configuration is designed in such a manner that we will have a safe and soft landing. We have addressed various things to achieve this goal of a safe and soft landing. We could not do it last time and we have addressed the issues that we faced in great detail,” he said, as per Indian Express.  It was in the final ’terminal descent phase’ that Chandrayaan-2 spun out of control and crashed. However, this time the ISRO is confident of a successful landing on the Moon’s unexplored south pole. With inputs from agencies