Chandrayaan-3: How ISRO's lander made it through final stages of descent to Moon

India’s Chandrayaan-3 mission successfully soft-landed on the Moon on Wednesday. The Indian Space Research Organisation (ISRO) lunar mission came just days after Russia’s Luna-25 spacecraft crashed into the Moon after spinning out of control and nearly four years after the Chandrayaan-2’s Vikram lander crashed into the surface of the Moon in September 2019. **Also read: Brothers in Arms: How NASA and ESA are helping ISRO with Chandrayaan-3’s moon landing** **Also read: Not so soft: Chandrayaan-3 will touch down on moon at speed that will crush a human to pulp** Let’s take a look at how the final minutes of descent – which experts had dubbed “20 minutes of terror”–  went What will happen? The Chandrayaan-3 lander touched down in four phases – the rough braking phase, the attitude hold phase, the fine braking phase and the terminal descent phase. The lander entered the first phase around 30 kilometres above the lunar surface. It then gradually reduced its speed by ‘retro firing’ its four thruster engines.

This was done to ensure the lander doesn’t crash due to the Moon’s gravity.

  ISRO’s Space Applications Centre director Nilesh Desai earlier explained to ANI why this was done, “The lunar gravity will pull it down as a result of which we will have to retrofire so that its speed lessens. Its speed will be reduced to zero as it will reach the moon’s surface. We have to fire thruster engines to reduce its speed as it lands." Then around 7.42 kilometres from the lunar surface, the lander went into its second phase – this lasting just a few seconds and covering 3.48 kilometers. Then came the fine-breaking phase. [caption id=“attachment_13026092” align=“alignnone” width=“640”] An illustration showing Chandrayaan-3’s lander in 25 km x 134 km orbit. PTI[/caption] Around 6.8 kilometres above the lunar surface, two engines were shut down. This was done to allow the lander a reverse thrust as it kept descending. Then, the lander shifted to a vertical position. ISRO chairman S Somanath had recently called this the most critical part of the landing.

“This is the trick we have to play here,” he said.

“The velocity at the starting of the landing process is almost 1.68 km per second, but (at) this speed (the lander) is horizontal to the surface of the Moon. The Chandrayaan-3 here is tilted almost 90 degrees, it has to become vertical. So, this whole process of turning from horizontal to vertical is a very interesting calculation mathematically. We have done a lot of simulations. It is here where we had the problem last time (Chandrayaan-2),” Somanath earlier explained. Then, the final phase of terminal descent begun. The lander, on reaching an altitude of about 150-100 metres, used its sensors and cameras to scan the lunar surface for obstacles.

The lander then proceeded to make its final descent.

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The lander module touched down on the lunar surface at 6.04 pm India time on Wednesday. What happens next? The rover Pragyan will now leave the lander module. It will descend from the lander’s belly onto the Moon’s surface using one of its side panels which will act as a ramp.

It will now begin studying the surface of the Moon.

This will be done through its payloads APXS – Alpha Particle X-Ray Spectrometer – to derive the chemical composition and infer mineralogical composition to further enhance understanding of lunar surface. The rover, which has a mission life of 1 lunar day (14 Earth days), also has another payload Laser Induced Breakdown Spectroscope (LIBS) to determine the elemental composition of lunar soil and rocks around the lunar landing site, ISRO said. “After powered descent on to the landing site, there will be deployment of ramp and rover coming out. After this all the experiments will take place one after the other – all of which have to be completed in just one day on the moon, which is 14 days,” Somanath earlier explained. Noting that as long as the sun shines all the systems will have its power, Somanath added, “The moment the sun sets, everything will be in pitch darkness, temperature will go as down as low as minus 180 degree celsius; so it is not possible for the systems to survive, and if it survives further, then we should be happy that once again it has come to life and we will be able to work on the system once again, and we hope like that to happen." However, ISRO officials do not rule out the possibility of them coming to life for another lunar day. Polar regions of the moon are a very different terrain due to the environment and the difficulties they present and therefore have remained unexplored. All the previous spacecraft to have reached the Moon landed in the equatorial region, a few degrees latitude north or south of the lunar equator. The Moon’s south pole region is also being explored because there could be a possibility of presence of water in permanently shadowed areas around it. Meanwhile, the Propulsion Module, whose main function was to carry the Lander Module from launch vehicle injection till lander separation orbit, will continue its journey in the current orbit for months/years, the space agency said. Apart from this, the Propulsion Module also has one scientific payload as a value addition. The SHAPE (Spectro-polarimetry of Habitable Planet Earth) payload onboard it, whose future discoveries of smaller planets in reflected light would allow us to probe into a variety of Exo-planets which would qualify for habitability (or for presence of life). The LM of Chandrayaan-3 had successfully separated from the Propulsion Module on 17 August –35 days after the satellite was launched on 14 July. Chandrayaan-3 entered into the lunar orbit on 5 August, following which orbit reduction manoeuvres were carried out on the satellite on 6, 9, 14 and 16 August ahead of separation of both its modules on 17 August. Earlier, over five moves in the three weeks since the July 14 launch, ISRO had lifted the Chandrayaan-3 spacecraft into orbits farther and farther away from the Earth. Then, on 1 August, in a key manoeuvre – a slingshot move – the spacecraft was sent successfully towards the Moon from Earth’s orbit. Following this trans-lunar injection, the Chandrayaan-3 spacecraft escaped from orbiting the Earth and began following a path that would take it to the vicinity of the moon. Noting that this is the work of a generation of ISRO’s leadership and scientists, ISRO chief S Somanath paid tribute to a generation of leadership and scientists of the country’s space agency. “This is the journey we started with Chandrayaan-1, continued in Chandrayaan-2 and Chandrayaan-2 craft is still working and doing lot of communication work with us– and all the team that contributed to building Chandrayaan-1 and 2 should be remembered and thanked, while we celebrate Chandrayaan-3.” With inputs from agencies