In Chandrayaan-3 lander’s eyes, and rover’s curiosity, the work of ISRO-SAC and PRL scientists

SAC has developed eight camera systems for the Chandrayaan-3 mission, four of which are on board the lander, and one on the rover — these five cameras will be primarily used for capturing images and their public dissemination. Three other cameras on the lander will play a crucial role in the landing.

ISRO SAC director Nilesh Desai told The Indian Express, “Of the three major cameras, the Lander Position Detection Camera (LPDC) will give the lander position while it is coming down, giving us the details of longitude and latitude during the 30-kilometre descent to the lunar ground. The other two cameras comprise part of the Lander Hazard Detection and Avoidance Camera (LHDAC) system, which will take pictures in real time while it is landing and correlate with stored images to find out if the landing is taking at the right place and adjust the track of the lander accordingly, so that it lands on the decided landing site. The LHDAC will be very crucial for the landing. Technically, of the two cameras, only one will be used while the other is redundant, kept as a backup.”

The Ka-Band altimeter (KaRA) — a radio-frequency radar altimeter — also built by SAC, will also play a crucial role during the landing. Desai said, “It will give us information of the height during the landing. This is crucial because we have to regularly feed the data of the height so that the thrust of the engine, the orientation etc. can be adjusted accordingly. It will start transmitting the height information when it is 10 km from the lunar surface, and will continue transmitting until its descent to a height of up to 10 metres above the lunar surface. The data will also be sent to the Hazard Detection and Avoidance (HDA) Processor (part of the Lander Hazard Detection and Avoidance Camera and Processing Algorithm technology).

“Based on the feedback, the thrusters are calibrated and fired for landing. Height sensor is crucial. The Japanese lander failure (Hakuto-R Moon landing mission which failed in April 2023) had occurred because it presumed it had landed on the surface when it was still kilometres away from the surface. It had started slow descent when the fuel was consumed and it ended up crash landing. Thus right information is very critical.”

Another major contribution from SAC has been towards the selection of the landing site. The landing is planned around 70 degree south latitude, closer to the lunar south pole than any mission has ever gone before.

Desai said, “All these years, by examining various images of the Moon from various sources, we did the preparatory work and we zeroed in on a 4 km x 2.4 km area as the landing site. Our landing site is a hazardous area, full of craters and boulders, and apart from controlling speed, we also have to take care of all the hazards.”

The identified landing site is bigger than the 500 m x 500 m area that was identified for the Chandrayaan-2 mission of 2019, which too, was selected by SAC. Images taken by sensors on board the Chandrayaan-2 orbiter, which remains in orbit around the Moon, aided in the selection of the landing site for Chandrayaan-3.

Desai said that 3,901 sections, each measuring 24 m x 24 m, have been identified as potential landing sites within the 4 km x 2.4 km landing area. An alternative landing site 400 km away has been identified as well, but it is “not as perfect as the currently identified site”, Desai said.

The mission is carrying two scientific payloads from the Physical Research Laboratory, Ahmedabad.

There is the rover payload called Alpha Particle X-ray Spectrometer, and the lander payload called Chandra’s Surface Thermophysical Experiment (ChasTE). Both payloads were part of the Chandrayaan-2 mission as well, but could not be utilised owing to the soft landing failure which resulted in the loss of the lander and rover and the associated five payloads.

PRL director Anil Bhardwaj told The Indian Express that the two payloads had been under development from around 2014-15. The Alpha Particle X-ray Spectrometer will provide on-site elemental and chemical composition of lunar soil and rocks, and is equipped to detect elements such as sodium, magnesium, aluminium, silicon, potassium, calcium, titanium, and iron. “This will help us understand the distribution of various elements on the lunar surface, a few metres apart, because the rover will stop, take measurements and again move and repeat the process, over a period of around 14 (Earth) days (which is equivalent to one lunar day),” Bhardwaj said.

The lander payload ChaSTE will serve two scientific purposes. One is to determine the vertical temperature gradient near the lunar south pole, measuring temperature with a probe at every one centimetre, up to a depth of 10 cm. “We wanted to do this kind of experiment because the lunar surface is known to be an insulator and the idea is to understand how the heat propagates to the interior of the Moon from the surface,” Bhardwaj said.

At its tip, the ChasTE probe has a heater, which will be turned on after a few days to measure thermal conductivity, he said.