‘17 minutes of terror’: Chandrayaan-3’s landing process when everything has to go right

Bengaluru, August 22

ISRO’s ambitious third Moon mission Chandrayaan-3’s Lander Module (LM) is all set to land on the lunar surface on Wednesday evening, as India eyes to become the first country to reach the uncharted south pole of Earth’s only natural satellite.

The LM comprising the lander (Vikram) and the rover (Pragyan), is scheduled to make a touch down near the south polar region of the Moon at 6:04 pm on Wednesday.

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

The Chandrayaan-3 landing will be broadcast live on DD National TV and several other news channels. The event will also be live-streamed on ISRO's official website at isro.gov.in. The live streaming of the event will also be available on ISRO's YouTube channel and Facebook page.

If the Chandrayaan-3 mission succeeds in making a touchdown on moon and in landing a robotic lunar rover in ISRO's second attempt in four years, India will become the fourth country to master the technology of soft-landing on the lunar surface after the US, China and the erstwhile Soviet Union.

17 Minutes of terror

A day before the scheduled touch-down, ISRO on Tuesday confirmed that the Chandrayaan-3 mission is on schedule.

The space agency said the Mission Operations Complex (MOX), located at the ISRO Telemetry, Tracking and Command Network (ISTRAC) here, is buzzing with energy and excitement.

"The mission is on schedule. Systems are undergoing regular checks. Smooth sailing is continuing," ISRO said in an update this afternoon, adding that the live telecast of the landing operations at MOX/ISTRAC begins at 5:20 pm on Wednesday.

ISRO's Space Applications Centre Director Nilesh Desai said, "If any health parameter (of the lander module) is found abnormal on August 23, then we will delay the landing by four days to August 27."  

The critical process of soft-landing has been dubbed by many including ISRO officials as "17 minutes of terror", with the entire process being autonomous when the lander has to fire its engines at the right times and altitudes, use the right amount of fuel, and scan the lunar surface for any obstacles or hills or craters before finally touching down.

After checking all the parameters and deciding to land, ISRO will upload all the required commands from its Indian Deep Space Network (IDSN) at Byalalu near here, to the LM, a couple of hours before the scheduled time touchdown.

According to ISRO officials, for landing, at around 30 km altitude, the lander enters the powered braking phase, and begins to use its four thruster engines by "retro firing" them to reach the surface of the moon, by gradually reducing the speed. This is to ensure the lander doesn't crash, as the Moon's gravity will also be in play.

Noting that on reaching an altitude of around 6.8 km, only two engines will be used, shutting down the other two, aimed at giving the reverse thrust to the lander as it descends further, they said, then, on reaching an altitude of about 150-100 metres, the lander using its sensors and cameras, would scan the surface to check whether there are any obstacles and then start descending to make a soft-landing.

After the soft-landing, the rover will descend from the lander's belly, onto the Moon's surface, using one of its side panels, which will act as a ramp.

The lander will have the capability to soft-land at a specified lunar site and deploy the rover which will carry out in-situ chemical analysis of the lunar surface during the course of its mobility. The lander and the rover have scientific payloads to carry out experiments on the lunar surface.

The lander and rover will have a mission life of one lunar day (about 14 earth days) to study the surroundings there. However, ISRO officials do not rule out the possibility of them coming to life for another lunar day.

South Pole mission

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.

The LM has payloads including RAMBHA-LP which is to measure the near surface plasma ions and electrons density and its changes, ChaSTE Chandra's Surface Thermo Physical Experiment -- to carry out the measurements of thermal properties of lunar surface near polar region-- and ILSA (Instrument for Lunar Seismic Activity) to measure seismicity around the landing site and delineating the structure of the lunar crust and mantle.

The rover, after the soft-landing, would ramp down of the lander module and study the surface of the moon through its payloads APXS - Alpha Particle X-Ray Spectrometer - to derive the chemical composition and infer mineralogical composition to further enhance understanding of the lunar surface.

The rover also has another payload Laser Induced Breakdown Spectroscope (LIBS) to determine the elemental composition of lunar soil and rocks around the lunar landing site.

Ahead of its scheduled landing on the moon, Chandrayaan-3's LM has established a two-way communication with Chandrayaan-2's orbiter which continues to orbit around the Moon. The two-way contact potentially offers ground controllers (MOX-Mission Operations Complex in Bengaluru) more channels for communication with Chandrayaan-3.

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