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ISRO on Wednesday said it is all set to initiate the Automatic Landing Sequence (ALS) for its ambitious third Moon mission Chandrayaan-3's Lander Module (LM) to touch down on the lunar surface this evening, reports PTI.

India is at the cusp of scripting history, as ISRO's ambitious third Moon mission Chandrayaan-3's Lander Module (LM) is all set to touch down on the lunar surface on Wednesday evening, making it only the fourth country to do so, and first 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. Isro has said it will livestream the landing attempt through its website and social media platforms starting 5.20pm on Wednesday.

"The mission is on schedule. Systems are undergoing regular checks. Smooth sailing is continuing. The Mission Operations Complex (MOX) is buzzed with energy & excitement!" ISRO said on Tuesday, also sharing visuals of the moon captured by cameras on the lander, according to PTI.

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.

Chandrayaan-3 is a follow-on mission to Chandrayaan-2 and its objectives are to demonstrate safe and soft-landing on the lunar surface, roving on the Moon, and to conduct in-situ scientific experiments.

Chandrayaan-2 had failed in its lunar phase when its lander 'Vikram' crashed into the surface of the Moon minutes before the touch down following anomalies in the braking system in the lander while attempting a landing on September 7, 2019. Chandrayaan's maiden mission was in 2008.

The Rs 600 crore Chandrayaan-3 mission was launched on July 14 onboard Launch Vehicle Mark-III (LVM-3) rocket, for a 41-day voyage to reach near the lunar south pole.

The soft-landing is being attempted days after Russia's Luna-25 spacecraft crashed into the Moon after spinning out of control.

After the second and final deboosting operation on August 20, the LM is now placed in a 25 km x 134 km orbit around the Moon.

The module would undergo internal checks and await the sunrise at the designated landing site, ISRO has said, adding that the powered descent -- to achieve soft-landing on the Moon's surface -- is expected to be initiated at around 5:45 pm on Wednesday.

ISRO's Space Applications Centre Director Nilesh Desai had 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."

'17 minutes of terror'

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 of 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.

Landing around 30km altitude

According to ISRO officials, for landing, 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.

ISRO Chairman S Somanath had recently said the most critical part of the landing will be the process of reducing the velocity of the lander from 30 km height to the final landing, and the ability to reorient the spacecraft from horizontal to vertical direction. "This is the trick we have to play here," he said.

Failure-based design

Instead of a success-based design in Chandrayaan-2, the space agency opted for a failure-based design in Chandrayaan-3, focused on what all can fail and how to protect it and ensure a successful landing, the ISRO chief said.

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. On landing the lander may have to face the challenge of lunar dust due to firing of onboard engines close to 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.

Soft landing capability

The lander has 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. Both have scientific payloads to carry out experiments on the lunar surface.

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.

The payloads

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.

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, giving the ground controllers more channels for communication with it.

The LM of Chandrayaan-3 had successfully separated from the Propulsion Module (PM) on August 17, which was 35 days after the satellite was launched on July 14.

Meanwhile, the PM, whose main function was to carry the LM 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 PM 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).

Lunar orbit

Post its launch on July 14, Chandrayaan-3 entered into the lunar orbit on August 5, following which orbit reduction manoeuvres were carried out on the satellite on August 6, 9, 14 and 16, ahead of separation of both its modules on August 17.

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 August 1 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.