"15 minutes of terror," Chandrayaan-3 landing process explained

“Fifteen minutes of terror” is what former ISRO chief K Sivan had termed the landing phase of Chandrayaan-2 when ISRO aborted the first attempt to launch the Chandrayaan-2 mission.  

Dr Sivan's explanation perfectly encapsulated the complexity of the mission's final stage, during which Chandrayaan-2 failed because the Vikram lander did not succeed to switch correctly from the horizontal to the vertical position and crashed onto the Lunar surface as it entered the "fine braking phase" 7.42 kilometres from the lunar surface. 

The Chandrayaan-3 lander will have to undertake a crucial technical manoeuvre on August 23 as it approaches the last 15 minutes of its effort to conduct a soft landing on the Moon. The manoeuvre will involve changing the craft's high-speed horizontal position to a vertical one, which will enable a slow touch-down to the lunar surface.

According to ISRO, the module will go through internal inspections and wait at the planned landing spot till sunrise. It was also stated that the powered descent, which aims to ensure a soft landing on the Moon's surface, is anticipated to begin at around 5:45 pm on Wednesday.

In a conversation with Republic, former ISRO deputy director, Arup Dasgupta said, “Every space scientist knows that space is very unforgiving..." as he highlighted the uncertainties of space. 

The mission's success will be determined during these final 15 minutes on Wednesday. India will become the first nation to softly land a probe on the moon's south pole when Chandrayaan-3 makes a momentous descent on the Moon on August 23, 2023, at about 6:04 pm IST. Before touchdown, there are crucial manoeuvres to be made. Initial preparation, velocity reduction, orientation shift, attitude hold phase, fine braking, final descent, and touchdown are the different stages of the touchdown.

At 6:04 p.m. on Wednesday, the Lander Module (LM), which consists of the lander (Vikram) and the rover (Pragyan), is supposed to make a soft landing close to the South Pole of the Moon.

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

Former ISRO deputy director Arup Dasgupta while explaining the Chandrayaan-3 landing process in a conversation with Republic said, “The craft should be autonomous; it should be able to decide for itself - what it has to do in those critical few minutes of the landing. The landing process starts 15 minutes before the touchdown, that's when it is in an orbit which is roughly 30 km. It is at that level, it starts doing what is called 'Braking Manoeuvre.”

Four descent phases

The lander must first slow down from its initial horizontal velocity of 1.68 km/sec (zero vertical velocity at this point) to 358 m/sec (roughly 1,290 km/h) and 61 m/sec (roughly 220 km/h) in the ideal "rough braking phase" (first phase) of 690 seconds, during which the lander will descend from an altitude of 30 km to 7.42 km. The lander will journey 713.5 kilometres across the lunar surface during this period to reach the touchdown point.

The lander will enter an "attitude hold phase" at an altitude of 7.42 km from the surface that will last for nearly 10 seconds. During this time, it will tilt from a horizontal to a vertical position while travelling 3.48 km. The height will be lowered to 6.8 kilometres, and the horizontal and vertical speeds will be 336 and 59 metres per second, respectively.

To maintain thrust consistency at the beginning of the second phase of landing, Chandrayaan-3's thrust requirement is larger during the second attitude hold phase—740X4 N as opposed to 400X4 N in Chandrayaan-2. The new mechanisms will make it easier for the probe to transition from a horizontal to a vertical posture while maintaining thrust and angle continuity.

The lander will entirely move into a vertical posture during the third phase of the landing procedure, known as the "fine braking phase," which will last around 175 seconds. It will travel the remaining 28.52 km to the landing area, descending to an altitude of 800-1,000 m, and reaching a nominal speed of 0 m/sec.

“From 30 km to 7.42 km (altitude) will be rough braking and at 7.42 km there will be an attitude hold phase where some of the instruments will carry out calculations; at 800 or 1,300 metres (altitude) it will start doing a verification of the sensors, at 150 metres (altitude) 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,” Dr Somanath said.

To guarantee that the desired dispersions are obtained in all of these phases, extensive simulations have been performed, the guidance designs have been modified, and numerous algorithms have been developed. Even if the nominal figures differ, the lander will still try a vertical landing, according to ISRO director Dr Somanath.

When the lander must operate its engines at the proper intervals and altitudes, use the proper amount of fuel, and scan the lunar surface for any hills or craters before finally touching down, the entire operation is autonomous.

At about 30 km altitude, the lander enters the powered braking phase and starts to use its four thruster engines by "retro firing" them to progressively lower the speed until it reaches the Moon's surface for landing. This will prevent the lander from crashing because the Moon's gravity will also be at work.

The lander would use its sensors and cameras to scan the surface to see if there are any obstacles before starting to descend to make a landing, ISRO officials told PTI, noting that on reaching an altitude of about 6.8 km, only two engines will be used, shutting down the other two to give the lander the reverse thrust as it descends further for a soft landing. 

Learning from the failures

Before the Chandrayaan-2 probe could approach its final "terminal descent phase," — it lost control and collided with the Moon's surface during the "attitude hold phase" and the "fine braking phase". The failure studies have been utilised to increase Chandrayaan-3's touchdown probability.

"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 an interesting calculation mathematically. We have done a lot of simulations. It is here where we had the problem last time (Chandrayaan-2)," Somanath explained.

Somanath stated that the lander's whole construction is done in such a way that it will withstand failures.

“If everything fails, if all the sensors fail, nothing works, still it (Vikram) will make a landing. That’s how it has been designed -- provided that the propulsion system works well," S Somnath was quoted as saying by PTI. 

The initial rough braking phase of Chandrayaan-2 used a first-order automated guidance system; whereas Chandrayaan-3 is using a second-order guidance system. In Chandrayaan-3, the rough braking phase also uses an immediate thrust regulation.

Chandrayaan-3 was launched on July 14 and entered lunar orbit on August 5. Thereafter, orbit reduction operations were performed on the spacecraft on August 6, 9, 14, and 16 in preparation for the separation of both of its modules on August 17 and the upcoming landing on August 23. 

Since its launch on July 14th, ISRO has manoeuvred the Chandrayaan-3 spacecraft more than five times in just three weeks, placing it in faraway orbits from the Earth.

Then, on August 1, the spacecraft was successfully launched from Earth's orbit towards the Moon in a crucial movement known as a slingshot. Chandrayaan-3 spacecraft left Earth's orbit after this trans-lunar injection and started travelling in a direction that would bring it close to the moon.

(With inputs from Agencies)