Explained: How Chandrayaan-3 Lander Will Navigate Through Its Final Descent To The Moon's Surface

The Chandrayaan-3 lander will need to perform a critical technical manoeuvre today (23 August) during its final 15 minutes of attempting a soft landing on the Moon.

This manoeuvre involves transitioning from a high-speed horizontal position to a vertical one, enabling a gentle descent onto the lunar surface.

The success of the mission hinges on these final 15 minutes, which will take place today evening.

Back in July 2019, when the first attempt to launch the Chandrayaan-2 mission was aborted by the Indian Space Research Organisation (ISRO), K Sivan, the former chairman of the space research body, referred to this phase as "15 minutes of terror."

Dr Sivan's description accurately captures the complexity of the mission's final phase.

It was during this phase that Chandrayaan-2 encountered failure when the Vikram lander failed to transition appropriately from horizontal to vertical, resulting in it crashing onto the Moon's surface during the "fine braking phase," around 7.42 km away from its intended landing site.

On Sunday (20 August), the final deboosting of the Chandrayaan-3 lander was successfully completed. This manoeuvre reduced its orbit to 25 x 134 km around the Moon.

During the landing process, the lander goes through a rough braking phase.

Another crucial aspect of the landing is reducing the lander's horizontal velocity from 1.68 km/sec to almost zero. This needs to be achieved at a height of 30 km from the lunar surface, for a soft landing at the designated site. The scheduled landing time is around 6.04pm IST on 23 August, as stated by ISRO.

When the landing process begins at 5.47pm today, the Chandrayaan-3 is tilted almost 90 degrees. However, it needs to be in a vertical position for a successful landing. Turning the lander to the correct position involves complex mathematical calculations and simulations.

This was an area where the previous Chandrayaan-2 mission faced difficulties, leading to its crash on 7 September 2019, according to ISRO Chairman S Somanath.

The transition from the horizontal position to the vertical position is a crucial challenge. It requires careful management of fuel consumption, accurate distance calculations, and the proper functioning of all algorithms, as explained by Dr Somanath.

Everything seemed to be going well during the landing in 2019, until just 3 minutes before the final descent phase.

At that point, the lander unexpectedly started spinning more than 410 degrees, deviating from its intended spin of 55 degrees. This unfortunate turn of events resulted in a crash landing on the Moon.

To control the speed and direction of the lander, there are 12 engines onboard. Four of these engines are used to decrease the velocity, while the remaining eight smaller engines are responsible for controlling the descent direction.

These engines can be adjusted to provide varying levels of thrust, ranging from 800 Newton to lower values. This allows the lander to hover in the Moon's gravity.

During the landing process, the lander needs to reduce its horizontal velocity from 1.68 km/sec to 358 m/sec, and its vertical velocity from zero to 61 m/sec. This is achieved during the "rough braking phase," which lasts for approximately 690 seconds.

Throughout this phase, the lander descends from an altitude of 30 km to 7.42 km, covering a distance of 713.5 km across the Moon's surface towards the intended landing site.

The lander will enter an "attitude hold phase" at a height of 7.42 km. This phase will last for about 10 seconds as the lander tilts from a horizontal to a vertical position, covering a distance of 3.48 km.

The altitude will be reduced to 6.8 km, with horizontal velocity at 336 m/sec and vertical velocity at 59 m/sec.

During the "fine braking phase", which lasts approximately 175 seconds, the lander will fully transition into a vertical position. It will travel the remaining 28.52 km to the landing site, bringing the altitude down to 800-1,000 m and reaching a nominal speed of 0 m/sec.

The journey from 30 km to 7.42 km will involve rough braking. At 7.42 km, the lander will enter the attitude hold phase, allowing certain instruments to perform calculations.

At altitudes of 800 or 1,300 metres, the sensors will undergo verification. When the altitude reaches 150 metres, a hazard verification will take place. Based on this, the lander will decide whether to land vertically or move laterally within a maximum range of 150 metres to avoid obstacles.

Unfortunately, it was during the transition from the attitude hold phase to the fine braking phase that the Chandrayaan-2 lander lost control and crashed.

ISRO has used the lessons learned from this failure to enhance the landing prospects of Chandrayaan-3 to a greater extent.

In Chandrayaan-2, a first-order automated guidance system was utilised during the initial rough braking phase. However, in Chandrayaan-3, a second-order guidance system is being employed. Additionally, Chandrayaan-3 incorporates an instantaneous thrust regulation during the rough braking phase.

To ensure thrust continuity at the start of the second phase of landing, Chandrayaan-3 demands a higher level of thrust at 740 x 4 N during the second attitude hold phase. This is in contrast to Chandrayaan-2, which required 400 x 4 N.

These new systems enable the lander to maintain thrust and angle continuity as it transitions from a horizontal to a vertical position.

Dr Somanath stated that extensive simulations have been conducted, resulting in changes to the guidance designs and the implementation of various algorithms. These measures were taken to ensure that the required dispersions are achieved in all phases of the mission.

Even if there are variations in the nominal numbers, the lander will still attempt a vertical landing.

The approach for landing on the surface of the lander is to go slower, lower, and closer.

To ensure the safety of the instruments on board, the lander is designed to touch down at a maximum speed of 3 m/sec (10.8 km/h). However, the optimal speed for landing is around 2 m/sec (7.2 km/hr). Additionally, the lander can safely land even with a tilt of up to 12 degrees.

While 3 m/sec may seem like a low speed, it is important to note that falling at this speed would result in severe injuries for a human. However, the sensors and measurements of the lander can guarantee this speed.

Landing at an ultra-low speed would require a significant amount of fuel, and there needs to be some velocity to ensure a successful touch down, which has been determined to be 1 m/sec. The lander's systems are built to handle speeds of up to 3 m/sec.

According to the ISRO chief, the landing speed threshold has been adjusted over time. Originally set at 2 m/sec (7.2 km/hr) with a small margin of increase, it has now been increased further. This adjustment has been made possible by the creation of an energy absorbing capability in the lander.

Once the lunar lander reaches the Moon's surface, it will deploy a rover equipped with two instruments. The rover's main tasks will be capturing images of the lunar landscape and conducting various experiments.