Targeting the lunar south pole, Chandrayaan-3 aims to explore a region rich in water ice, holding potential as a vital source of oxygen, fuel, and water for forthcoming lunar missions or even a sustained lunar outpost. Successful landing would grant Chandrayaan-3 an estimated operational span of two weeks, during which it will conduct an array of experiments, including mineral composition analysis using a spectrometer on the lunar surface.
Standing at approximately 2 meters tall and weighing just over 1,700 kg (3,747.86 lb), the Chandrayaan-3 lander bears a resemblance to an SUV’s dimensions. Its design includes the deployment capacity for a smaller lunar rover, weighing 26 kg.
On August 23, during the last 15 minutes of its mission, the Chandrayaan-3 lander from the Indian Space Research Organisation (ISRO) will need to execute a crucial technical maneuver. This maneuver involves transitioning from a high-speed horizontal position to a vertical one, which is essential to enable a smooth and controlled descent onto the lunar surface.
Having completed the final deboosting process on August 20, which effectively lowered the Chandrayaan-3 lander’s orbit around the Moon to 25 x 134 km, the primary focus now shifts to the impending final landing phase set for the evening of August 23. Success during this critical phase, where the Chandrayaan 2 mission previously encountered difficulties, is pivotal for the overall success of the Chandrayaan-3 mission.
In contrast to its forerunner, Chandrayaan-3 lacks an orbiter module. Its primary objective is to achieve what Chandrayaan-2 couldn’t: executing a successful lunar south pole landing and conducting topographical investigations using a rover. Recognising that landing-related damage could jeopardize the mission, ISRO has taken exceptional caution, building on the lessons learned from the Vikram lander mishap. Employing a “failure-based design,” ISRO has comprehensively addressed potential failure points, including sensors, engines, algorithms, and calculations. Furthermore, the designated landing area has been expanded fourfold compared to Chandrayaan-2, offering a wider margin of error to accommodate unforeseen changes in conditions. With these considerations in mind, ISRO aims to accomplish a flawless soft landing, a feat that would elevate India to a select group alongside the United States, China, and the former Soviet Union.
The lander possesses the capability to execute a gentle touchdown at a designated lunar location and subsequently deploy a rover. This rover will conduct on-site chemical analyses of the Moon’s surface as it navigates. The lander incorporates three distinct payloads:
RAMBHA-LP (Langmuir Probe): Designed for the measurement of near-surface plasma density, including ions and electrons, along with tracking changes over time.
ChaSTE (Chandra’s Surface Thermophysical Experiment): Tasked with measuring thermal characteristics of the lunar surface in proximity to the polar region.
ILSA (Instrument for Lunar Seismic Activity): Intended to gauge seismic activity in the vicinity of the landing site and to provide insights into the structure of the lunar crust and mantle.
Landing parameters for Chandrayaan 3
On Sunday, the Chandrayaan-3’s Lander Module successfully completed its final deboosting maneuver, resulting in an orbit of 25×134 km around the Moon, according to Isro. The powered descent is scheduled to commence at 5:45 pm on August 23, encompassing four main phases: the Rough Braking Phase, the Attitude Hold Phase, the Fine Braking Phase, and the Terminal Descent Phase.
Let us understand the Landing parameters for Chandrayaan 3 in four broad phases:
Rough braking phase
On August 9, ISRO chairman S Somnath said, “as the landing procedure initiates at 17:47 pm on August 23, the Chandrayaan 3 is almost perpendicular to the ground, at an angle of around 90 degrees. However, for a successful landing, it must assume a vertical orientation. The intricate process of transitioning the lander from horizontal to vertical is a fascinating mathematical calculation. We have extensively conducted simulations to refine this procedure. It was at this very juncture that we encountered challenges during the previous attempt, resulting in the crash of Chandrayaan 2 on September 7, 2019, upon the lunar surface.”
The pivotal maneuver we must execute here is transitioning from a horizontal to a vertical position to facilitate the landing. It’s a delicate balance where we strive to minimise fuel consumption, ensure accurate distance calculations, and verify the flawless functioning of all algorithms, he added.
Chandrayaan 3’s lander maneuverability relies on 12 onboard engines to control its speed and direction. Among these, four engines are responsible for velocity reduction, while eight smaller engines manage the descent’s trajectory. These engines offer adjustable thrust, ranging from 800 Newton to lower values, even allowing the lander to hover under Moon’s gravity, as explained by the ISRO chairman.
At the outset of the landing process, the horizontal velocity starts at nearly 1.68 km/sec or approximately 1680 m/sec, while vertical velocity remains zero. In the ideal “Rough braking phase” lasting 690 seconds, the objective is to bring down the horizontal velocity to 358 m/sec and vertical velocity to 61 m/sec. During this phase, the lander descends from an altitude of 30 km (at a distance of 745.5 km from the landing site) to 7.42 km, covering a total distance of 713.5 km across the lunar surface toward the designated landing area.
Attitude Hold Phase
Upon reaching a height of 7.42 km above the lunar surface, the lander will initiate an “Attitude Hold Phase,” spanning approximately 10 seconds. During this interval, the lander will execute its initial tilt, transitioning from a horizontal to a vertical orientation. This maneuver will transpire while advancing a distance of 3.48 km, lowering its altitude from 7.42 km to 6.8 km. Simultaneously, the velocity will decrease to 336 m/sec (horizontal) and 59 m/sec (vertical).
Fine braking phase
The third landing phase on the lunar surface, referred to as the “Fine Braking Phase,” will endure approximately 175 seconds. During this phase, the lander will seamlessly transition to a fully vertical position while covering the remaining 28.52 km distance to the intended landing site. Altitude will diminish from 6.8 km to 800/1000 meters, while maintaining a nominal speed of zero m/sec.
“At altitudes spanning from 30 km down to 7.42 km, the process will involve rough braking. At 7.42 km, the attitude hold phase will take place, enabling certain instruments to perform calculations. As the altitude reaches 800 or 1300 meters, the sensors will undergo verification. At 150 meters altitude, a hazard assessment will occur. The lander will then decide whether to execute a vertical landing at that point or adjust laterally up to 150 meters to avoid potential boulders or craters,” added by Somnath.
Terminal descent phase
The critical phase between the second “Attitude Hold Phase” and the subsequent “Fine Braking Phase” marked the point where the Chandrayaan 2 lander encountered a loss of control, leading to its unfortunate crash before it could enter the conclusive “Terminal Descent Phase.”
Drawing on the lessons gleaned from the Chandrayaan 2 setback, ISRO has substantially enhanced the likelihood of Chandrayaan 3’s successful landing.
One notable improvement is the shift from a first-order automated guidance system utilised in Chandrayaan 2’s initial rough braking phase to a second-order guidance system in Chandrayaan 3. Furthermore, Chandrayaan 3 incorporates instantaneous thrust regulation during its rough braking phase.
The lander has the capability to achieve a gentle touchdown with a maximum speed of 3 m/sec (equivalent to 10.8 km/hr) without posing any risk to the onboard instruments. However, the preferred and most optimal speed for landing is approximately 2 m/sec (equivalent to 7.2 km/hr). The lander is designed to accommodate a tilt of up to 12 degrees to ensure a safe and secure landing.
After the soft landing, what will be the role of Vikram?
“The initial design of Chandrayaan 2 targeted a landing speed of 2 m/sec (equivalent to 7.2 km/hr) with a slight allowance for variation, but we have now elevated the acceptable landing speed. We have incorporated an energy-absorbing capability,” stated the ISRO chairman in recent remarks.
“Our prime objective is to achieve a gentle and secure landing. The functionality of none of the instruments would be viable in the event of a crash. The mission involves five experiments – three on the lander and two on the rover. These experiments will only yield results if the landing is safe and smooth,” he added.
Upon successful settling onto the lunar surface, the lander will deploy a rover it carries. This rover is equipped to capture images of the moon’s surface and conduct experiments using two onboard instruments.
