If there’s one thing that Russia’s Luna-25 mission and countless other space missions before it has shown, it is that a soft landing on the Moon, especially around its poles, is a tricky thing to do.
With Chandrayaan-3’s Vikram, ISRO plans to do the impossible – be the first space mission to successfully execute a soft landing procedure on the Moon’s south pole. But how exactly is it planning to go about this?
On August 23, during its last 15 minutes of attempting to land on the Moon softly, the lander has a tricky move to pull off. It must shift from flying horizontally at high speed to being vertical, setting itself up for a gentle descent onto the lunar surface.
Everything will have to be done within 15 minutes this Wednesday evening, and every step during these 15 minutes will be a make or break for the whole mission. In July 2019, when the first try for the Chandrayaan-2 mission got canceled by the Indian Space Research Organisation (ISRO), K Sivan, the big boss at India’s space research organization at the time, called this phase “15 minutes of terror.”
“Chandrayaan-3 is tilted almost 90 degrees at this time (when the landing process begins at 5.47 pm on August 23), but it needs to be vertical (for a landing). This process of turning the lander…is an exciting calculation mathematically. We have done a lot of simulations. This is where we had a problem the last time (which led to the crash of Chandrayaan-2 on September 7, 2019),” ISRO Chairman S Somanath told reporters a few days ago.
“The transfer from the horizontal to the vertical position…is the trick we have to play over here. We have to ensure that the fuel consumed is less, the distance calculation is correct, and all the algorithms are working properly,” he added.
How the landing is supposed to go
The lander is moving horizontally at a blazing 6048 km/h. The first big challenge is to get that horizontal speed down to 1,290 km/h. At the same time, the vertical speed needs to be brought down to 61 m/s, or about 220km/h.
This maneuver, known as the “rough braking phase,” should ideally take about 690 seconds. During this phase, the lander will make its way down from a height of 30 kilometers to 7.42 km above the Moon’s surface. Along the way, it’s covering a distance of about 713.5 km across the lunar landscape, heading toward its landing spot.
When the lander gets to 7.42 km up, it will enter the “attitude hold phase.” This stage lasts about 10 seconds. During this time, the lander has to change its orientation from horizontal to vertical. And while it’s doing this twist, it will move about 3.48 kilometers across the Moon’s surface. The altitude drops slightly more to 6.8 km, and the speeds decrease to 336 m/s (horizontal) and 59 m/s (vertical).
Now, we move to the third phase, the “fine braking phase.” This is about a 175-second stretch where the lander ultimately transitions to a vertical position. It’s the final approach, covering the last 28.52 km across the Moon’s surface to the landing site. At this point, the altitude gets down to around 800-1,000 meters, and the speed ideally drops to a calm 0 m/s.
What did Chandrayaan-2’s failure teach ISRO?
The crucial point where things went wrong for the Chandrayaan-2 lander was right after the “attitude hold phase” and before the “fine braking phase.” It was here that control was lost, and the unfortunate crash occurred. The Indian Space Research Organisation (ISRO) has taken the lessons learned from that failure and applied them to improve the landing prospects for Chandrayaan-3.
The design is so robust that even in worst-case scenarios where all sensors and systems seem to fail, the lander will still attempt landing as long as the propulsion system functions correctly. That’s how solidly it’s engineered. The design accounts for such contingencies.
If, for instance, a couple of engines don’t work, the lander is designed to handle it and still make a successful landing. It’s all about resilience. As long as the algorithms do their job as intended, the Chandrayaan-3 lander should be capable of pulling off that vertical landing they’re aiming for.
Landing impact could kill a human being.
Even if things go well, the Vikram Lander will land with such a force that had it been a human, all of its bones would have been crushed.
When the lander contacts the lunar surface, it’s designed to do so with a maximum speed of 3 meters per second, equivalent to about 10.8 km/h. This landing speed ensures the safety of the instruments on board so they won’t get damaged upon touchdown. However, the sweet spot for an ideal landing is around 2 meters per second, roughly 7.2 km/h.
Additionally, the Vikram Lander module and the Pragyaan Rover have a combined mass of about 1750 kgs. That’s well over 2000 kgs, even on the Moon.
“Although three m/s looks like a low speed, all our bones will be crushed if a human falls at that speed. But it is a speed we can guarantee with our sensors and measurements. Attempting to land at ultra-low speed also requires a lot of fuel, and theoretically, there has to be some velocity to touch down, which has been identified as one m/s. The system is built to handle velocity of up to 3 m/s,” the ISRO chief had said.
The lander is built to handle a certain degree of tilt during landing. It can tilt up to 12 degrees and still manage a safe landing. This flexibility in running tilt adds an extra assurance that the landing process can accommodate some variability while ensuring a successful and safe touchdown.