Lessons from 2019 play out in 18 mins
Isro chief S Somanath said scientists working on Chandrayaan-3 spent years analysing each contingency and rectifying the errors of Chandrayaan-2
When Russia’s Luna-25 crashed during its attempt to touchdown on the lunar surface on Saturday, it brought global attention on the grave challenges that even space superpowers face in missions of such magnitude.
Indian scientists were very well-versed with these challenges.
Not one Indian Space Research Organisation (Isro) scientist working on Chandrayaan-3 needed reminding that the final moments before the craft’s landing would be the most perilous. In the four years since Chandrayaan-2, they had spent sleepless nights reliving these final moments of that craft — moments that were perhaps the ones that brought them the most pain.
This time, they knew exactly what they wanted: Braking commands had to be far more precise to ensure that the autonomous landing was executed in a controlled manner and velocity, adequately dropped. And ground stations had to ensure continuous uninterrupted contact with the lander module to keep track of the progress of the descent and make any corrections, if needed.
This time, they knew how to get it done to avoid a repeat of September 2019.
This is why the landing of Chandrayaan-3 marks a pivotal milestone in India’s colossal space ambitions. It redraws the global list of space superpowers – placing India among the very elite group consisting of just three other nations that have landed a spacecraft on the Moon.
Chandrayaan-2, and the lessons from it
Speaking to reporters after the landing, Isro chief S Somanath said scientists working on Chandrayaan-3 spent years analysing each contingency and rectifying the errors of Chandrayaan-2, strengthening the hardware and software of the craft, and have prepared for worst-case scenarios.
“The unsuccessful attempt to soft land with Chandrayaan-2 has really helped us to perfect the landing methodology [for Chandrayaan-3]. We were also able to have a large number of experiments that helped us perfect the process of landing. And today, it is efforts like these that have paid dividends,” Somanath said.
“Most of the people who were involved in Chandrayaan-2 are working with us and helping us on Chandrayaan-3. And they have gone through such an agony of what went wrong [Chandrayaan-2]. They spent a year, thrashing through the data of Chandrayaan-2... The credit goes to those people,” he added.
“Four years is not a short period [of time], and we have utilised every bit of it in bettering our mission, to map all contingencies and prepare backup plans. In fact, we have prepared backups of our backup plans as well,” he told HT in an interview on Tuesday.
How the landing was executed
Chandrayaan-3’s descent started, as planned, at 5.45pm IST. The entire landing process largely comprised seven steps, with one additional step at the end that marks the release of the Pragyan rover.
During this 18-minute-long landing procedure, Chandrayaan would drop 30km in altitude, and it would fluctuate between speeds of 5,760 km per hour, to a near-still hovering at around 150m from the surface, to finally dropping down.
The first phase of the Chandrayaan-3’s descent was a phase called “rough braking”. As it was orbiting the Moon at an altitude of 30km, and around 750km away from the landing spot, all four main engines of the craft were activated, plummeting it down towards the lunar surface. In the next 11 minutes, Chandrayaan dropped nearly 23km and reduced around 4,500 kmph of horizontal speed (at which it was headed to the landing spot).
In the second phase, where altitude dropped from 7.4km to 6.8km, eight smaller thrusters fired on the spacecraft, tilting its orientation from 90° to 59° – giving it the ability to photograph the surface and identify its final landing spot.
Still barrelling towards the landing location at over 1,300kmph, the spacecraft entered its third stage, with the main rockets firing once again to reduce horizontal speed. Meanwhile, the small thrusters worked to bring the orientation of the Vikram lander to near-straight. During this stage, altitude of the craft dropped from 6.8km to 800m.
Stage four, known as “fine braking” stage, is where Chandrayaan-2 had struggled. As altitude drops from 800m and 150m, the craft uses its cameras to scrutinise an obstruction-free path to reach the selected spot to land – in the case of Chandrayaan-3, this was a 4km by 2.5km patch of highland between the Manzinus and Boguslawsky craters. Four years ago, the engines of Chandrayaan-2 ended up providing slightly more thrust than needed due to an error, which led to the craft to spinning around this time. Another issue with Chandrayaan-2 was that its target landing zone was much smaller – just 500m by 500m, giving the spacecraft very little room for error in final manoeuvre.
The Isro chief said that additions of new world-class sensors on Chandrayaan-3 helped them overcome the obstacles they struggled with the last time.
“The technology we have deployed in Chandrayaan-3 is no less complex and advanced than any other technology that goes to the Moon [by other countries]. We have the best of the sensors in the world, and we have used them in Chandrayaan-3. One of the main differences between Chandrayaan-2 and Chandrayaan-3, is an instrument has been added that is called laser doppler velocimeter. This is a world-class instrument developed by one of the labs of Isro and it is capable of measuring minute changes in velocity,” Somanath said.
Stage five was when most scientists could feel they were within seconds of history. In this stage, Chandrayaan successfully managed to drop to an altitude of 150m and then hovered for about half a minute. During this hover, it was able to make final adjustments and deviated to a slightly safer location to land.
From here, it was easy cruising.
In stages six and seven, the craft dropped to an altitude of 10m, from where thrusters powered down, dropping the lander on the lunar ground. In doing so, Somanath said, Chandrayaan-3 managed a far safer speed than they were prepared for.
“We were able to achieve most of the optimal conditions required for landing. The final landing velocity we achieved was far less than 2 metres per second (around 7km per hour), which gives us a lot of confidence that the health of the craft will be very good. This also tells us that we will be able to roll out Pragyan and conduct our experiments as planned.”
Next steps
The Vikram lander is carrying the Chandra’s Surface Thermophysical Experiment (ChaSTE) to measure thermal conductivity and temperature; Instrument for Lunar Seismic Activity (ILSA) for measuring the seismicity around the landing site; Langmuir Probe (LP) to estimate the plasma density and its variations. A passive Laser Retroreflector Array from the National Aeronautics and Space Administration (Nasa) is also accommodated for lunar laser ranging studies.
The Pragyan rover has the payload of the Alpha Particle X-ray Spectrometer (APXS) and Laser Induced Breakdown Spectroscope (LIBS) for deriving the elemental composition in the vicinity of landing site. It rolled out around four hours later without any problems.
Both modules – the lander and rover – will now activate their sensors one by one and start measurements in the hours after the landing and will continue for one full lunar day – about 14 days on Earth.
As Somanath said: “We are looking forward to very exciting 14 days for Chandrayaan-3!”
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