Chandrayaan 3’s rigorous Moon landing approach reminds us of the testing regime key to future global success in space

On August 23, 2023 at 12:33 UTC, India’s robotic Chandrayaan 3 robotic lander completed the 19 minutes of its nerve-wracking autonomous descent to the Moon. In one piece and upright, it now lay near the lunar south pole at the coordinates 69.37°S, 32.32°E. The triumphant touchdown made India only the fourth nation to achieve the feat of a soft lunar landing, following the former Soviet Union, the US, and China. Success wasn’t a given, especially considering that four out of the previous five lunar landing attempts had failed: namely the Israel-based SpaceIL’s Beresheet craft, India’s own Chandrayaan 2 lander, private Japanese firm ispace’s Hakuto-R, and Russia’s Luna 25.

Launched by ISRO about four years after Chandrayaan 2’s crash on the Moon, Chandrayaan 3 saw the agency pursue and clinch its goal with a completely different approach. “Instead of a success-based design, ISRO has this time opted for a failure-based design,” said ISRO’s then-Chief S. Somanath during a press briefing. Chandrayaan 3 focused on all that can fail, and how those failures can be prevented. “We looked at sensor failure, engine failure, algorithm failure, calculation failure,” added Somanath. But when the mission’s LVM3 rocket, the Chandrayaan 3 spacecraft’s mass across its modules, and the mission’s budget as approved by the Indian government all remained roughly the same as Chandrayaan 2, how can redundancies against failures be built in for the second attempt?

This is where the Chandrayaan 2 orbiter comes in, which has been operating nominally in lunar orbit since 2019 and mapping the Moon with its many instruments. The orbiter had ended up with ample fuel margin after carrying the Chandrayaan 2 lander from Earth orbit to lunar orbit in 2019. Because of its good performance, ISRO figured it could lend some of the Chandrayaan 3 orbiter’s mass to the lander instead, which includes fuel reserves.

The 1752-kilogram Chandrayaan 3 lander, named Vikram after the originator of the Indian space program Vikram Sarabhai, was 280 kilograms heavier than its predecessor. This provided greater redundancy, such as more fuel to better stay on its intended trajectory onto the lunar surface, strengthened legs to absorb the mechanical shocks of touchdown, extended solar panels to ensure power generation in more orientations post-landing, a new velocity sensor for enhanced navigation measurements during descent, and software improvements to accommodate sensor failures. “Getting the reconfigured lander as a robust system on paper itself was crucial because you can’t really redo hardware in advanced development phases. This is where Chandrayaan 2’s flight was invaluable. Its many in-house developed systems that did work allowed us to arrive at a Chandrayaan 3 configuration that wasn’t overdone,” said Kalpana Kalahasti, the mission’s Associate Project Director, during an interview I conducted for Nature.

Kalahasti and Project Director Palanivel Veeramuthuvel led the mission through ISRO’s Bangalore-based U R Rao Satellite Center (URSC), which integrated the Chandrayaan 3 spacecraft by coordinating with various ISRO centers and suppliers across the nation. Kalahasti and Veeramuthuvel said they spent the bulk of Chandrayaan 3’s development time coming up with and overseeing comprehensive ground tests to increase the likelihood of a safe landing. These efforts spanned testing landing sensors on a helicopter to mimic different descent phases, examining the performance of engines which need to dynamically throttle, and assessing the navigation system’s ability to avoid hazards before touchdown using crane-based setups on Moon-like terrain. ISRO also conducted lander leg drop tests on lunar simulant beds and other surfaces to ensure it can handle varied slopes and a range of velocities during the crucial moment of touchdown. The agency developed a lunar soil simulant called LSS-ISAC-1 by sourcing naturally occurring, Moon-like anorthositic rock fragments from the Sittampundi Anorthosite Complex in southern India. This aided the simulant’s fidelity, and therefore built confidence in the hardware test results translating to success when at the real Moon.

ISRO then topped off the efforts with another measure. “You can only go so far with hardware tests. You can’t subject hardware to many failure scenarios, and so we formed a dedicated simulation group to characterize our lander’s ability to recover from off-nominal trajectories,” added Kalahasti. This group also simulated each redundant path the lander could’ve taken during autonomous descent to be confident about the desired outcome.

Lunar descent, mode autonomous

Chandrayaan 3’s entire lunar descent had to be fully autonomous. During this crucial stage of the mission, signals take about three seconds to go from the lander to Earth and back again—a delay too long for earthbound ISRO engineers to reliably guide the landing. So Vikram’s task was to reduce its high orbital velocity to zero, such that it stays as close to its intended trajectory as possible all the way until a soft touchdown. To do so, it needed to orchestrate the firing of its engines based on continuous measurements of distance, velocity, and orientation. Chandrayaan 3’s guidance, navigation, and control abilities were thus improved. “There were improvements to 21 subsystems for Chandrayaan 3. These changes have been reinforced by numerous helicopter- and crane-based ground tests,” said Nilesh Desai, the then-director of ISRO’s Space Applications Center (SAC) in Ahmedabad, for an article I wrote for Scientific American.

Vikram’s roughly 19-minute-long lunar descent comprised four major phases. The first, the “rough braking” phase, began when the spacecraft was 30 kilometers above the Moon in its orbit and about 750 kilometers downrange from its landing site. By firing all of its four 800-newton main engines for about 12 minutes until it was at a 7-kilometer altitude, Chandrayaan 3 reduced its high horizontal velocity of about 1.7 kilometers/second by some 80 percent. Next up came a brief but crucial 10-second “attitude hold” phase, wherein the lander stabilized itself using its eight smaller thrusters to gain a steady view of the looming lunar surface for its various landing sensors. Here, Chandrayaan 3 could throttle its engines and correct deviations from its intended trajectory more quickly than Chandrayaan 2.

For height measurements, Chandrayaan 3 relied on two altimeters, one using lasers and the other using microwaves. While laser altimeters are commonly employed by several lunar landers, they can be prone to failures or report anomalous heights at times if, say, a lander passes over a mountainous terrain or a large crater. Dysfunctional altimeters have led to crashed landings for ispace Japan’s first and second Mooncraft as well as hard landings for US-based Intuitive Machine’s IM-1 and IM-2 missions, all in this decade itself. Chandrayaan 3 would benefit from a redundancy here. Unlike laser rangefinders, “the microwave altimeter’s wider footprint allowed Chandrayaan 3 to better tolerate abrupt changes in altitude,” explained Priyanka Mehrotra of SAC, who was the lead system designer of Chandrayaan 3’s Ka-Band microwave altimeter.

Chandrayaan 3’s lander also used a new instrument called a Laser Doppler Velocimeter (LDV) to navigate more precisely in the first place. “While there are other ways for a lunar lander to measure its velocity, an LDV provides a direct measurement with respect to the ground, which allows a lander to greatly reduce accumulation of navigation errors,” said William Coogan, lunar lander chief engineer at Firefly Aerospace, a US-based company which landed the Blue Ghost spacecraft on the Moon for NASA’s CLPS program later in 2025. However, in a post-landing interview with Republic World, Somanath said that Chandrayaan 3’s descent and landing went so nominally that the LDV didn’t end up being necessary. The development nevertheless enables ISRO to use the advanced sensor on upcoming missions like Chandrayaan 4 sample return and Chandrayaan 5 / LUPEX, which have greater precision requirements.

After the attitude hold phase, Chandrayaan 3 entered a three-minute “fine braking” phase in which it used only two of its four main engines to descend up to roughly 850 meters above the Moon’s surface. It then briefly hovered there, so as to capture pictures of the surface and compare them to preloaded onboard satellite images to determine whether it was above its desired landing region. The preloaded maps were primarily sourced from the Chandrayaan 2 orbiter and NASA’s Lunar Reconnaissance Orbiter (LRO). High-resolution images from them were also used to select Chandrayaan 3’s landing site. “The target landing zone spans 4 by 2.5 kilometers. ISRO scientists and engineers divided it into 3900 equal-sized subsections, meticulously assessed the safety level of each for a landing, and loaded it into the lander as reference information,” noted Desai.

Triumphant touchdown

After hovering over the surface and comparing snapped images to preloaded maps, the Vikram lander determined that it was indeed above the intended landing zone. Its onboard computer then identified the safest feasible subsection area and accordingly proceeded toward touchdown. Had Vikram found itself elsewhere, it would’ve still proceeded with an autonomous landing but based on self-identified hazard levels from captured imagery instead of a predetermined subsection.

In the final “terminal descent” phase, Chandrayaan 3 lowered itself to about 150 meters above the surface and then hovered again for about half a minute to assess the area below for landing hazards. At this point, since the surface right below the lander didn’t look safe, the lander sought a safer adjacent area and deviated to touchdown there. “The processing system for hazard avoidance was sped up for Chandrayaan 3 to make the lander’s decision-making during the critical final phases significantly faster than Chandrayaan 2,” said Rinku Agrawal of SAC, who led the team that developed the processing unit of the hazard detection and avoidance system.

Finally, Chandrayaan 3 made contact with the Moon’s surface. Sensors on the lander’s legs detected the same and triggered the shutdown of its main engines. ISRO designed the legs to absorb most of the mechanical shock from the touchdown. The agency’s leg design tests on lunar simulant test beds on Earth ensured that the lander could tolerate a high vertical velocity of three meters per second—and even a horizontal velocity of one meter per second if it were to touch down askew. Vikram’s touchdown was smooth, hitting the surface with a vertical velocity of 0.983 meters/second, notably less than the nominally expected upper bound of 2 meters/second. The final location of 69.37°S, 32.32°E placed Chandrayaan 3 just ~350 meters away from its targeted ideal spot.

Chandrayaan 3’s landing was the most pole-ward at the time. The near-polar lunar surface and environment had never been measured in-situ before Chandrayaan 3 arrived. Chandrayaan 3 was intentionally landed a day after local dawn to maximize the solar-powered lander and rover’s surface operations lifetime to most of the lunar daylight period of 14 Earth days. ISRO powered on the lander’s three active instruments on August 24, each of which returned valuable data for 11 days since. From August 25, the rover began exploring the geologically rich landing region and traversed 100 meters on the lunar surface over 10 Earth days. As usual, r/ISRO subreddit admin u/Ohsin had compiled a detailed rundown of post-landing events, something which ISRO did not adequately, contextually provide or even list later on in one place. Three years after Chandrayaan 3’s lunar surface operations, science from the mission has grounded our understanding of a distinct region on the Moon just as we prepare to send a wave of lunar missions.

Meticulous Moonshot

Chandrayaan 3 should be remembered for how the team meticulously emerged from failure. Moon landings remain hard even on retries, evident by the failed soft landing of Intuitive Machines’ second lunar lander IM-2 and the crash of ispace Japan’s second Hakuto-R craft, both flying after Chandrayaan 3 and within two years of their respective first missions. At a critical time, Chandrayaan 3 kept the momentum for the Moon going by feeding into the global frenzy of sending robotic missions to Luna for long-term space exploration. “India giving the hard problem of a lunar landing a second try soon after its first attempt is an appreciated investment the whole world will benefit from,” said Jessy Kate Schingler, an outer space policy researcher and a senior advisor at the Open Lunar Foundation (a Moon Monday sponsor).

Chandrayaan 3’s tedious developmental efforts evidently paid off. However, conducting so many tests and integrating their results while also working towards the flight model was a giant, iterative task. Swiftly coordinating the dozen ISRO centers involved countrywide wasn’t easy. “It was as if we were building five to six different satellites together,” remarked Kalahasti. To take on the responsibility, Kalahasti leaned on her past experiences in project management and systems engineering, which notably included leading roles in ISRO’s Earth observation satellites such as the Indo-French SARAL craft. “The goal was to have a well documented, well understood system. There was no compromise in demonstrating the system’s performance down to its specifics,” said Kalahasti, who represents not just herself but Mehrotra, Agrawal, the over 100 such women who worked on the mission, and the nation-wide Chandrayaan 3 team. India now celebrates August 23 as National Space Day.

The world is now mounting numerous Moon missions to directly explore the treacherous lunar south pole and its water ice. In this new frontier, Chandrayaan 3’s rigorous testing approach must remain a reminder of not leaving success to chance as much as possible. There is another important reason to remember Chandrayaan 3. The country is facing an orbital launch crisis this decade and next while its national needs and ambitions have scaled and spanned across the spectrum. And so the time is here again to reinforce, scale, and apply Chandrayaan 3’s diligence to the most fundamental technology driving space activities.

The article is a synthesized and edited composition of my coverage of Chandrayaan 3 across Moon Monday, Scientific American, Nature, and Indian Space Progress, done so to provide a complete picture of the most critical aspect of the mission: the nail-biting descent and landing.