BENGALURU, India—Quiet moments of nail-biting tension gave way to cheers of joy in the Indian Space Research Organization (ISRO) mission control center as the space agency sent its lunar lander—and India—into the annals of history. On August 23 at 12:33 P.M. UTC India’s Chandrayaan-3 mission’s robotic lander, named Vikram, touched down on the moon near its south pole. Launched on July 14, Chandrayaan-3 was the result of ISRO doubling down on its bet on lunar landing after the unfortunate crash of its Chandrayaan-2 mission in 2019. With the spacecraft now safely on the moon, ISRO’s efforts have paid off, and India has become the fourth country to achieve a soft lunar landing, following the former Soviet Union, the U.S. and China.

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 would stay as close to its intended trajectory as possible, all the way until a safe touchdown. To do so, it needed to orchestrate the firing of its engines based on continuous measurements of distance, velocity and orientation.

To stick the landing this time around, ISRO built far more redundancies and safeguards into Chandrayaan-3 than it had for Chandrayaan-2. In an August 5 talk detailing these changes, ISRO’s chief S. Somanath emphasized how Chandrayaan-3 carried more fuel and a better guidance, navigation and control system to correct even major deviations from the intended paths. “There were improvements to 21 subsystems for Chandrayaan-3. These changes have been reinforced by numerous helicopter- and crane-based ground tests,” says Nilesh Desai, director of ISRO’s Space Applications Center (SAC) in Ahmedabad, India.

Evidently, these improvements have culminated in the triumphant touchdown of Chandrayaan-3. This success wasn’t a given, especially when considering that four out of the previous six lunar landing attempts within the past five years have failed. The latest failure occurred on August 19, when Russia’s Luna-25 spacecraft misfired its engines and crashed into the moon—a brutal reminder that getting to the lunar surface in one piece remains risky. Luna-25 thus joins the ruins of the Israel-based company SpaceIL’s Beresheet, India’s Chandrayaan-2 and the private Japanese firm ispace’s Hakuto-R spacecraft. Thankfully, at least Chandrayaan-3’s outcome has instead followed those of China’s Chang’e 4 and Chang’e 5 landers, the only other recent successes.

“We now have a tremendous responsibility to inspire India and the world at levels no less than this landing,” said Sankaran Muthusamy, director of the U. R. Rao Satellite Center (URSC), the ISRO center that led the construction and integration of the Chandrayaan-3 spacecraft and mission.

How Chandrayaan-3 Made It to the Moon

Chandrayaan-3’s about 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 km 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-km altitude, Chandrayaan-3 reduced its high horizontal velocity of about 1.7 kilometers per second by some 80 percent.

Next 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.

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 report anomalous heights at times if, say, a lander passes over mountainous terrain or large craters. “Instead the microwave altimeter’s wider footprint allowed Chandrayaan-3 to better tolerate abrupt changes in altitude,” explains Priyanka Mehrotra of SAC, who is lead system designer of Chandrayaan-3’s Ka-Band microwave altimeter.

Where Past Landings Faltered

Chandrayaan-3’s redundant altimetry is especially pertinent because of the role laser altimetry played during the failed April 25 touchdown of ispace’s first lunar lander. As that lander passed over the rim of the Atlas Crater to approach the target landing site that lay within, its laser altimeter correctly reported an increased elevation of roughly 3 km, corresponding to the crater’s depth. But onboard software designed to filter out certain abrupt values to keep the ispace lander’s motion stable rejected the measurement as erroneous. The Japanese lander, thinking it was closer to the surface than it really was, continued decelerating slowly until it ran out of fuel and fell to a ruinous crash landing.

It was during the attitude hold phase that Chandrayaan-2 faltered. Its engines provided a slightly greater thrust than expected because of an inadequately functioning thrust control valve, which accumulated navigation errors over time. ISRO had designed the onboard computer to correct such “off-nominal” paths only after the attitude hold phase ended. But the deviation quickly grew to be so large that the lander couldn’t correct it in time despite its ability to throttle its thrust.

In response, ISRO ensured that Chandrayaan-3 could determine and correct such deviations from its intended trajectory far faster than its failed predecessor. 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 of velocity with respect to the ground, which allows a lander to greatly reduce accumulation of navigation errors,” says William Coogan, lunar lander chief engineer at Firefly Aerospace, a private company that has partnered with NASA via the space agency’s Commercial Lunar Payload Services (CLPS) program to deliver science and technology payloads to the moon in 2024 and 2026,.

A Fine Hover or Two

After its fraught 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 and briefly hover there. This pause gave the lander a chance to capture pictures of the surface and compare them to preloaded onboard satellite images to determine whether it was above its desired landing region.

“Chandrayaan-3’s target landing zone spans four by 2.5 kilometers. ISRO scientists and engineers divided it into 3,900 equal-sized subsections, meticulously assessed the safety level of each for a landing and loaded it into the lander as reference information,” Desai says. At this point, Chandrayaan-3 must have taken one of these two decisions: If it found itself above this predetermined landing zone, the onboard computer would have identified the safest feasible subsection area, then accordingly proceeded toward touchdown. If Chandrayaan 3 found itself elsewhere, it would have proceeded with an autonomous landing based on self-identified hazards from its imagery instead of the preprogrammed subsection-based landing. Confirmation of which decision was taken will be known after ISRO determines the landing site.

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,” says Rinku Agrawal of SAC, who led the team that developed the processing unit of the hazard detection and avoidance system.

“Hazard detection and avoidance allows for a critical divert maneuver if needed during the final moments to ensure a safe touchdown,” says Ander Solorzano, flight director of aerospace company Astrobotic Technology’s first moon landing mission, which will carry NASA CLPS and international payloads.

Finally, on touchdown, sensors on the lander’s legs triggered the shutdown of its main engines. Chandrayaan-3 now stands tall on the moon.

ISRO designed the lander’s legs to absorb most of the mechanical shock from the touchdown. The agency tested the legs on lunar simulant test beds on Earth to ensure 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.

“The touchdown was smooth; the vertical velocity was notably less than even the nominal upper bound of 2 meters per second,” said ISRO chief S. Somanath in a post-landing press event.

Chandrayaan-3 landed near the lunar south pole shortly after local sunrise. Doing so maximizes the mission’s surface operations lifetime to an entire period of lunar daylight (14 Earth days) because the lander and the rover it will deploy are both solar-powered. To begin Chandrayaan-3’s surface science mission, Vikram will activate its four onboard instruments and deploy the rover via a ramp to start exploring the geologically rich landing region.

India’s Next Moonshot

Chandrayaan-3 feeds into the global frenzy of sending hardware to the moon, particularly to its south pole. The U.S.’s upcoming Artemis crewed missions, China’s Chang’e robotic craft and the majority of other governmental as well as private endeavors (such as those under NASA’s CLPS program) plan to explore this valuable lunar region. They eventually aim to extract its water ice and other resources to sustain long-duration missions and perhaps even to commercialize aspects of such operations.

It was thus quite the timing when, on June 21, India signed the Artemis Accords, a U.S.-led framework for cooperative lunar exploration. As a signatory, India can now accelerate its lunar endeavors by better collaborating with the U.S. and other signatory nations. Astrobotic CEO John Thornton says, “I’m encouraged by India’s signing of the accords. It’s certainly a signal for extended partnerships and co-developments between the two countries. The more we can do that as a species, the better chance we have of succeeding together.”

For its next moon mission—targeting launch before the end of this decade—India may partner with Japan, another Artemis Accords participant. The pair’s planned LUPEX rover would directly study the nature, abundance and accessibility of water ice on the moon’s south pole and could provide vital data for future crewed missions launched there as part of NASA’s Artemis program. “LUPEX requires a more precise touchdown with a much bigger lander. Chandrayaan-3’s success will act as a stepping stone toward India building LUPEX’s lander and thus playing a key role in the future exploration of our moon,” says S. Megala, deputy director of ISRO’s lunar science and exploration program.

First, however, India’s government must formally approve the nation’s involvement. (Japan has already given the green light for its own contribution.) And in the meantime, Japan will launch another lunar mission of its own: the nation’s Smart Lander for Investigating Moon (SLIM) is slated for liftoff on August 26, with a goal of lunar touchdown later this year to demonstrate new technologies for precise and affordable moon landings amid complex terrain.