As the space agency sent its lunar lander—and India—into history, quiet moments of nerve-wracking apprehension gave way to cries of elation in the Indian Space Research Organisation (ISRO) mission control centre. The robotic lander called Vikram from India’s Chandrayaan-3 project splashed down on the moon near its south pole on August 23 at 12:33 PM UTC. After the catastrophic Chandrayaan-2 mission crash in 2019, ISRO decided to stake more money on lunar landings, which led to the launch of Chandrayaan-3 on July 14.With the spacecraft now securely on the moon, ISRO’s efforts have paid off, and India has joined the United States, China, and the erstwhile Soviet Union as the fourth nations to successfully make a soft lunar landing.The whole lunar descent of Chandrayaan-3 had to be independent. Signals from the lander to Earth and back again during this key phase of the mission take around three seconds, which is too long for ISRO engineers on Earth to accurately direct the landing. In order to keep it as near to its original trajectory as possible all the way to a safe descent, Vikram had to lower the spacecraft’s high orbital velocity to zero. It had to coordinate the starting of its engines based on ongoing observations of altitude, speed, and direction.

Chandrayaan-3 has many more redundancy and safeguards than Chandrayaan-2 did in order to successfully land this time. ISRO’s director S. Somanath highlighted how Chandrayaan-3 carried more fuel and a stronger guidance, navigation, and control system to rectify even significant deviations from the intended routes in a lecture outlining these advancements on August 5. For Chandrayaan-3, 21 subsystems saw upgrades. Numerous ground experiments using helicopters and cranes have helped to solidify these adjustments, according to Nilesh Desai, head of the ISRO’s Space Applications Centre (SAC) in Ahmedabad, India.Evidently, these advancements have resulted in Chandrayaan-3’s successful touchdown. This achievement wasn’t certain, particularly in light of the fact that four of the previous six lunar landing attempts during the preceding five years have ended in failure. A cruel reminder that reaching the lunar surface unharmed is still dangerous occurred on August 19 when Russia’s Luna-25 spacecraft misfired its engines and crashed onto the moon. Thus, Luna-25 joins the wreckage of the Israeli corporation SpaceIL’s Beresheet, the Indian spacecraft Chandrayaan-2, and the Hakuto-R spacecraft built by a private Japanese business, ispace. The only other recent achievements, China’s Chang’e 4 and Chang’e 5, were followed, thankfully, by Chandrayaan-3’s performance.

“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

The roughly 19-minute lunar descent of Chandrayaan-3 was divided into four main segments. The “rough braking” phase, the first, started when the spacecraft was 750 kilometres away from its landing spot and 30 kilometres above the moon in its orbit. Chandrayaan-3 lowered its high horizontal velocity of roughly 1.7 kilometres per second by around 80% by firing all four of its 800-newton main engines for about 12 minutes, until it was at a 7-km altitude.

After then, the lander stabilised itself for 10 seconds during the vital “attitude hold” phase so that its numerous landing sensors could have a stable image of the approaching lunar surface.Chandrayaan-3 used two altimeters, one that used lasers and the other that used microwaves, to measure height. Although many lunar landers use laser altimeters, these devices occasionally show unusual heights, for example when a lander crosses a hilly area or a large crater. As the principal system designer for Chandrayaan-3’s Ka-Band microwave altimeter at SAC, Priyanka Mehrotra says, “Instead, the microwave altimeter’s wider footprint allowed Chandrayaan-3 to better tolerate abrupt changes in altitude.”