ISRO Chandrayaan-3: How India Landed on the Moon (The Engineering Behind the Feat)

At 18:04 IST on August 23, 2023, a pin-drop silence fell over the Mission Operations Complex at the ISRO Telemetry, Tracking and Command Network (ISTRAC) in Bengaluru.

Four years earlier, in September 2019, the Chandrayaan-2 lander had suffered a software and braking glitch in its final seconds, crashing into the lunar plains.

This time, as the telemetry graphs flattened and the altitude readout reached zero, ISRO Chairman S. Somanath walked to the microphone and delivered the words that sparked celebrations across 1.4 billion people:

"India is now on the Moon."

How did the Indian Space Research Organisation achieve one of humanity's most difficult aerospace feats — landing near the cratered, treacherous lunar south pole — on a budget smaller than the production cost of Hollywood movies like Interstellar or Oppenheimer?

Let us dissect the orbital mechanics, sensor systems, propulsion, and lessons learned that made Chandrayaan-3 historic.

The Mission Architecture: What Was on Board?

Launched on July 14, 2023, from the Satish Dhawan Space Centre in Sriharikota aboard the mighty LVM3-M4 rocket (formerly GSLV Mk III), Chandrayaan-3 weighed 3,900 kg and was divided into three critical modules:

┌────────────────────────────────────────────────────────┐
│               Chandrayaan-3 Integrated Craft           │
│                                                        │
│  1. Propulsion Module (2,148 kg)                       │
│     - Carried lander to lunar orbit                    │
│     - SHAPE payload (studies Earth's spectro-profile)  │
│                                                        │
│  2. Vikram Lander (1,726 kg with rover)                │
│     - 4 throttleable liquid thrusters (800 N each)     │
│     - Autonomous hazard detection cameras & sensors    │
│     - Payloads: ChaSTE, ILSA, LP                       │
│                                                        │
│  3. Pragyan Rover (26 kg)                              │
│     - 6-wheeled robotic exploration vehicle            │
│     - Payloads: LIBS (laser spectrometer), APXS        │
└────────────────────────────────────────────────────────┘

The Trajectory: The Genius of the Orbital Slingshot

NASA’s Apollo missions used the colossal Saturn V rocket to blast astronauts directly toward the Moon in just three days via a brute-force trans-lunar trajectory.

ISRO did not have a super-heavy rocket with enough thrust to push Chandrayaan-3 directly to lunar orbit. Instead, ISRO engineers used an elegant, fuel-efficient orbital mechanics strategy known as Earth-bound orbit raising using gravitational slingshots:

[Earth] ──► Orbit 1 ──► Orbit 2 ──► Orbit 3 ──► Orbit 4 ──► Orbit 5
          ▲ (Perigee rocket burns pump energy into the ellipse)
          └──────────────── Trans-Lunar Injection (TLI) ─────────► [Moon Orbit]

1. Earth Orbit Raising (July 15–August 1): The spacecraft was placed into an elliptical Earth orbit. Each time it reached perigee (the closest point to Earth), the onboard liquid engine fired for a few minutes. This pumped orbital energy into the craft, stretching the apogee (farthest point) wider and wider like pulling back a rubber band.
2. Trans-Lunar Injection (August 1): A final powerful burn hurled Chandrayaan-3 out of Earth's gravity well onto a path intercepting the Moon.
3. Lunar Orbit Insertion (August 5): When approaching the Moon at high speed, Chandrayaan-3 executed a retrograde burn (braking burn) to allow the Moon's gravity to capture it.
4. De-orbiting and Circularization (August 6–17): Gradual burns stepped the orbit down from $164 \times 18,074\text{ km}$ to an almost circular $100 \times 100\text{ km}$, and finally down to a perilune of just $25\text{ km}$ above the surface.

This trajectory took 40 days instead of 3 days, but it saved enormous amounts of propellant and weight, enabling a medium-lift rocket to deliver a heavy lander.

"19 Minutes of Terror": The Autonomous Powered Descent

Because radio signals take roughly 1.3 seconds to travel between Earth and the Moon (creating a 2.6-second round-trip communications lag), humans in Bengaluru could not pilot the lander with a joystick.

From an altitude of 30 km, the onboard computer had to make hundreds of split-second decisions completely autonomously.

The descent was executed in four distinct phases:

Phase 1: Rough Braking Phase (Altitude: 30 km down to 7.4 km)

• Velocity: Travelling horizontally at 1.68 km/s (over 6,000 km/h).
• Vikram fired four 800 N throttleable liquid engines in the reverse direction of flight.
• Over 690 seconds, the engines cancelled out nearly 80% of Vikram's horizontal orbital speed, bringing the velocity down to ~358 m/s while descending from 30 km to 7.4 km.

Phase 2: Altitude Holding Phase (7.4 km down to 6.8 km)

• The lander tilted its orientation from horizontal to 50 degrees.
• Instant laser altimeters and Doppler radar sensors scanned the lunar terrain to recalibrate height and ground speed.

Phase 3: Fine Braking Phase (6.8 km down to 800 m)

• Vikram rotated into a completely vertical posture.
• Velocity slowed to virtually zero horizontal speed, with vertical descent at ~60 m/s.
• The Lander Hazard Detection and Avoidance Camera (LHDAC) took high-resolution photos of the ground, looking for boulders larger than 12 cm or slopes steeper than 12 degrees.

Phase 4: Terminal Descent & Touchdown (800 m down to 0 m)

• The computer processed images in real time, compared the terrain to onboard danger maps, nudged the lander horizontally to find an ultra-flat patch of regolith, and throttled the engines down to gentle vertical deceleration.
• At 2 metres above the surface, the engines were cut to prevent blowing clouds of abrasive lunar dust onto the solar panels and sensors.
• Vikram dropped under the Moon's gentle gravity ($1.62\text{ m/s}^2$) and settled securely on its four reinforced titanium legs.

Why the Lunar South Pole?

Before Chandrayaan-3, all previous lunar landings (Apollo, Soviet Luna, Chinese Chang'e) occurred near the equator. Why did ISRO aim for the hostile, shadowy south pole?

1. Water Ice in Permanently Shadowed Regions (PSRs): Deep craters near the lunar poles never receive direct sunlight because the Moon's axial tilt is only $1.5^\circ$. Temperatures inside these craters plunge to $-240^\circ\text{C}$ (colder than Pluto). These "cold traps" have preserved billions of tons of water ice for over 2 billion years.
2. Fuel for the Deep Space Economy: Water ($H_2O$) on the Moon can be split into hydrogen and oxygen to manufacture rocket propellant and breathable air for future crewed bases and missions to Mars.
3. Scientific Pristine Archive: The unbaked craters hold an unaltered geological record of the early Solar System and asteroid bombardment.

What Pragyan and Vikram Discovered

Shortly after touchdown, Vikram lowered a ramp, and the six-wheeled Pragyan rover rolled onto the lunar dirt, imprinting the ISRO emblem and Ashoka Lion into the regolith.

During their active 14-day operational lunar day:

1. Unambiguous Detection of Sulphur: Pragyan’s Laser-Induced Breakdown Spectroscopy (LIBS) instrument fired laser pulses at the regolith, vaporizing surface minerals into plasma and analyzing the emitted light. It conclusively detected Sulphur (S) on the lunar surface, along with Aluminum, Calcium, Iron, Chromium, Titanium, and Oxygen. Sulphur provides critical clues about ancient lunar volcanism.
2. First Soil Temperature Profile (ChaSTE payload): A temperature probe penetrated 10 cm deep into the soil. While the surface baked at $+50^\circ\text{C}$ in the lunar sun, just 8 centimetres below the surface the temperature dropped drastically to $-10^\circ\text{C}$ — proving lunar regolith is one of the most remarkable thermal insulators in nature.
3. Lunar Micro-Seisms (ILSA payload): Vikram’s seismometer recorded tiny vibrations from rover movement and logged a suspected natural lunar seismic event.
4. The Historic "Hop Experiment": On September 3, ISRO commanded Vikram to fire its engines, lift off 40 cm above the surface, move 30 cm laterally, and land again. This demonstrated in-situ takeoff capability — the crucial prerequisite for future lunar sample-return missions (Chandrayaan-4).

Summary: A Benchmark in Global Space Exploration

Chandrayaan-3 proved that cutting-edge deep-space exploration does not require unlimited billions. With mathematical elegance, rigorous failure-mode testing, and unyielding perseverance, ISRO established India as an indisputable global aerospace superpower and opened the gateway to humanity's permanent presence on the Moon.