15 mins of terror: How Chandrayaan-3 will make its final descent on Moon on Aug 23

Fifteen minutes of terror await Chandrayaan-3 on August 23. From a height of 30 kilometres, the craft will soft-land in about 15 minutes. Landing on the Moon is a tricky affair. Since it is devoid of air, a parachute or such contraption cannot be used to reduce the speed of the descending craft. Four rockets in the underbelly of the lander have to be used to reduce the rate, change the lander's orientation, and make it soft-land. Seven crucial manoeuvres will be performed to bring the craft safely down to the lunar surface.

A to B: Rough Braking Phase

Objective: To reduce the horizontal speed of the craft for a soft-landing.

Duration: 690 seconds

Events:

• The ship descends from 30 km to 7.4 km above the lunar surface.

• The legs of the vessel are horizontal.

• The four rockets in the underbelly of the CY3 burn. As the exhaust gas flows towards the direction of the craft’s motion, an impulse is generated against the direction of the motion.

• Impeded by the resisting impulse, the craft's horizontal speed drops from about 6,000 km per hour to approximately 1,288.8 kmph. It gains vertical speed from zero to 219.6 kmph.

• The craft starts about 745 km from the target landing location. By the end of the event, the ship is just 32 km from the landing site.

• The craft uses the navigational instrument to sense its location on the lunar surface and navigates towards the landing site.

• All four engines are active.

• Micro-Star sensor and the Lander Position Detection Camera (LPDC) provide navigational information.

 B to C: Attitude Holding Phase

Objective: To reduce the vertical velocity, turn the craft's orientation.

Duration: 10 seconds

Events:

• Horizontal velocity drops from about 1,288.8 kmph to 1,209.6 kmph; vertical velocity drops from 219.6 kmph to 212.4 kmph

• The craft comes down from 7.4 km to 6.8 km above the lunar surface.

• The craft's orientation is tilted by about 50 degrees, with the legs pointing sideways.

• At the end of the phase, the vessel is just 28.52 short of the landing site.

C to D: Fine Braking Phase

Objective: To decelerate the craft and bring it to a grinding halt at about 800 metres above the lunar surface.

Duration: 175 seconds and hovering for about 12 seconds at 800 metres altitude.

Events:

• The craft comes down from 6.8 km to about 800 metres above the lunar surface.

• The craft's horizontal and vertical velocities at the endpoint are zero. The craft hovers above a point on the lunar surface. It moves neither up nor down, front or back, or sideways.

• The craft hovers above this point for 12 seconds.

• The craft’s orientation is now 90 degrees turned, with the legs pointing downwards.

• The craft reaches the landing site and is almost on top of it.

• The hazard detection and avoidance manoeuvre is performed. The AI-powered software identifies a safe location within the landing site, devoid of large boulders, deep holes, and steep slopes.

• Laser Altimeter (LASA) and Ka-Band Altimeter (KaRA) measure the craft's altitude. The Lander Position Detection Camera (LPDC) provides information on its location. Laser Doppler Velocimeter (LDV) measure the speed of the ship. The craft navigates towards the landing site using these parameters.

 D to E: Terminal Descent Phase 1

Objective: To identify a safe landing spot

Duration: 131 seconds to descend and 22 seconds to hover 150 metres above the ground.

Events:

• The craft slowly descends to 150 metres above the lunar surface.

• The Moon’s gravity pulls the craft downwards, while the ground-facing rockets add an impulse in the opposite direction. By increasing and decreasing the throttle of the rockets, the descent’s speed is controlled. The craft descends at a leisurely 18 kmph.

• It takes 131 seconds for the craft to come down from about 800 to 150 metres.

• During this phase, the landing site is investigated with several instruments. The hazard avoidance camera is hyperactive.

• Hovering above the point for about 22 seconds, the AI-powered system scans the surface to identify safe spots for landing within a 150-metre radius of that point.

• The Lander Hazard Detection & Avoidance Camera (LHDAC) becomes active to identify hazards in the landing site and suggest multiple safe spots.

• Safe spots will be compared, and the best among them will be chosen.

 E to F: Terminal Descent Phase 2

Objective: Bring the craft to a 60-metre altitude and choose a safe landing location.

Duration: 52 seconds.

Events:

• At the commencement of this event, the AI-powered software identifies safe and viable landing spots.

• The throttle of the rockets will be modulated to give a net downward force. As the craft descends slowly, it will also move sideways to reach the spot above the chosen landing spot.

• The hazard-detection and navigation systems and several instruments that measure the rate of descent come into action.

• The Laser Doppler Velocimeter (LDV) and Lander Horizontal Velocity Camera (LHVC) help check the speed of the descent.

 F to G: Terminal Descent Phase 3

Objective: Reduce the speed of the craft for a soft-landing

Duration: 38 seconds

Event:

• The ship will descend straight down towards the chosen landing spot.

• Once again, the throttle of the rockets will be modulated to make the craft descend slowly. The vessel will move at a crawling speed of 4.7 kmph.

• The craft will come from 60 to 10 metres above the lunar surface.

• Upon reaching 10 metres above the lunar surface, all the lander's engines will be shut down.

G to H: Terminal Descent Phase 4 (Free Fall)

Objective: To land the lander safely on the lunar surface.

Duration: 9 seconds

Events:

• Once the craft is 10 metres above the lunar surface, all rockets will be shut down, and the Moon's gravity will be allowed to take over. The ship will fall towards the lunar surface like a stone.

• Upon touching the ground, the sensors on the lander's legs will be triggered.

• The ISRO ground control in Bengaluru will have confirmation after about 1.25 seconds of the touchdown.

• The craft's fall will trigger a puff of dust to rise from the lunar surface.

H: On Lunar Surface: Rover Rolls Out

Objective: To bring the rover to the lunar surface

Duration: A few hours after the landing

Events:

• The freefall landing will result in a dust envelope rising from the ground. The craft will wait for a few hours until the dust settles.

• After a few hours, one of the walls of the lander craft will unfold and drop down. It will become a slanting ramp for the rover to roll out.

• The rover will be slowly released to the ground using a pulley system. Once the rover touches the lunar surface, the rope will be disconnected.

• The lander will take an image of the deployed rover, and the rover will capture a photo of the lander. These images will tell us that the lander and the rover are healthy.

Image credit: Wikimedia Commons