Bengaluru / Vikram lander went incommunicado 335 metres above lunar surface, not 2.1 km

In a new revelation, Chandrayaan-2's Vikram lander was way closer to the lunar surface than 2.1 km when it went incommunicado. ISRO sources told Express that the lander communication link was intact until 335 metres above the moon's surface.

Though ISRO scientists failed to soft-land Vikram lander safely, their valiant efforts were praised by all the global space agencies.

So far, the national space agency has been tight-lipped about what caused the last-minute glitch which denied India the credit of becoming the fourth nation to soft-land on moon's surface.

Initially, ISRO chairman K Sivan had said: "Vikram lander descent was as planned and normal performance was observed up to an altitude of 2.1 km. Subsequently, the communication from the lander to the ground centre was lost".

This gave the impression that the communication link was lost at an altitude of 2.1 km. Later on Tuesday, the agency issued a statement saying, "Vikram lander has been located by the orbiter of Chandrayaan-2, but no communication with it yet. All possible efforts are being made to establish communication with lander".

However, ISRO senior adviser Tapan Mishra, who was instrumental in building India's radar satellites, on Wednesday explained the scenario in which the lander tumbled and crashed on the moon during

the last 15 minutes of 'terror' while attempting to soft-land.

Writing a detailed post on his Facebook page, Mishra said Chandrayan 2 had five big (800 Newton) thrusters and eight small thrusters.

Thrusters are essentially small rockets, usually mono or bi-propellant based. Big thrusters are kept for braking/hovering and small thrusters are meant for orientation change and hovering.

Without directly saying thrusters of the lander malfunctioned, Mishra said: "Five big thrusters (four at corners and one in the centre), if fired equally, will combine in a vertical direction, providing opposing

force and the resultant vertical axis of the vector will pass through the centre of gravity, providing stability. If an imbalance is created by throttling four engines, i.e. by varying fuel injection rate, the resultant uncompensated horizontal force will spin the lander in the horizontal plane."

"If spinning in two orthogonal plane goes out of control, it will essentially tumble down the lander. Tumbling of the lander with thrusters on, will make things very complex, like firework burnt in Diwali, called spinning wheel or 'Charki'. The result will be simultaneous tumbling and zig-zag random motion of lander, beyond the control of the on-board control system. So throttling of the four thrusters is a critical activity," Mishra said.

He also elaborated on the depletion of fuel in the lander's fuel tank. "A very large component of the lander is the fuel tank. When lander accelerates, decelerates, because of inertia, the liquid fuel gets into sloshing, akin to splashing of water in a tub. Sloshing becomes severe as more and more fuel depletes in the fuel tank, making life difficult. It may so happen that engine nozzle feed will be starved of fuel resulting in uncontrolled throttling," he said.

Sources said the first phase of braking phase lasting from 30 km altitude to 400 meters altitude where velocity is reduced from 1.66 km/sec (6000 km/hr) to 60 m/sec ( 200km/ hour) has been successfully carried out. In this phase, the orientation of lander is changed from horizontal to vertical. Throughout this period four corner thrusters are operated to brake and central thruster is switched off.

At 400 metres height, the second phase of braking started and the communication link was lost when the lander was 335 metres above the lunar surface, sources said.