Data frozen on giant screens at the Mission Operations Complex at ISRO’s Telemetry Tracking and Command Centre early on Saturday are the basis on which the space agency has begun its analysis of the Vikram lander’s failure to soft-land on the Moon.
The data suggest a failure in the “Fine braking phase” in the final part of Vikram’s journey (an altitude of 5 km to 400 m), which kicked in when the lander was 5 km from the surface of the Moon.
In its statement, ISRO said that “normal performance (of Vikram) was observed up to an altitude of 2.1 km”, and “subsequently, communication from Lander to the ground stations was lost”.
The frozen screens at mission control have shown that communication was lost when the lander was barely 335 metres (0.335 km) from the surface of the Moon. The screens show that the green dot representing the lander started to deviate from the time its altitude was just above 2 km, and continued to deviate before stopping at a point that was clearly below 1 km altitude, and somewhere near or below 500 m.
At that time, the module was still moving with a vertical velocity of 59 metres per sec (or 212 km/hr) and a horizontal velocity of 48.1 m/sec (or 173 km/hr). The lander was at that point around 1.09 km from its designated landing spot on the Moon.
As per plan, Vikram should have lost most of its velocity by the time it was 400 m from the surface of the Moon, and should have been hovering above the intended landing site — set to make a soft vertical descent at “walking pace”.
“Data received at mission control showed that the landing was going as intended until the 2 km altitude. The communication link was lost when the lander was a few metres from touchdown,” a former head of an ISRO centre, who was at mission control on September 7, said.
It is likely that touchdown occurred at a much higher velocity than the “stringent touchdown requirement of 5 m/s in vertical and horizontal velocity” or less, as specified by ISRO in the planning phase. “In the final touchdown phase, the velocity should have been only 1 or 2 metres per second, something like a walking pace,” the former senior ISRO scientist said.
Early analysis suggests that Vikram began to experience a “high pitch rate” (spinning rate) after it attempted — around an altitude of 7 km — to manoeuvre into position to pick up images of the lunar surface in order to select a place to land, the scientist said.
Snatches of conversation that occurred in mission control among the mission director and the centres monitoring the parameters of the lander through the landing process, also indicated that a glitch occurred in the final “fine braking phase” of the descent.
“Please confirm the parameters,” mission director Ritu Karidhal radioed several minutes after the screens froze up indicating loss of communication with the lander. “Braking ended at around two km,” was the message radioed back to mission control.
The 15-minute process of reducing the velocity of the lander from 1680 m/sec (6048 km/hr) to zero m/sec was in the 13th minute when the screens at mission control froze. Until that point, the Navigation Guidance and Control System — working in auto mode on the basis of final data fed into its systems (including data fed around four hours before the descent, at a distance of 30 km from the Moon’s surface) — had performed precisely to plan.
A “rough braking phase” had reduced the velocity of the lander from 1680 m/sec to 146 m/sec, when it was 7.42 km from the Moon’s surface — resulting in screens at mission control showing the mission was going as per “pre flight indications”.
From 7.42 km to 5 km, the lander coasted in the “attitude and absolute navigation control phase”, reducing velocity almost to the intended 96 m/sec (346 km/hr). Around 9.52 minutes into the landing operation, Vikram’s imager is understood to have been switched on.
When ISRO Chairman K Sivan approached Prime Minister Narendra Modi to convey the status of the mission after the loss of the communication link, he said “the telemetry link has been lost to the ground”. P Kunhikrishnan, the director of the U R Rao Space Centre, told officials that “they are getting data only from Madrid (ground station), no other telemetry links”.
The braking thrust for reducing the speed of the lander from 1680 m/sec to the range of zero m/sec was being provided by four 800 N liquid fuel engines (each having eight thrusters) on board, using new “throttleable technology” developed at ISRO.
The lander was being guided to the surface of the Moon by an Inertial Navigation System onboard, in which Vikram was taking decisions by itself, without intervention from ground stations, on the basis of data obtained from cameras, sensors, and altimeters on board.
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