Chandrayaan-2: Is Vikram’s excess velocity towards terminal phase the villain?

Chandrayaan-2: Is Vikram’s excess velocity towards terminal phase the villain?
A representative design work of the possible soft-landing scenario of Vikram, had it been successful.
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Bengaluru: Space scientists have begun the task of digging deep into the data to map the exact cause that tripped lander Vikram just moments before soft-landing on the South Pole region of Moon early Saturday morning.

While there are remote chances of the Chandrayaan-2 mission now getting back on its feet, scientists are finding solace in the fact that the Orbiter will perform its duties beyond the expected period. They now claim that the Orbiter will be serving for seven years (2026) with enough fuel on board to perform various tasks.

The space agency says that the Orbiter will enrich the understanding of the Moon’s evolution and mapping of the minerals and water molecules in the polar regions, using its eight state-of-the-art scientific instruments.

“The Orbiter camera is the highest resolution camera (0.3m) in any lunar mission so far and shall provide high resolution images which will be immensely useful to the global scientific community. The precise launch and mission management has ensured a long life of almost 7 years instead of the planned one year,” says the space agency.

The Indian Space Research Organisation (ISRO) claimed that till date 90 to 95 per cent of the mission objectives have been accomplished and despite the loss of communication with the lander Vikram, the lunar sciences missions would continue.

Vikram lander
Left: Vikram lander during Chandrayaan 2 spacecraft integration at launch centre. Right: Control Centre at ISTRAC, Bengaluru.

Re-link efforts on

Insiders in ISRO say that efforts are on to re-establish the link with the lander Vikram and they haven’t ruled out the possibilities of a ‘revival.’

“It's a very complex mission and right now we are looking at all the available data to narrow down on the reasons. This would take time and we cannot pin-point what would have gone wrong now. There could be a single reason or a combination of multiple factors that caused the snapping of the link,” an official said.

One theory that’s gaining ground among investigators is the velocity of Vikram, after the 7.4 km descend.

“The deviation of flightpath has been noticed after the 5.5 km period. From 30 km to till 7.5 km everything was perfect. Later we noticed deviation. There some wiggles and finally lost the signals at this juncture,” an official said.

Another official too corroborated some issues during the ‘velocity kill’ phase during the power descend phase.

“Up to 2.1 km all was fine. Soon the rotation rates increased and the engine thrust went up to 100 per cent in place of around 70 per cent. The velocity too shot up indicating tumbling and crashing. At this stage, the telemetry link was lost,” says another top scientist part of the crucial mission activities.

Data critical

Sources said that the story would have been different had Vikram managed to reach beyond the 400 m hovering point above the Moon’s surface.

“The lander had a planned 12 second hovering at 400 meter and then another 22 second hovering at 100 meter. This would have been followed by a re-targeting at 60 m and at 10 m all throttle-engines (four) would have been switched off. Here the central engine was planned to be on that would have ensured a soft-landing. Now everything was perfect till 27.9 km of descend and we have to see the scenario what went wrong after that,” says an official.

He said most critical component onboard the lander – a radar-altimeter – would have come to play to assess the exact landing spot with the help of sensors.

“We will narrow-down the cause during the fault investigation phase. Even for the radar-altimeter we had built-in redundancy systems in place. It’s a matter of days before we arrive at the crucial findings,” a scientist said.

“We had to plot every bit of data including the algorithm profile. In the next 3-4 days we should have an answer. The lander after rotation (90 degree, at around 25 km) seems to have gained more velocity than required. Things might have gone wrong here,” he said.

Space agency chief K Sivan had told Onmanorama recently that all operations are done carefully with multiple rehearsals and simulations, before the final command is executed.

“Every bit of the results are analysed with extra care so that there’s no scope for any error,” he had said.

Principal Scientific Adviser K Vijay Raghavan tweeted saying that the project review teams of the space agency made presentations to mission management, Chaired by Sivan right through the early hours of Saturday onward to address the causes.

Interestingly, out of the 40 soft-landing attempts including the latest one by ISRO, only 20 had succeeded so far, pointing at the complexities of these lunar missions.

(The writer is an independent aerospace and defence journalist, who blogs at Tarmak007 and tweets @writetake.)

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