Chandrayaan-2 mission: So near yet so far

Several factors going wrong at the very last minutes of the Chandrayaan-2 mission led to the lander Vikram crash-landing on the moon as the nation watched with bated breath.

Published : Oct 03, 2019 07:00 IST

Scientists working on Vikram at the ISRO Satellite Integration and Test Establishment in Bengaluru on June 12.

Scientists working on Vikram at the ISRO Satellite Integration and Test Establishment in Bengaluru on June 12.

IT is a tragedy that India is unable to come to terms with. Everything had worked with clockwork precision as India’s Chandrayaan-2 spacecraft with a lander and a rover aboard travelled 3.84 lakh kilometres on its 48-day journey towards the moon. The series of manoeuvres that the Indian Space Research Organisation (ISRO) executed as the spacecraft headed towards the moon had gone off perfectly. It looked as if nothing would go wrong with ISRO’s ambitious mission to soft-land the lander, called Vikram, on the moon’s surface at 1:55 a.m. on September 7. Four hours later, the rover, named Pragyaan, was to emerge from Vikram on to the lunar surface. These were the core objectives of the prestigious mission.

At 2:43 p.m. on July 22, India’s most formidable rocket, the Geosynchronous Satellite Launch Vehicle-Mark III M-1 (GSLV-Mk III M1), lifted off from Sriharikota and put the Chandrayaan-2 mission’s composite module into a perfect, initial orbit around the earth more than 16 minutes later. The composite module consisted of an orbiter/spacecraft, Vikram and Pragyaan. Vikram sat on top of the orbiter, and Pragyaan was ensconced inside Vikram. While the spacecraft would go into an orbit around the moon, Vikram was to detach itself from the orbiter and land softly on its four legs on the moon’s South Polar region. Four hours later, Vikram’s door would open, a ramp would get deployed and the rover, which is a robotic car, would roll out on to the moon’s surface on its six aluminium wheels. The orbiter, the lander and the rover carried a total of 14 science instruments, including high-resolution cameras, to gather information on the moon, its minerals, the presence of water-ice and helium gas, the occurrence of quakes, and so on. While the orbiter will have a life of one year, Vikram and Pragyaan were to last for 14 earth days, or one lunar day. This was ISRO’s mission plan.

After GSLV-Mk III M1 put the composite module around the earth on July 22, the propulsion system on board the orbiter was fired successfully five times on different days to raise the module’s orbit towards the moon. Then, the module shot out of the earth’s orbit and started heading towards the moon. On August 20, ISRO fired the orbiter’s propulsion system again to insert the module into an orbit around the moon. As the module’s velocity decreased, the moon’s gravity captured the module into an orbit around itself. It was a tricky, crucial manoeuvre executed without any hiccup by ISRO’s ground controllers at the Mission Operations Complex (MOX) at the ISRO Telemetry, Tracking and Command Network (ISTRAC) in Bengaluru. If it had not been done correctly, the composite module would have crashed on the moon or vanished into deep space. Then, more manoeuvres were done to shrink the module’s orbit around the moon. On September 2, Vikram separated itself, without any hitch, from the orbiter and started going round the moon. There were thus two contraptions of Chandrayaan-2 separately in orbit around the moon: the orbiter and the lander with the rover inside it. For the next four days, the ground controllers at the MOX monitored Vikram’s health. It was in a perfect orbit around the moon, with a perilune of 35 km and an apolune of 100 km.

D-day was September 7. Perhaps, no scientific mission other than the Chandrayaan-2 mission fascinated the entire country and had crores of people identifying themselves with it the way the Chandrayaan-2 mission did. They were thrilled with every one of its series of successful manoeuvres. The media—print, television and online—had gone to town about the mission for several months. From September 7 midnight, crores of Indians kept awake, riveted to various television channels. Several hundred reporters from various parts of the country had gathered under a big tent near the ISTRAAC campus to watch on a big screen the events leading up to Vikram landing. Prime Minister Narendra Modi had flown from Russia to Bengaluru and was present at the MOX.

At 1:40 a.m. on the dot, when Vikram was at an altitude of about 35 km above the moon, its four throttleable engines started firing and it started descending as planned. Vikram was to complete the descent in 15 minutes and land on the moon’s surface at 1:55 a.m. Applause filled the MOX when the lander’s descent began. Vikram completed its rough braking phase and began its absolute navigation phase, which was completed without any hitch. It descended to about 33 km above the moon. As Vikram entered the fine braking phase with only 2 km to go before touchdown, it seemed to encounter a problem with its control. One of the four throttleable engines seemed to have produced a higher thrust than it should have. So Vikram’s velocity of descent started increasing instead of reducing. Vikram, therefore, tumbled out of control. Instead of turning 90 degrees before coming down to soft-land on the moon, it swung 180 degrees. It “twisted”, or deviated from its fine trajectory, came down fast and crash-landed on its side. ISRO Chairman K. Sivan, who was scanning the console in front of him, shot out of his seat. Former ISRO Chairman A.S. Kiran Kumar, too, quickly got up from his seat. Both looked crestfallen. The Chandrayaan-2 mission had failed in its core objective. Mission Director Ritu Karidhal and Project Director, Chandrayaan-2, M. Vanitha, looked pale.

There was stunned disbelief in the MOX and the mood was sombre. Gloom engulfed the country. Schoolchildren from across the country who had been selected to be present at the MOX looked disappointed. Sivan quickly strode to Modi and motioned to him with his hands that Vikram had toppled and crashed on the moon. He spoke briefly to the Prime Minister. Later, Modi hugged Sivan and patted him on his back. Sivan, who is admired for his technological prowess in rocketry, including cryogenic engines, had tears in his eyes. The Prime Minister tried to lift the mood in the MOX. He told the ISRO personnel: “There are ups and downs in life. This is not a mean achievement. The nation is proud of you…. I am with you. Move forward bravely.”

Sivan read out a terse statement: “Vikram lander’s descent was as planned and normal performance was observed up to 2.1 km. Subsequently, communication from the lander to the ground station was lost. Data is being analysed.” At a press conference on June 12 in Bengaluru, Sivan described the 15 minutes of Vikram’s descent as “the most terrifying moments”. He added: “These 15 minutes will form the most complex mission that ISRO has ever undertaken. The throttleable engines are a new [technology] development for us.” The 15 minutes exacted their toll.

The propulsion system

The propulsion system ISRO developed for the lander consisted of four throttleable engines, a first for ISRO, and one fixed steering engine. The four throttleable engines were to fire in a balanced manner and break the lander’s velocity in a controlled fashion to bring the lander down slowly for its soft landing.

Vikram’s 15 minutes of descent was totally autonomous, called “autonomous descending sequence”, executed by a computer on board. It took over at 1:40 a.m. The entire descent was to be done in four phases. The first phase was the rough braking phase when Vikram came down in a controlled manner from an altitude of 30 km to 7.4 km. The next was the absolute navigation phase when the lander descended from a height of 7.4 km to 5 km. The third was the fine braking phase when Vikram climbed down from a height of 5 km to 400 metres above the moon’s surface. From 400 m, it was the terminal, vertical descent phase to final touchdown. Each of these phases was a powered phase.

Opinion is sharply divided among ISRO engineers on whether one of the four throttleable engines produced more thrust than required, leading to an increase in the velocity of descent.

An informed ISRO engineer said: “The failure happened at the end of the fine braking phase when a manoeuvre had to be executed. We saw that something happened suddenly at the end of this phase. The horizontal phase was almost over. The next phase is hovering [of Vikram over the moon’s surface]. After that comes the identification of the landing site. Before these things could happen, there was a problem with a manoeuvre related to Vikram. We are trying to identify what failed and what has gone wrong.”

He said the velocity of Vikram’s descent had not reduced. Velocity should have been brought down to some metres a second. “At that point, there was a loss of communication and data from the lander,” he said. Since the velocity of descent did not reduce, Vikram came down fast and crashed on the moon’s surface. “There is no doubt that it crashed. It is very disappointing,” he added.

According to a former ISRO engineer, the four throttleable engines should have worked in a balanced manner during the fine braking phase. However, an engine produced a higher thrust than required, so the velocity of descent shot up. Vikram, therefore, started deviating from its path and it did so fast. Instead of turning 90 degrees to land on its four legs, the lander turned 180 degrees. The engineer said: “Owing to the twist from 90 degrees to 180 degrees, communication with the lander was lost. The higher thrust from one of the engines led to Vikram coming down fast and it crashed. It lay on its side, with its four legs jutting out.” He termed as “a face-saving strategy” ISRO officials claiming that the Chandrayaan-2 mission had achieved “95 per cent success”. It had not met its important objective of the lander soft-landing on the moon, he said.

Another top engineer asserted that what led to the disaster was “basically a software error” that occurred during the fine braking phase. “Problems will arise if the algorithm formulation is not done properly,” he said. There was an error in computer coding. “We will verify whether all the computer codes were running,” he added. “Vikram had to be manoeuvred to some degree. How it was controlled [then] is what is important. Vikram will turn. I do not know the exact angle by which it will turn. During this manoeuvre, the problem occurred…. There was a disturbance which could not be controlled,” he said.

The engineer described as “earlier thinking” the version that one of the four throttleable engines generated a higher thrust, which led to the velocity of Vikram’s descent increasing and its crash-landing. He asserted: “Vikram’s propulsion was perfect. It worked all right. Its sensors worked well. Its navigation was good. Everything worked well. But there was some software problem such as we had in ASLV [Augmented Satellite Launch Vehicle] flights [in 1987 and 1988] and the first PSLV [Polar Satellite Launch Vehicle] flight in 1993. It is definitely not a propulsion problem. It is an algorithm formulation problem. We are strong in propulsion. We are strong in sensors. We should have concentrated a bit on the software. We should have done more simulation.”

Another top ISRO official also said that both the liquid engine and the cryogenic engine on board the GSLV-Mk III M1, the propulsion system on the orbiter and the four throttleable engines of Vikram—all developed by the Liquid Propulsion Systems Centre—had worked perfectly during the mission.

A combination of several factors—that is, an “anomaly in the performance of one of the four throttleable engines’’, an error in the overall software configuration, and the sloshing around of the liquid fuel in the tank on board Vikram as the fuel was getting depleted—could have led to the crash-landing on the moon, a top spacecraft engineer said.

“When the orientation of Vikram changed drastically, its propulsion should not fire in that wrong orientation of the lander. This is the normal mission requirement. If the spacecraft goes to the wrong attitude [orientation], the propulsion should stop firing. When the lander is coming down, it should not accelerate,” he added. When a problem occurred in controlling the lander, “the control stability will not come on its own”, he explained. The stability should be created by the software. If a disturbance occurred in Vikram during its fine braking phase, the software should take care of it.

He summed up the issue thus: several things added up. The intended orientation of the lander went wrong because of an anomaly in the performance of an engine. The software was not able to take care of it. The fuel in the tank was getting depleted because Vikram had already completed two manoeuvres. The fuel in the tank was sloshing around, which created another problem. Since the software was not robust enough, the problems got compounded.

Hopes revived briefly on September 8 when ISRO was able to locate the lander, which was lying a few hundred metres close to its intended touchdown spot on the moon’s South Pole. ISRO was able to locate it with the help of the thermal images taken by a high-resolution camera on the orbiter. Despite valiant attempts, ISTRAC was not able to communicate with the lander up to September 18. Time was running out to communicate with the lander because its intended lifespan was only for 14 earth days, that is, up to September 21.

ISRO has set up a Failure Analysis Committee to find out the root cause of the problem that led to Vikram’s crash-landing. Sivan has gone on record as saying that “90 per cent to 95 per cent of the Chandrayaan-2 mission objectives have already been achieved” and that “we should not call the mission failure or a setback”. He was confident that the orbiter would have a lifespan of seven years and a half instead of its projected life of one year because “there is a lot of fuel left” on it.

What is worrying space specialists is that with ISRO fully concentrating on the Chandrayaan-2 mission in the past few months, there has been no launch of remote-sensing, communication or navigation satellites. The last lift-off that took place before the Chandrayaan-2 launch was of PSLV-C46, which put a radar-imaging satellite called RISAT-2B into orbit on May 22, 2019. “A backlog has built up. Many satellites are getting old. We are behind the requirements as far as communication satellites are concerned. There is a demand for more communication channels,” one of them said,

Big-ticket missions

ISRO will have a busy schedule in the coming years with big-ticket missions such as the Gaganyaan, which aims to send three Indian astronauts, including a woman, into space in 2022; a probe to Venus; and another mission to Mars.

As Modi noted on September 8: “On the night of September 7, 100 seconds after 1:50 a.m., an incident awakened and united the entire country. Like sportsman spirit, an ISRO spirit is running in the entire country. Countrymen are not ready to accept negativity.”

On September 17, ISRO tweeted a picture of a man with a backpack jumping from one hill to another. The picture carried the legend “Chandrayaan 2” on top and it added: “Thank you for inspiring us to always aim for the sky.” The tweet touchingly said: “Thank you for standing by us. We will continue to keep going forward—propelled by the hopes and dreams of Indians across the world!”

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