All was set for the lander, Vikram, of India’s Chandrayaan -2 mission to touch down gently on the moon’s surface at 1.55 a.m. on September 7, but it crash-landed instead. The 15 minutes of “terror” that the Indian Space Research Organisation (ISRO) was anxious about in its audacious mission ultimately ended up breaking the hearts of crores of Indians.
At 1.40 a.m., the lander began its descent at an altitude of 30 km from the surface of the moon with the activation of its autonomous landing sequencer and ignition of four throttleable engines that control the descent. The descent and eventual soft touchdown on the South Polar region of the moon were to take 15 minutes.
The 1.4-tonne lander with a rover, named Pragyaan, inside had descended in copybook fashion to an altitude of 2.1 km from the moon during what is called its rough braking phase. A top ISRO official told Frontline , “Vikram’s descent was very good during the rough braking phase. During the transition from the rough braking phase to the fine braking phase, there was a problem. The velocity of the descent could not be controlled. We are trying to identify what failed and what went wrong. It is very disappointing.”
The official said the lander would have crash-landed on the moon’s surface, a point that he emphasised more than once. “There is no question of its soft-landing. From the data available, it is clear that it has crashed,” he said. “We have a lot of data available and they are being analysed. In a couple of days, we will be able to come out with what the problem was,” he added. The data included telemetry data, including on-board data from the lander.
Another top official of ISRO said a committee consisting of specialists in propulsion, control, guidance, navigation and sensors had been set up to identify the precise reason why contact was lost with the lander when it was only 2.1 km above the moon’s surface. He added: “In a few more minutes, we would have touched down on the lunar surface. What is the reason why no data were available when the lander had only 2.1 km to come down we have no clue as yet. However, enormous amounts of data are available and we will analyse them to find out what exactly went wrong.” He also stressed that a glitch occurred during the transition phase of the lander from the rough braking phase to the fine braking phase.
That something had gone wrong was clear when ISRO Chairman K. Sivan shot out from his seat in the Mission Operations Complex at the ISRO Telemetry, Tracking and Command Network (ISTRAC) in Bengaluru. He strode towards where Prime Minister Narendra Modi was seated, rolling his hands over each other as if to say that the lander had crashed and toppled. Modi consoled and hugged him and repeatedly patted him on his back. Sivan and ISRO engineers looked crestfallen. Modi told them: “There are ups and downs in life. This is not a small achievement. The nation is proud of you. I congratulate you. All of you have done a big service to the nation. I am with you. Move forward bravely.”
Some minutes later Sivan read out a terse statement, which said, “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.” He cancelled a press conference he was scheduled to address around 8 a.m.
Everything had progressed smoothly with ISRO’s 48-day mission aimed at putting a rover on the surface of the moon. An orbiter, or spacecraft, carrying a lander with a rover inside it had been put into an orbit around the moon, and the lander had separated from the orbiter and it was to touch down softly on the South Polar region of the moon and lower its ramp for the rover to emerge from it and roll on to the lunar surface to do science experiments.
The orbiter, the lander and the rover constituted the composite module. ISRO’s most formidable rocket, the Geo-synchronous Satellite Launch Vehicle (GSLV-Mark III M1), lifted off from its launch pad at Sriharikota on July 22 and put the composite module into a precise earth-bound orbit 16 minutes later. Then began the composite module’s 3.84 lakh-km journey to the moon. A propulsion system on board the orbiter was fired five times to make the composite module’s orbit around the earth more elliptical. The ignition of the propulsion system on board the orbiter for the fifth time shot the composite module out of the earth’s orbit and put it on its journey towards the moon. Everything went smoothly with all these five manoeuvres.
On August 20, the mission passed an important test. The propulsion system was fired for 30 minutes to slide the composite module into its orbit around the moon. The composite module’s velocity was reduced, allowing it to be captured by moon’s gravity. If this crucial manoeuvre had not been executed properly, the module could have vanished into deep space or crashed on the moon.
After the composite module was put into an orbit around the moon, the orbiter’s propulsion system was fired a few more times to shrink its orbit around the moon. After this the lander detached itself from the orbiter on September 2. This crucial manoeuvre, too, was a success. At this stage the orbiter and the lander were going around the moon separately in different orbits. From September 2 to 6, ground-controllers at ISTRAC monitored the health of the lander containing the rover inside it. The lander was in fine fettle, going around the moon with a perilune of 30 km and an apolune of 101 km.
On Sepember 7, the lander began its descent to the moon at 1.40 a.m. as planned when it was at an altitude of 30 km above the moon’s surface. The descent was to take 15 minutes, which the ISRO Chairman had described as “the most terrifying moments”. As the lander’s autonomous landing sequencer went into action there was applause in the Mission Operations Complex. The four throttleable engines on board ignited to brake the descent. Again applause. The descent consisted of four phases: rough braking phase; absolute navigation phase; fine braking phase; and terminal, vertical descent phase.
The lander’s descent was good and progressed as planned during the rough braking phase. During the transition from the rough braking phase/the navigation phase to the fine braking phase something went wrong. The velocity of the descent could not be controlled. These are autonomous actions done by sophisticated sensors on board the lander. Contact with the lander was lost when it was 2.1 km above the moon’s surface. With its velocity of descent going haywire, the lander, with the rover inside it, crashed on the moon’s surface. Why the velocity could not be reduced and data from the lander was lost when it was 2.1 km above the moon are matters to be found out.
A top ISRO official said, “It is not clear why it happened. We are unable to decipher anything out of it so far. We have set up a team consisting of specialists in propulsion, control, guidance, navigation and sensors to find out what went wrong. So far, we do not have any clue about what went wrong.” He was confident that ISRO would zero in on the exact cause of the failure because enormous data were available and they would be analysed. All the data would be correlated. What was clear was that the lander had crashed on the moon’s surface.
A silver-lining amid this gloom is that the orbiter is doing well, orbiting around the moon at an altitude of 100 km. It may not have been in line of sight for its two high-resolution cameras to take pictures of the crash. The orbiter will have a life of one year.
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