Chandrayaan-2: Historic mission to the moon

GSLV-Mk III M-1 puts the Chandrayaan-2 composite module into a perfect orbit on July 22, signalling the beginning of India’s 48-day journey to the moon, a journey replete with mind-boggling technological challenges for ISRO.

Published : Aug 03, 2019 07:00 IST

  The spacecraft  and the lander before being enclosed in the heat shield.

The spacecraft and the lander before being enclosed in the heat shield.

IT was a moment of delirious joy not only for about 7,500 people gathered at the newly built Viewers’ Gallery outside the Sriharikota spaceport but also for the rocket and spacecraft engineers intently looking at their consoles in the Mission Control Centre (MCC) inside the spaceport. As a voice from the MCC boomed around 3 p.m. on July 22 that the Geosynchronous Satellite Launch Vehicle-Mark III (GSLV-Mk III M-1) had put Chandrayaan-2 into orbit, loud applause reverberated in the MCC. The Indian Space Research Organisation (ISRO) Chairman, K. Sivan, and rocket and spacecraft technologists did not hide their joy. There was much rejoicing in the Viewers’ Gallery, too. For it was a mission to the moon, a mission that had caught the imagination of people across the country.

Among those who travelled to Sriharikota to watch the launch was a couple from Gondia near Nagpur in Maharashtra, Niraj Verma and Rashmi Verma, who proudly cradled in their hands a model of the GSLV-Mk III rocket they had made themselves.

“ISRO bounced back with flying colours,” Sivan declared, soon after the Chandrayaan-2 composite module went into orbit. There was reason for him to be delighted because a technical snag in the vehicle had forced ISRO to cancel the lift-off on July 15, 56 minutes before ignition at 2:51 a.m. The snag was tackled on a war-footing and the launch was rescheduled for 2:43 p.m. on July 22.

On that day, when the GSLV-Mk III M-1 put the Chandrayaan-2 composite module into a perfect orbit, more than 16 minutes after lift-off at 2:43 p.m., it signalled the beginning of India’s 48-day journey to the moon. But it will be a journey replete with technological challenges, because ISRO will not only put a spacecraft/orbiter called Chandrayaan-2 into orbit around the moon but land a contraption called lander at the South Pole of the moon. The lander, named Vikram after Vikram Sarabhai, the charismatic founder of India’s space programme, will touch down gently on the moon on September 7. From the lander will emerge a rover called Pragyaan (“knowledge”) and it will roll out to the lunar surface. This robotic vehicle will wander to a maximum distance of 500 metres on the south polar region of the moon. Both Vikram and Pragyaan will perform experiments on the moon for 14 earth days, or one lunar day, with their science payloads. Simultaneously, the Chandrayaan-2 orbiter, from its orbital perch 100 kilometres above the moon, will take pictures of the lunar surface. Its instruments will look for minerals and buried water on the moon. The orbiter’s life span is one year.

On July 24, a second success came ISRO’s way. ISRO Telemetry, Tracking and Command Network (ISTRAC) fired Chandrayaan-2’s onboard propulsion for about 48 seconds and raised the composite module’s orbit to 241.5 km x 45,162 km around the earth. On July 26, the propulsion systems were fired again for more than 883 seconds and Chandrayaan-2’s orbit raised to 251 km x 54,829 km.

(The Chandrayaan-2 spacecraft /orbiter, the lander and the rover are together called the composite module. It weighs 3.8 tonnes, with the spacecraft weighing 2.4 tonnes, Vikram 1.4 tonnes and Pragyaan 27 kilogrammes. While the lander sits on top of the orbiter, the rover is ensconced inside the lander.)

Ambitious and complex mission

Chandrayaan-2 is not only India’s most ambitious mission but technologically the most complex because it involves the separation of the lander Vikram from the orbiter, Vikram’s slow descent and its soft landing on the moon’s surface on four legs on September 7. To ensure the controlled descent of Vikram and its soft landing, Vikram has on board five throttleable engines and “minute” sensors, all developed by ISRO. It will take 15 minutes for the lander to descend the 30 km from its orbit above the moon and settle down on the lunar soil. These 15 minutes will constitute the most complex mission for ISRO in its 56-year history.

Soon after the composite module was put into an orbit around the earth on July 22, Sivan said: “Today is a historic day for space science and technology in India. I am extremely happy to announce that the GSLV-Mk III M-1 has successfully injected Chandrayaan-2 into an orbit 6,000 km more than the desired orbit. This is the beginning of India’s historic journey to the moon and to land at a place near the South Pole to carry out scientific experiments to explore the unexplored.”

If the target was to put the composite module into an earth-bound orbit with a perigee of 170 km and an apogee of 39,120 km, the vehicle’s cryogenic stage’s performance was so good that it injected the module into an orbit with a perigee of 170 km and an apogee of 45,475 km. “This meant that the satellite will have more fuel, more life and more time to do the manoeuvres,” the ISRO Chairman said. Ironically, it was in the cryogenic engine chamber that a helium gas leak occurred on July 15.

V. Narayanan, Director, Liquid Propulsion Systems Centre (LPSC), ISRO, was proud of the performance of the vehicle’s cryogenic engine, which uses 25 tonnes of liquid hydrogen and liquid oxygen. “It was an excellent performance, a very good performance,” said Narayanan, who was earlier the Project Director of C-25 cryogenic engine development for the GSLV-Mk III.

The Project Directors for the Chandrayaan-2 composite module and the Mission Director are two women, M. Vanitha and Rithu Karidhal respectively.

Sivan paid tributes to ISRO engineers and technicians who overcame the technical glitch on July 15 when helium gas leaked from one of the metallic bottles in the engine chamber in the vehicle’s topmost cryogenic stage. Helium gas started leaking after the propellant tanks had been filled with liquid hydrogen, which is the fuel, and liquid oxygen, which is the oxidiser. The purpose of helium gas is to maintain the pressure in the engine chamber. What was puzzling was that helium did not leak when the liquid hydrogen and liquid oxygen were being pumped into the propellant tanks but it haemorrhaged only at a certain pressure, at a certain bar. The leak occurred from a manual joint. As the gas kept leaking, the pressure in the engine chamber did not hold. This meant that the control valves, the hydraulic systems, the pneumatic systems and the electro-voltage systems in the stage would not function. “The rocket may go up but it will put the spacecraft into an orbit lower than the targeted orbit,” said a rocket technologist.

After the launch was called off, ISRO engineers did not rush to the vehicle, whose hardware temperature was high. The cryogenic stage was drained of its 25 tonnes of liquid hydrogen and liquid oxygen. The core, main liquid stage, was drained of its 110 tonnes of liquid propellants. A committee was formed to find out what led to the helium leak. ISRO engineers waited for the vehicle’s hardware to cool down to the ambient temperature before they approached the vehicle. Since the vehicle was on the launch pad, there were platforms to access it at its different stages. A list of procedures—on who should do what, and who should be where—was prepared meticulously. Investigations revealed that helium had leaked from a fluid joint. A rocket technologist explained that “the problem was identical to a leak from a joint in the water line in one’s residence. You take out the joint and put a new joint. Wherever helium gas leaked here, we corrected it.”

S. Ramakrishnan, former Director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, said: “We knew that the problem was not complex because we face these kinds of leaks in every vehicle. They occur in launch vehicles that use liquid propellants. Hydrogen leaked from the United States’ space shuttle. We identified the problem and corrected it…. People worked round the clock.”

Sivan commended those who ensured the mission’s remarkable comeback. He said: “On July 15, ISRO intelligently observed a technical snag. Team ISRO worked hard, fixed and corrected the snag within 24 hours. For the next day and a half, the required tests were done to ensure that the corrections made were proper and in the right direction. Today, ISRO has bounced back with flying colours. This was possible only because of the hard work of ISRO engineers, technicians and technical support staff of SDSC [Satish Dhawan Space Centre, Sriharikota], VSSC, LPSC and IPRC [ISRO Propulsion Complex at Mahendragiri near Nagercoil in Tamil Nadu].

“They [the team] forgot their families and sacrificed their interests. The experts’ team constituted was on the job for the last seven days to ensure that every system functioned properly. I salute them.”

To make up for the loss of one week, ISRO has now revised its schedule of the composite module heading to the moon. The composite module will now spend 23 days in its earth-bound orbit instead of the original 16 days if the launch had taken place on July 15. Instead of spending 27 days orbiting the moon, the module will spend 13 days in the lunar orbit.

Now that the composite module has been put into an orbit around the earth, ISRO will face a series of daunting challenges before Vikram touches down on the moon. ISRO until now has mastered the technology of sending up rockets to put satellites into a variety of orbits. That is, these missions were headed towards the space. However, in the Space Capsule Recovery Experiment (SRE-1) in January 2007, ISRO recovered the satellite it had put into orbit 12 days earlier, using a Polar Satellite Launch Vehicle (PSLV-C7). The satellite splashed down in the Bay of Bengal near Ennore, Chennai, and the Coast Guard recovered it. In the Crew Module Atmospheric Re-entry Experiment (CARE) in December 2014, a GSLV-Mk III rocket put a 3.7 tonne empty crew module into a sub-orbit at an altitude of 126 km. It landed safely later in the sea near the Andaman archipelago and was retrieved.

“But this is the first time we are attempting to gently land a lander on the surface of another heavenly body. This makes the mission very special because the challenges here are mind-boggling,” said an ISRO engineer. The first challenge was to launch the GSLV-Mk III M-1 and put the Chandrayaan-2 orbiter with the lander and rover into an initial orbit around the earth. That was done on July 22. The next challenge is to enhance the composite module’s orbit progressively ( Frontline , July 19). This will be done by firing the propulsion system on board the module five times during the 23 days when the module will be going around the earth. The first firing was done on July 24. On the 23th day, the module will enter the lunar transfer trajectory when the propulsion is fired for the fifth time. In other words, the module will escape from the earth’s gravitational pull and start heading towards the moon for the next seven days. Thus, it will reach the moon’s vicinity.

“We will face another challenge now,” said the engineer. “We have to reduce the speed of the spacecraft to slow it down enough so that the relatively weak gravity of the moon captures it in its orbit. So we will decelerate the spacecraft.” After the composite module is captured by the moon’s weak gravity, the module’s orbit will be refined to enable it to get into a 100 km by 100 km circular orbit around the moon so that it passes over the lunar poles. This will involve a series of complex manoeuvres. For 13 days, the module will circle the moon at an altitude of 100 km.

Until now, the composite module was a single entity, with the lander Vikram sitting on top of the orbiter and the rover, Pragyaan, inside the lander. On the 43rd day (the lift-off day is counted as day one), commands will be given from the ground for the lander to separate from the orbiter. There will be two entities now: the orbiter and the lander. Controlling both is another challenge.

ISTRAAC engineers will give commands for Vikram’s circular orbit of 100 km to be reduced to 30 km x 100 km. In this orbit, its various systems and its health will be observed for four days. When all this is happening, the orbiter will be taking pictures of the moon’s surface with its high-resolution cameras. At an opportune moment, when the lander is at an altitude of 30 km, ISTRAAC engineers will give commands to the five throttleable engines on board the lander to fire so that the lander begins its descent, performs a series of braking manoeuvres to control its descent and lands softly on the moon. Each of the five throttleable engines has a thrust of 800 Newtons. They will decelerate Vikram in stages as it comes down towards the moon’s surface.

As Propulsion Today , an ISRO publication, says: “This is the most crucial phase of the mission when the soft-landing is executed.”

The throttleable engines are used for the lander’s “orbit correction, de-boost, and rough, precision and final braking manoeuvres”, explains Propulsion Today . The final touchdown is from “a height of two metres where the engines will be shut off and the lander is allowed to fall free.” The lander, with the rover inside, should touch down with a sufficiently low velocity. The IPRC developed the throttleable engines. There are sensors on board the lander, which will decide where Vikram should touch down: in an area where there are no craters, boulders and steep slopes. The slopes should not have an inclination of more than 12 degrees. Otherwise, Vikram will topple. It will take 15 minutes for Vikram to descend the 30 km touchdown.

“These 15 minutes of terror,” as Sivan called them, would constitute the most complex mission for ISRO. After the lander remains on the lunar surface for four hours, a door will open and a ramp will unfold. Pragyaan will emerge from Vikram, slide down the ramp and wander about on the moon’s surface in the South Pole. “How to bring down the lander, how to make it soft-land on the moon and how to bring out the rover from inside the lander are all new to us,” Sivan said. Everything will be autonomous once the lander begins its 15 minutes of descent. The ISRO Chairman said: “We cannot do anything…. We cannot interrupt. It is total autonomy. We are doing these for the first time. The rover will be autonomous. Some kind of interaction will be there from the ground with the rover. We will be telling it where to take the images. Otherwise, the total system is autonomous.”

The orbiter has eight payloads, including two high-resolution cameras. Its other instruments will look for minerals such as magnesium and iron, search for buried water ice on the moon and study its exosphere. Four instruments on the lander, including one from the National Aeronautics and Space Administration (NASA), will study the properties of possible landing sites on the moon, analyse the lunar seismic activity and measure the distance from Vikram on the moon to the earth. The rover carries two payloads. It will do in situ mapping around the landing sites.


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