India starts historic march to Moon, yet crucial manoeuvers remain

Amidst the countdown, take-off and majestic soaring of the GSLV Mk III rocket, with Chandrayaan -2 aboard, a composed, confident and relaxed K Sivan, chairman of ISRO remained glued to the computer terminal displaying the vitals of the flight. At T+17 minutes, as the display panel at the launch command centre flashed C25 shut off, indicating that the recalcitrant Cryogenic Upper Stage (CUS) rocket engine has performed as expected, a small smile spread across his face. At T+993.8 seconds, as the display flashed the successful separation of ‘Chandrayaan-2’ spacecraft from the spent, now defunct, CUS, Sivan could be seen springing from his seat beaming with a wide smile.

After hugging, handshaking and patting the backs of his fellow engineers and scientists who made the mission successful, a buoyant K Sivan declared, “The launch was more successful than we could have expected” and did not forget to thank his team and said “this is possible only because of the hard work by ISRO … it is my duty to salute all the people who have done the work”, yet did not fail to underscore the unfinished intricate journey ahead.

After a snag, GSLV gives a huge surprise

The technical snag in the CUS engine developed during the fuelling, just T minus 56 minutes, resulting in the abort of the previous launch attempt was uppermost in the mind of all. The strain was visible in the control room, as well as among the visitors gathered to cheer for the mission. Once the performance parameters of the launch trickled in, and Sivan declared that the GSLV Mk III has outperformed, there was visible relief and spontaneous applause.

The CUS engine was expected to burn until T+959.30 providing boost to the spacecraft and Chandrayaan-2 was supposed to separate at T+ 974.30 seconds and hurl into a highly elliptical orbit with apogee (farthest point from the Earth in the elliptical orbit) of 39120 km and perigee (closest point in the oval orbit around Earth) of 170 km with an orbital inclination of 21.4°. This was the expectation.

However, the Indian made C25 CUS engine, tweaked for this prestigious mission, performed better than expected, burning until T+978.7 seconds, giving longer impulse than expected and taking the orbit much higher, by 6000 km. Using the telemetry data, ISRO has computed the initial injected orbit to be 169.7 × 45475 km, with the apogee of the orbit much higher than the design.  An elated Sivan described this as “whatever had to happen tomorrow has already happened today”.

With the apogee, farthest point from the Earth in the elliptical orbit, going up by 6000 km, the orbit raising manoeuvers will be able to save precious space fuel which could come handy in unforeseen circumstances along the treacherous deepspace odyssey as well as help prolong the life of the orbiter around the Moon. Indeed, this was a much-awaited good news for the jinxed Chandrayaan-2 mission which had been seeing setbacks one after another.

Also read: Mission accomplished: Chandrayaan-2 in earth’s orbit, India over the moon

Moreover, until now the earlier two GSLV Mk III launch has carried only 3.1 and 3.4 tons of payload. The GSLV Mk III M1 flight carried 3.8 ton Chandrayaan-2, the heaviest so far, which itself was seen by some as a calculated risk taken by ISRO. The improvised C25 engines giving a 15 per cent increased performance imply that the ISRO can go for its designed 4 ton to GTO capacity in its future missions with GSLV Mk III rockets.  Despite the snag on July 15 causing sleepless nights for the ISRO engineers, ultimately the C25 engine proved to be reliable.

Miles to go

With the successful injection of Chandrayaan-2 into orbit, the mission to soft land on the Moon has commenced, which may place India in the league of the US, Russia and China.

Nevertheless, this is only the first step in the tricky and arduous long march towards the Moon. That is why while declaring the launch of Chandrayaan-2 as a “historical journey of India towards Moon and to land at a place near the south pole to carry out scientific experiments to explore the unexplored”, Sivan did not flinch from cautioning about the 15 crucial manoeuvers that lay ahead before the mission becomes a success.

In the next 22 days, Chandrayaan will make as many orbits, and the thruster rockets have to be activated for a short duration, called ‘burns’, at a precise time and location. Gaining impulse from the burns, the apogee of the orbit go higher and higher. The first burn will be initiated in the third orbit to increase the apogee to 47000 km. In the fourth orbit, the burn will be initiated to increase not the apogee but perigee to 240 km from the current 170 km. Further additional four burns will be performed during the 6, 11, 16 and 19 the orbit, to raise the apogee to 55800 km, 71500 km, 89700 km and finally to 144000 km. When the spacecraft makes its 22nd orbit on August 14, 2019, the Trans Lunar Burn will be performed, and the probe will lunge towards the Moon. By then it would have gained the sufficient velocity to escape the ever-present clutches of the Earth’s gravity.

Tango in space

The Moon is going around the Earth which in turn is revolving around the Sun. Thus the relative position of the Sun, the Moon and the Earth always change as well as their absolute place in space. Therefore one has to compute the precise location in the space where the Moon will meet the spacecraft at the designated time and aim at that point. One space expert described the difficulty level as throwing a ball from Delhi aiming at a one rupee coin held high at Kanyakumari. It is that intricate. The angle, direction and the orbit of the spacecraft must be precisely choreographed to meet the Moon at the designated location in the space.

Coasting along the Trans Lunar Orbit, once the Orbiter-Lander-Rover combo reaches a point 62,630 km away from Moon, it will enter into the sphere of influence of Moon. On August 20, six days after the Trans Lunar injection, Chandrayaan-2 will fall into the spell of Moon and commence its highly elliptical 120×18000 km lunar orbit around the Moon. From now on the probe will be Lunar bound.

Four burns will be performed while going around the Moon to make Chandrayaan-2 lower its orbit and obtain a circular 100×100 km lunar orbit. As per the flight plan, ISRO hopes to achieve this by September 1, 2019.  If the launch had taken place in the first attempt, on July 15, ISRO would have had ample 28 days for this manoeuver. With the launch delayed for eight days, now they will have to accomplish this within just 13 days.

Fifteen minutes of terror

Candidly admitting that two out of the three components of the Chandrayaan 2, Lander and Rover, are dark horses for ISRO, Sivan termed the soft landing of the Lander-Rover combo as ‘fifteen minutes of terror’. The Orbiter of Chandrayaan-2 is similar to Chandrayaan-1. Not much surprise is anticipated in the operation of the Orbiter.

Forty-three days from the launch, the Orbiter and the Lander-Rover combo will be separated. While the Orbiter is maintained at the 100×100 km orbit, the Lander-Rover will be ‘deboosted’, that is the rockets will be fired against the direction of the motion to slow it.

Through various manoeuvers, Lander-Rover will be made to go in an elliptical orbit of 30×100 km around the Moon.  For the next four days, the systems and instruments in the Lander-Rover will be tested and calibrated. The orbit will be fine-tuned to go above the designated landing spot. On the D-Day, as the Lander-Rover reach a 30 km above the surface of the Moon, once again it will be ‘deboosted’ to drop further towards the Moon.

In the next fifteen minutes, the Lander-Rover combo will descent and touchdown on the surface of the Moon. The acceleration under the gravity of Moon is just 1.6 km/s per second compared to 9.8 m/s per second on Earth. Yet if the Lander-Rover free fall from 35 km above the surface the Moon, the terminal velocity as hits the surface will be as high as 1200 km per second. Total time taken to fall will be just 3.4 minutes. Imagine an object moving at 1200 km per hours colliding with the Moon. Collision at this high velocity is out of the question – all the components of the Lander-Rover will break into pieces.

By retro firing the thruster rockets that is firing in the direction of the motion, the gravitational attraction would be countered, and the Lander-Rover will be made to touchdown gently with a velocity of just 2 m/s. The whole operation will take 15 minutes. If all goes well the soft landing will occur on 02:58 IST on September 7, 2019.

The mission

Once the dust settles, a few hours after the soft landing, the Rover will roll out from the Lander. As both the Lander and Rover use the sunlight to harness the power, they will be operational for one Lunar daytime, which is equal to about 14 Earth days. However, the Orbiter is expected to operate for more than one year orbiting in the Polar orbit enabling remote sensing and 3D mapping of the lunar terrain. On the science front, a press release by ISRO states that the ‘mission aims to further expand our knowledge about the Moon through a detailed study of its topography, mineralogy, surface chemical composition, thermo-physical characteristics and atmosphere, leading to a better understanding of the origin and evolution of the Moon’.

The South polar region of the Moon has been receiving keen interest by all space-faring nations in the recent past, as the possibility of the presence of water in the form of ice is high in this region.  One of the scientific instrument aboard the Orbiter will look for the telltale sign of water on the surface of the Moon. On successful launch, NASA tweeted that it looks forward to Chandrayaan-2 to learn about the south polar region, as it is planning to send astronauts to this region as part of the Artemis mission in a few years.