As the famous proverb ‘for want of a nail the shoe was lost’, bespeak, a simple nipple joint shrinking in the freeing sub-zero temperature halted the launch of the much-awaited Chandrayaan-2 on early hours of July 16, 2019. Helium stored under high pressure, as high as 350 bars, to drive the fuel and the oxidiser into the combustion chamber, leaked.
According to insider information in the public domain, the problem was not daunting; however exercising ‘abundant caution’, rightly so, alert engineers of ISRO decided to call off the launch. Better safe than sorry; no point in a haughty 1000 crore firework.
While the perplexed ISRO officials not knowing the actual extent of the problem at hand hurriedly sought ten days to come up with the next step. However, without dismantling the rocket assembly, the Cyro fuel was drained. Then the ISRO engineers approached and tightened the leaking nipple join through the access door to the helium gas bottle atop the oxygen tank, fixing the problem in record time.
Elated ISRO wasted no time, they revised the mission schedule and worked out a revamped flight plan to rendezvous with the Moon at the same point as planned earlier. Working backward, they found the appropriate launch window to be July 22 14:43 hrs.
However, one small hitch; the launch window is just one minute long.
Flight plan tweaked
ISRO has publicly announced that the launch rehearsal and Mission Readiness Review took place on July 20, and the ‘performance was normal’ at 11:04 pm. Now, the customary 20 hrs countdown will begin at 18:43 on Sunday (July 21). If all goes well, 16 min.14 sec after the launch, the Chandrayaan-2 must be orbiting in its parking orbit of 170×38,000 km elliptical orbit, with an orbital inclination of 21.4°.
— ISRO (@isro) July 18, 2019
The delay in launching also has made ISRO come up with a revised flight plan. For the lander to have a full life of one lunar day, that is 14 days, it has to touchdown as intended previously, on September 6, 2019, precisely when the designated landing point at the Moon experience the Sunrise. With the launch date of July 16, ISRO had ample 54 days in its hand to go from Earth to Moon and land comfortably. With the launch date revised, quite a few adjustments have been made in the flight plan.
Six days longer around the Earth
In the original flight plan, the Lander-Orbiter combo was to go around the Earth in a highly elliptical orbit for about 17 days to gain enough impulse to be hurled at Moon. In the revised flight plan, Chandrayaan-2 will be Earthbound for six additional days. During this period, each time the spacecraft comes to the point near the Earth (called perigee), the rockets will be fired, technically called as ‘burns’, making the path slowly spiral outwardly and progressively raising its apogee (farthest point in orbit from the Earth). With about five ‘perigee burns’, the mission hopes to place the spacecraft in a 150×1,41,000 km orbit. Once this orbit is achieved, the spaceship would have gained the adequate escape velocity of 11.2 km per second in contrast to launch speed of just 10.3 km per second.
Towards the Moon
On August 13, 23rd day from the launch, injected into trans lunar trajectory, the spacecraft will commence its actual 3,80,000 km long journey to the Moon. Like a bullet aimed at flying bird, the space probe must be sent in its course to meet the Moon on the appointed day at a designated place in the space. As the launch date has shifted, to reach the Moon at a specific location in space, the spacecraft has to travel two more days than the original mission plan. Cruising in the severe cold and radiation filled deep space for the next seven days, on August 20 the spacecraft is slated to enter into the orbit of Moon.
The earlier mission plan had envisaged the Vikaram-Prgyaan (Orbiter- Lander) combo to orbit the moon for sumptuous 28 days. With the launch date altered, it has to now hurry and just revolve around the Moon for only 13 days before the Lander-Rover combo is separated from the Orbiter. The Orbiter will be stabilised in a 100×100 km circular orbit within this 13 days. Next day, the Lander-Rover will be detached from the Orbiter, and the decoupled Lander-Rover will be placed in a 100×30 km elliptical orbit. Permitting the Lander-Rover combo to orbit the Moon many times, the powered descent will commence on the 48th day of the launch, that is September 7, 2019, On the same day, the Lander-Rover combo will touchdown the surface of the Moon and after a few hours the Rover, Prgyaan will crawl out of the Lander Vikram.
Confidence in sunshine
The tryst with Moon has to be September 6, 2019, when it is in its first quarter. It is on that day the Sun starts to shine towards the designated landing site. Landing during the freezing night or in the shaded region is not an option for Vikram-Pragyan combo. While most lunar lander and rover, like the recent Chinese one, incorporate radioisotope passive heating units (RHU) to heat up the electronics and batteries, Indian Lander-Rover lack one. The extreme cold swing of temperature on the Moon could freeze the electrolytes of the electrical cells and cause permanent failure. Therefore, Chandrayaan-2 cannot do without sunlight. At the proposed landing site 70.90°S, 22.78°E, on the plains near the lunar south pole, on September 6, it dawns, and hence will experience full sunshine for fourteen days. Consequently, the Lander-Rover combo has to land on this date, if it has to have a whole operational period of 14 days.
ISRO Chairman, K Sivan is confident that the time lost could be somehow made up during the journey and touchdown on Moon as planned on September 6, 2019.
(T V Venkateswaran is science communicator with Vigyan Prasar, New Delhi.)