The much-awaited D-day is around the corner. The nation is excited. All have their fingers crossed. Milling around the Isro campus the excitement in the air is perceptible. Also the anxiety. If on the one hand, the tricky landing procedure is making the Isro engineers and managers jittery the media frenzy is making them nervy. Media from around the world has descended at Bengaluru. Security has been beefed up anticipating the arrival of Prime Minister Narendra Modi at the Mission Operations Complex (MOX) at Isro Telemetry, Tracking and Command Network (ISTRAC) in Bengaluru.
What will be the role of controllers during the crucial landing process? Of course, none will sleep. For the whole 15 minutes, until the Vikram lands on the surface of the Moon, they would be just sitting at the edge of their chairs, biting their nails. Yes, indeed, they can do nothing. If the trust in science, like most Isro scientist do, they will be hoping that they have not missed anything in their computations. If they believe in God, like some do, then they may be just praying.
During these crucial time period, famously described as “fifteen minutes of terror” by Isro chairman, Dr K Sivan, the controllers at the Mission Operations Complex (MOX) will not be playing video game with the Vikram lander. Split-second decisions have to be made and hence the whole operation will not be in human control, but autonomously executed by the onboard computers of the Vikram lander. The computers have been prepared to face all types of challenges and programmed to locate, navigate and identify safe landing spot in the designated landing site automatically. Hence once the proverbial button is pushed, Isro engineers push the go button at 1:38 am on September 7th, they have nothing in their hands. Thus can just peer at the telemetry and monitor the progress.
All preparations done
After the second deorbiting manoeuvre on September 4, 2019 the Vikram Lander attained an orbit of 35 km x 101 km. The powered decent earlier proposed at 1:40 am has been advanced to 1:38 am taking into view the current orbital parameters. Even a small anomaly in an angle of impulse will result in many kilometres of deviation in the craft position. Watching the orbital parameters of the craft in the last two days, Isro engineers have made the last minute computations, prepared the final landing sequence and uploaded the commands to the craft. Just pushing the command button remains.
What can go wrong?
There are four important challenges in soft landing on the Moon; reduce the orbital velocity to zero, control the terminal velocity of fall to gentle speeds, tilt the horizontal spacecraft to vertical position for landing, ensure that the craft avoids hazard and lands at safe spot.
At 30 km above the surface of the Moon, the orbital velocity of the Vikram lander is about 1,600 km per hour. This velocity has to be reduced to zero before the craft can land. The Moon has no atmosphere and hence one cannot use parachutes to break the speed. Therefore by firing the four out of the five rocket engines towards the direction of the motion the speed of the craft has to be retarded. The first 12 minutes and 37 seconds of the total 15 minutes of the landing operation will go into this aspect.
Meanwhile, the orientation of the Vikram lander will be tilted. While orbiting the moon the four legs of the lander would be facing forward and the craft itself will be in horizontal position. Slowly the craft will be first tilted to about 50 degrees, to enable to navigational camera and altimeter a better view of the ground below.
The last three minutes
By then the spacecraft would have arrived at 400 meters above the ground and few meters short of the landing spot. The orbital velocity is reduced to zero. It left to fall from this height the terminal velocity of the craft will be around 130 km per hour. The craft will not survive the hit. By firing the two of the five rockets facing down, the craft will be made to counter the pull of the Moon’s gravity. The craft will hover on the spot for 12 seconds.
For 15 minutes and 26 seconds, the craft would be just 10 meters above the chosen safe spot. The four engines would be shut and the central engine will be kicked in. In the next thirteen seconds, the craft will fall to the ground at 2.76 km per hour.
Finding safe spot
The four legs must rest on a plane surface and the surface of the Moon is uneven with pit here and a mount there. Identifying and navigating to a secure spot is a crucial step in the landing sequence. When the craft is at an altitude of 400 meters, onboard landing camera and other instruments would scan the surface to identify a safe spot for landing. Once again when the height of the craft is 100 meters, it will hover above the landing site to scan a safe spot. Once a space spot is found the Vikram lander will slowly descent in a parabolic path towards the chosen landing spot. All these operations will take place in just three minutes.
After years of engineering and science, finally, the day has arrived. Surely no one is going to sleep. May be for the first time, for science and not for religion, Indian people are going to be awake whole night and keep vigil. How will September 7 dawn? An Isro scientist gave a confident smile. That says all.