A month-and-half after it took off from the launch pad at Sriharikota, traversing a distance of over 3,84,000 km on a pre-defined path, Chandrayaan-2 is ready to face its moment of truth. Later tonight, its lander module, called Vikram, which has already detached itself from the main spacecraft and has been moving independently for the last three days, will begin what ISRO Chairman K Sivan has been repeatedly describing as the most terrifying 15 minutes of its journey the final descent to the moon s surface from the nearest point in its current orbit, which is just 35 km in vertical distance.
At the time it begins the descent, sometime after 1:30 am Saturday, Vikram would be travelling at about 6 km per second, or about 21,600 km per hour. That is about 30 to 40 times the average speed of commercial airliners, which usually travel at speeds between 500 to 900 km per hour. Within 15 minutes, Vikram would need to bring down its speed to 2 metres per second (about 7 km/hr) or lower to enable a safe landing.
If successful, this would be India s first soft landing on the moon s surface. Only the US, the former USSR and China have been successful in landing humans or machines on the moon.
Chandrayaan-2 has had a smooth journey so far, but it is not difficult to see why Sivan s description of the final descent captures the essence of this journey. Just five months back, in April this year, an Israeli attempt to make a soft landing on the moon ended in failure. Its Beresheet spacecraft was unable to slow down sufficiently enough and crash-landed on the moon. Of a total of 109 missions to the moon so far flybys, orbiters, landers, rovers and human landings 41 have been unsuccessful. But it is also a fact that after the moon exploration resumed in the 1990s following a near two-decade lull, Beresheet has been the first and only failure.
Three of these missions, all from China, have made soft landings on the moon. Others were flyby or orbiter missions and did not involve landing.
More safety measures
K Sivan s description of the final descent as the most terrifying part of the journey is mainly because ISRO is carrying out this operation for the first time. However, Chandrayaan-2 has incorporated additional safety measures to deal with heightened risks during the descent.
To apply brakes on its speed in deep space, Vikram will fire thrusters in the direction of its movement. If fired in the opposite direction, these thrusters provide acceleration. This is similar to the recoil that a gun experiences after firing a shot. This is exactly how a rocket also takes off by burning and ejecting propellants in downward direction. But if fired in the direction of movement, the thrusters would slow down the spacecraft. Vikram has an onboard propulsion system that will operate according to a pre-defined profile.
The thrusters onboard Vikram use two chemicals mono methyl hydrazine and nitrogen tetroxide as fuel. These are mixed and burnt in the thruster chamber and expelled in gaseous form. There are four thrusters on Vikram and all of them would be firing simultaneously, almost throughout the time of the descent. Each thruster is required to expel equal amounts of energy to keep the spacecraft balanced. However, if there is a deviation from the pre-defined profile and the thrusters do not fire as programmed, due to problems such as blockage, Vikram has the provision of a fifth thruster as well. This is one of the several additional safety features in Chandrayaan-2 that Beresheet lacked.
Vikram s four legs have also been enabled with a much greater ability to absorb shocks. It is programmed to make a landing after decelerating to a speed lower than 2 metres per second. But, scientists say, it is designed to withstand an impact with the moon s surface even if it touches down at a speed of 5 metres per second (18 km/hr).
When it is about 100 metres above the surface, Vikram will begin what is called a hazard avoidance manoeuvre. It is supposed to land at a site located between two large craters. But it is only after it reaches so near the surface that it would be able to assess whether the chosen location is flat enough, and free of any stones or boulder, to land safely. This kind of granular detailing of moon s surface cannot be seen from the earth. At this point, if everything goes according to the programmed profile, Vikram s downward movement would almost come to a halt, the firing of the thrusters providing enough upward lift to balance the gravitational pull. If its onboard cameras detect any problem with the location, Vikram can move around a few metres horizontally to locate and land at a safer place. It is designed to withstand a slope of 15 degree inclination, but it would try and avoid any of its legs landing on a stone or a boulder.
About three hours after landing, the six-wheeled robotic vehicle called Pragyan would come out of the lander and slope down to the moon s surface to roam around and collect data. The Pragyan rover, which has two instruments on board, would crawl at a speed of 1 cm per second, collecting information about the elemental composition of the moon s surface, and the relative abundance of different minerals present there. It can communicate with the lander, which in turn, is equipped to communicate directly with the control facility in Bengaluru.
Both the lander as well as the rover are designed to be operational for 14 days, the duration of a lunar day. The main spacecraft, or the Orbiter, would continue to go around the moon for at least a year, its eight onboard instruments collecting different kinds of data from a distance of 100 km.