India’s second lunar mission, was successfully launched from the Satish Dhawan Space Centre (formerly Sriharikota Range) SDSC SHAR on July 22, 2019. The 3,840 kg spacecraft was launched by GSLV MkIII-MI. This was the first operational flight of the GSLV MkIII. The Chandrayaan-2 project consisted of three components–the Orbiter, the Lander (Vikram), and the Rover (Pragyan).
The lander was named after Dr Vikram A. Sarabhai, considered to be the Father of the Indian Space Programme. The rover Pragyan (‘wisdom’ in Sanskrit), carried within the lander, was to be released with the help of a ramp on Vikram’s landing.
The mission was aimed at helping to demonstrate the ability to soft land on the lunar surface and operate a robotic rover on the surface.
The broad objectives, in the words of ISRO, were to expand India’s footprint in space; inspire a future generation of scientists, engineers, and explorers; and surpass international aspirations.
The scientific objectives included the following:
(i) Extensive mapping of the lunar surface was to be carried out so that variations in lunar surface composition could be studied in order to trace the origin and evolution of the Moon. The Moon is understood to provide the best link to Earth’s early history, offering an undisturbed historical record of the environment of the inner solar system.
(ii) Further studies were to be undertaken on the basis of the evidence for water molecules found by Chandrayaan-1 to find out the extent of water molecule distribution on the surface, below the surface, and in the tenuous lunar exosphere to address the origin of water on Moon. The choice of the landing site of South Pole region was directed towards this end, as the shadowed portion could be conducive to the presence of water.
(iii) Detailed topographical studies, comprehensive mineralogical analyses, and several other experiments on the lunar surface were to be conducted to better understand the origin and evolution of the Moon. The landing site was to help in this regards as the craters in this region contain a fossil record of the early solar system.
(iv) Other objectives were to study the lunar ionosphere as well as the lunar crust and mantle, and to measure moonquakes.
The spacecraft entered into Lunar Transfer Trajectory (LTT) on August 14, 2019. With the Lunar Orbit Insertion (LOI) manoeuvre being performed on August 20, 2019, Chandrayaan-2 was successfully inserted into an elliptical orbit around the Moon. After a series of lunar-bound orbit manoeuvres, the orbit was reduced to a circular polar orbit around the Moon. On September 2, the Vikram lander separated from the orbiter. The landing of Vikram was attempted on September 7 and it followed the planned descent trajectory from its orbit of 35 km to around 2 km above the surface. Subsequent to this, communication between the lander and ground station was lost. All the systems and sensors of the lander functioned as expected until this point and proved many new technologies such as variable thrust propulsion technology used in the lander. Later, NASA is reported to have found the debris of the lander, suggesting it had crashed.
A successful landing of Vikram (lander) would have placed India in the fourth place after the Soviet Union, United States, and China to have achieved the landing of an unmanned spacecraft on the Moon.
Though the mission could not achieve the desired goal due to the damage caused to the lander, the orbiter continues its seven-year mission to study the Moon. Moving in a 100 km x 100 km orbit around the Moon, its eight experiments continue to function, their studies ranging from surface geology and composition to exospheric measurements. These studies and observations will continue to help scientists understand various aspects of the Moon.
According to ISRO, the Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS), an X-ray fluorescence spectrometer, aims to map the amounts of elements at 12 km spatial resolution at its best. CLASS detected charged particles and its intensity variations during its first passage through the geotail. Once every 29 days, the Moon moves through the geotail for about 6 days, centred around the full Moon. The instruments on the orbiter can then study the properties of the geotail, ISRO said.
The best observations are enabled in the geotail region in space, a region that exists as a result of the interactions between the Sun and Earth. The solar wind is made up of a continuous stream of charged particles that the Sun emits. These particles are embedded in the extended magnetic field of the Sun even as the Earth’s magnetic field obstructs the solar wind plasma. Due to this interaction, a magnetic envelope is formed around Earth. On the side of the Earth facing the Sun, the envelope is compressed into an area approximately three to four times the radius of Earth. On the other side of the Earth, the magnetic envelope is stretched into a long tail extending beyond the Moon’s orbit. This tail is called the geotail.
The Solar X-ray Monitor (XSM), a companion payload to CLASS, has observed solar flares with intensity variations much beyond the sensitivity limit of GOES (geostationary operational environmental satellite).
Chandrayaan-2 detected solar proton events due to high intensity solar flares in January 2022. In March 2022, a spent rocket booster hit the lunar surface near the Hertzsprung crater on the far side of the Moon. The Terrain Mapping Camera-2 onboard Chandrayaan-2 imaged the far side of the Moon in April 2022 and identified the impact site.
Why Such a Long Journey
It took Luna-2 of the USSR just 34 hours to reach the Moon in 1959. Then, in 1969, NASA’s Apollo-11 mission landed humans on the Moon after a journey of a few hours more than four days. Apollo-11 was not just the world’s first manned Moon mission, but it was also the fastest trip of astronauts to the Moon. Chandrayaan-2 journeyed for 48 days before attempting a landing on the Moon. The discrepancy seems glaring.
The explanation lies in the build of the rocket, the amount of fuel it carried, and the speed of the spacecraft. To cover long distances in space, high speeds and straight trajectories are required. The Saturn V launcher that was used by NASA to place the Apollo-11 in orbit was a powerful heavy-lift launcher, with a lift capability of more than 40 tonnes that included the lunar module, service module, and command module housing the crew capsule. It could travel at more than 39,000 km per hour. India does not have a rocket of that calibre, or even one powerful enough to shoot Chandrayaan-2 onto a straight path to the Moon. The GSLV MkIII launcher, though called ‘Bahubali’, comes nowhere near Saturn V, and its role was to place Chandrayaan-2 in geostationary transfer orbit (GTO). ISRO thus chose a circuitous route to take advantage of Earth’s gravity, which would help slingshot the craft towards the Moon, where the Moon’s gravity was to be made use of by the lander to touch down.
Any spacecraft requires a minimum velocity of 11km/s to escape the Earth’s pull to go to the Moon. Of that, 10.3 km/s is provided by the vehicle and 700 m/s is provided by the craft’s propulsion system, as the chairperson of ISRO explained. As the engine of GSLV is small, it was being burnt not continuously but in short bursts so as to manoeuvre the craft. A powerful engine like Saturn V would have enabled Chandrayaan-2 to have reached the Moon in a single shot. The Moon’s gravitational pull was being used to take the craft to lunar orbit. Also, it is immensely cost effective to take the circuitous path even if it takes longer. The total investment of ISRO in the Chandrayaan-2 mission is Rs 978 crore, of which Rs 375 crore was spent on building the launcher. That is a very small amount compared to what NASA spent on the Saturn V launcher.
From the parking orbit, the spacecraft is manoeuvred into an extended orbit around the Earth. When the apogee is close to the Moon’s orbit, fuel is burnt to move into the Moon’s gravitational force, after which the spacecraft enters the lunar orbit.
About ‘Launch Windows’ and Their Importance
When the launch of Chandrayaan-2 scheduled for July 15, 2019 was cancelled, there was much speculation about the availability of a suitable ‘launch window’ during which the spacecraft could be sent into space.
There is a period of time which is considered best for launching a spacecraft, of course depending on where it is intended to reach. A ‘launch window’ is the period of time, usually a short one, within which a particular space mission must be launched in order to ensure that it reaches its destination. This may be to meet another satellite in space, reach the space station, or reach another planet.
Earth is travelling around the Sun at 1,07,000 km/h even as it spins on its axis at a speed of some 460 m/s at the equator (and slower by various degrees elsewhere). At the same time, the Moon is also travelling, though at a slower rate of 3,683 km/h, around Earth, besides spinning about its axis, again slower than Earth. In the case of Chandrayaan, the journey was between Earth and Moon. Here, too, several factors determine the ‘launch window’, including the ways in which the celestial objects are moving.
An important factor to consider is that a region chosen for the landing on the Moon receives sunlight for approximately 14 Earth days, as the other fourteen days would be in the dark. The timing for a spacecraft’s touchdown on the Moon has to be such that it uses the sunlit days on that spot to maximum advantage, as the lander and the rover need solar energy both to power them and keep them warm enough. If the landing spot has to be seen from Earth, the time of landing needs to be during the sixth phase of the Moon (first quarter) as seen from Earth. Another factor to consider is that, once placed in lunar orbit, Chandrayaan-2 must be visible to the ground station, which will determine when the operation of the landing takes place. Much of the time between launch and landing is taken up in making various orbital manoeuvres and operations; there is just a little time to take some decisions. The launch has to keep in mind these factors.
Launch windows are defined for two different time intervals: a ‘daily window’ exists, of a few hours in a given 24-hour period; a ‘monthly window’ consists of a few days during a given month (or, in the case of Chandrayaan, lunar cycle). But a monthly launch window is not continuous. To allow for operational flexibility, there has to be as large a daily and monthly launch window as is possible. A longer daily launch window allows time for managing delays during the countdown. July 15 was chosen because the best launch window for Chandrayaan-2 was considered to be between July 9 and July 16; on these days, the daily window during which the mission could be launched was of a comfortable duration of more than 10 minutes or so. It is not as if no launch window was available later, but they would be of much shorter duration. The July 22 window was barely a couple of minutes, and required all operations to be completed with extreme precision; there was no room for a hold.
Launch windows can be found every month; indeed, it was reported that a suitable window would be available, after working in all the factors, in September if the technical snag had taken longer to remedy. However, given the complex schedule drawn up for the mission, including the planned lunar orbit, the soft-landing on the lunar surface, and the rover experiments, a major overhaul of the plans and technical aspects would have been required. That would have increased the cost of the mission greatly.
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