Prasanta Kumar reported in September, 2019 that in a setback for India’s ambitious plans to become the first country to explore the lunar south pole, the Chandrayaan-2 Vikram lander lost contact when it was trying to land on the moon on Sept. 7. More than a decade ago, India’s Chandrayaan-1 spacecraft successfully conducted the country’s first lunar mission, which found evidence of water at the Moon’s poles. On July 22, the world’s second most populous country launched Chandrayaan-2 from the Satish Dhawan Space Centre in southeastern India in a bid to be the first to explore the moon's south polar region with a rover.
After a week-long delay due to a technical snag in the launch vehicle system, a GSLV MK-III rocket, India’s most powerful launch vehicle, sent the Chandrayaan-2 into space, kick-starting the spacecraft’s 48-day journey to the moon’s surface. The Chandrayaan-2 is made up of two modules — the orbiter and the lander. Among other payloads, the lander houses a six-wheeled robotic rover which can travel up to 500 metres from the landing site at a speed of 1 centimeter per second and was to conduct missions for 14 days.
A variety of government entities located in Bengaluru were monitoring the mission from the ground. For example, antennas at the Indian Deep Space Network’s (IDSN) Bengaluru campus were set up to receive Chandrayaan-2’s health data and information from its payloads. The mission also involved ground stations located around the world that are a part of the Indian Space Research Organization’s Telemetry, Tracking and Command Network. Deep space communication with the spacecraft was provided on demand by the U.S. Jet Propulsion Lab Deep Space Network’s facilities.
A long journey
Unlike most other moon missions, Chandrayaan-2 traveled for nearly 30 days before injecting itself into the Moon’s orbit. After entering the lunar orbit, the spacecraft orbits the celestial body for another 13 days before the lander detaches itself from the orbiter module. The lander took another week before it eventually began its descent down on the moon. In comparison, China’s most recent Chang’e-4 mission took approximately 25 days from launch to landing. Even though the launch was delayed by a week, the landing date is unaffected. The trajectory was planned so that the spacecraft could meet the moon at a particular time and predetermined position to maximize the time available for lander and rover operations.
Orbiting Earth multiple times allows Chandrayaan-2 to raise its orbit across multiple engine burns. Even though this method lengthens the time it takes to make the journey, it allows for the use of a smaller engine and consumes less fuel, which in turn helps to bring down the weight of the composite module and cost of the entire mission. The lander began descending to the lunar surface in the early hours of Sept. 7 but scientists lost contact with it during the penultimate stage of the descent.
Where to land?
While a number of nations have sent missions to explore Earth’s natural satellite, the Soviet Union, the United States and China are the only three countries that have successfully soft-landed on the moon. Most of these landings occurred close to the equatorial region.
Note: China successfully landed Chang'e-4 on the far side of the moon in January 2019.
Chandrayaan-2, however, attempted a landing close to the south pole. This presented a unique challenge as the orbit of the spacecraft had to be perpendicular to the lunar orbit when it was injected into an orbit around the moon. That left little room for changes in trajectory or orbital maneuvers as every correction would eat up precious spacecraft fuel.
Gravity plays an important role when determining the spacecraft’s trajectory and orbital maneuvers. The uneven distribution of mass below the lunar surface makes the gravity of the moon ‘lumpy’, posing extra challenges for the scientists. Variations in local gravity needs to be accounted for in the lunar descent trajectory during the braking phases so that the lander can touch down at the proper site.
Lump Gravity has been measured in parts of the Moon by NASA’s GRAIL mission. Choosing a suitable landing site on the moon can be tricky due to its rugged surface. Chandrayaan-2 not only required a landing site that was close to the south pole, it also needed to have a landing slope of less than 12 degrees with an acceptable distribution of craters and boulders. The angle of the slope was important as steeper slopes can damage or topple the lander during touch down.
…if there is a dispersion of 10 cm/s (in the velocity of Chandrayaan-2 as it reaches the lunar orbit)... we will not be able to meet the requirement for soft-landing at the lunar south pole. The site also had to be located on the near side of the Moon so that the spacecraft could communicate with mission control on Earth and be sunlit for a full lunar day (14 Earth days).
Extreme conditions
The lunar environment is harsh. Powdery lunar dust, formed when micrometeorites pulverize moon rocks into fine particles, blankets the surface. Besides being highly abrasive, lunar dust carries an electrostatic charge that lets the particles cling onto any surface they come in contact with, making it particularly difficult to remove from surfaces of spacecraft and mission equipment. The particles pose a high risk to rover operations and may disrupt the performance of solar panels and on-board sensors.
Spacecraft and mission equipment also need to operate in a vacuum environment and withstand extreme temperatures which can range anywhere between -248 and 123 degrees Celsius on the lunar surface. NASA’s Diviner Lunar Radiometer Experiment recorded temperatures inside a shadowed crater to be as low as -238 degrees Celsius, which is among the lowest recorded temperatures in the solar system. In comparison, temperatures in several hollows on a high ridge on the East Antarctica Plateau — oft-considered the coldest place on Earth — can dip below -92 degrees Celsius on a clear winter night. Calibrating sensors, insulating lander and rover electronics and preparing for various thermal conditions at different orbital altitudes so that equipment can operate under extreme conditions are critical to the success of the mission.
Chandrayaan-1 faced abnormally high temperatures during operations in 2008, forcing it to operate only one scientific instrument at a time for part of the mission. The failure of power supply systems, likely due to overheating, caused the spacecraft to abort its mission in August 2009, more than a year short of its planned two-year lifetime. At least 95% of Chandrayaan-1’s objectives had been accomplished by then, according to the Indian Space Research Organization (ISRO).
“15 minutes of terror”
After Chandrayaan-2 achieved a final orbit of about 100 km from the surface of the moon, the Vikram lander separated from the composite module on Sept. 2 to begin its descent onto the moon. After separation, the systems on board the lander were powered on for the first time since its journey began. An integrated navigation system guided the lander autonomously towards the landing site through a crucial sequence of braking phases.
“We are going to experience 15 minutes of terror, to ensure that the landing is done safely near the south pole,” ISRO’s chairman, Kailasavadivoo Sivan, told reporters after Chandrayaan-2 successfully lifted off, describing the final moments before the spacecraft was expected to touch down on the moon. But the lander eventually lost contact with ground stations after it reached an altitude of 2.1 km above the lunar surface. ISRO subsequently said that the lander has been located on the moon and that it is trying to communicate with it.
Vikram lander separates from the orbiter near the North Pole 100 km above lunar surface Deboost phase. The lander travelling at about 1.6 km/s undergoes de-orbiting maneuvers using thruster burns.
Rough braking phase
As the lander reaches a point closest to the Moon near the South Pole, further de-boost burns are used for horizontal braking of the lander using four thruster engines.
Precision braking phase. Using the throttle control in the the thruster engines, the lander is slowly prepared for vertical descent. The lander scans the lunar surface and compares the topography with on-board imagery. It then makes the required calibrations and decides the final safe site for soft landing.
Powered descent
Once the lander is hovering above the landing site, the four engines are shut off. A central thruster is used to slowly reduce the vertical velocity for a soft landing. The central thruster ensures that the lunar dust is blown away from the site without letting it cloud the lander after touchdown.
To the South
After Chandrayaan-1’s impact probe discovered ice in the lunar south pole in 2008, the region has garnered increased interest. NASA and others have also set their sights on the moon’s south pole because it is a region believed to hold enough recoverable ice water for use in synthesizing additional rocket fuel as well as for drinking water to sustain astronauts. Compared to the north pole, a larger area remains shadowed in the south pole, which increases the possibility of finding water in some of those permanently shadowed areas. Moreover, cold traps in the craters in the south pole region contain a fossil record of the early solar system. Surface ice distribution at the poles as detected by NASA’s Moon Mineralogy Mapper instrument aboard India’s Chandrayaan-1 lunar spacecraft.Images courtesy of NASA
Thirteen payloads, including spectrometers and high-resolution cameras aboard Chandrayaan-2’s orbiter, lander and rover, had aimed to extensively study the occurrence and distribution of minerals and water as well as the chemical, thermal and seismic characteristics of the surface and the lunar exosphere. A passive experiment from NASA was also on board to understand the dynamics of the lunar system and find clues about the moon’s interior.
Experts said the lander and rover were only a part of the moon mission and Chandrayaan-2’s orbiter was still orbiting the moon and will collect and send back valuable data for one year. India also has further ambitions in space. Last year, Prime Minister Narendra Modi announced a 100 billion rupee project — named ‘Gaganyaan’ — that aims to send a three-member crew to low earth orbit by 2022.
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