Newly designed cars are tested for road-worthiness on terrain where they would be driven, while new aircraft are test-flown in the skies. But where on earth did the Chandrayaan-2 mission’s lander and rover, which will head for the moon on July 15, check out their legs and wheels?
More than a decade ago, even as the Chandrayaan-1 orbiter mission of 2008 was being readied, the Indian Space Research Organisation (ISRO) created a proto-Lunar Terrain Test Facility (LTTF) at its advanced satellite testing unit, ISITE, in Bengaluru. This, it did, by modifying a balloon research lab, about 30-40 m high, long and wide.
At the time, ISRO was grappling with the task of indigenously executing the cryogenic stage for its GSLV MkII rocket. Any thought of sending a moon lander was a distant dream of low priority. Equipping the LTTF and making it look and feel like being on the moon was the first challenge. It needed lunar ‘soil’ with almost all its features and texture, lunar temperatures, low gravity and the same amount of sunlight as on the moon.
For recreating the terrain, an option was to import simulated lunar soil from the U.S. — at an exorbitant $150 a kg (the then prevailing price). The facility needed about 60-70 tonnes of soil.
ISITE’s parent, the U.R. Rao Satellite Centre, or URSC (it was called the ISRO Satellite Centre or ISAC at the time) did buy a small amount of simulated lunar soil from the U.S., but soon decided to find its own solution at a lower cost.
M. Annadurai, who as URSC Director oversaw activities related to the Chandrayaan-2 spacecraft until he retired in August 2018, recounts that geologists of various national agencies had found that a few sites near Salem in Tamil Nadu had the ‘anorthosite’ rock that somewhat matches lunar soil in composition and features. The URSC’s lunar soil simulation studies team zeroed in on Sithampoondi and Kunnamalai villages for the soil.
It turned out to be a ₹25 crore project: experts from the National Institute of Technology in Tiruchi, Periyar University in Salem, and the Indian Institute of Science, Bengaluru, joined in, working without any fee.
Professional crushers broke down the rocks and soil to the micro grain sizes sought by the ISRO-led team. Transporters moved the tonnes of this ‘lunar earth’ to ISITE, all free of charge, Dr. Annadurai recalls.
These challenges were not there when he led the first lunar orbiting-only mission, Chandrayaan-1, as its project director.
At the LTTF, the team spread the soil trucked in from Salem up to a height of about 2 metres. Studios were hired to illuminate the facility exactly as sunlight would play on the lunar terrain.
On the Moon, the metre-long rover, weighing 27 kg, must move for about 500 metres during its expected life of 14 Earth days (one lunar day). Rover tests began as early as in 2015. The ISRO team had to reckon with the weak lunar gravity, about 16.5% of Earth’s. The rover’s weight was artificially reduced using helium balloons.
Previous missions by other countries have suggested that the southern part of the Moon is mineral rich with the promise of water, which was first confirmed by the Chandrayaan-1 mission.
Lunar south pole
ISRO Chairman K. Sivan recently said the Indian lander Vikram would be the first ever spacecraft to land at the lunar south pole.
It has two site options, the craters Manzinus-C and Simpelius-N.
The sites were picked after scouring through a few thousand lunar images from Chandrayaan-1 and other missions.
For testing the lander, ISRO had a large test bed created at its new R&D campus at the Challakere Science City, some 400 km from Bengaluru. Vikram’s set of sensors, called the Hazard Detection and Avoidance (HDA) system, is a critical part of the mission.
In the actual descent to the Moon, the lander hovers for a few seconds over a site and the sensors must assess whether the spot is flat enough for the lander’s legs: whether it has rocks that might topple the lander, and whether the lander can be steady to release the rover within it. If the spot is not safe, it must quickly rise and shift to a neighbouring spot and again assess if it is suitable to land on, all in seconds.
Sometime in 2016, the URSC created several artificial ‘lunar’ craters at the Challakere site. Late that year the team put a test bed of lander sensors in a small ISRO plane and flew it over the craters to see if the sensors could read the terrain and find the right landing spot.
According to Dr. Annadurai, the success of the landing depends on the sensors’ correctly guiding the lander to a safe site; and the fuel in the lander lasting for duration of the whole exercise.
Other tests were conducted to clear the working of the lander’s propulsion system, its actuator and legs, and the rover’s movement.
In a joint paper presented at the International Academy of Astronautics symposium in June 2017, Dr. Annadurai and his co-authors wrote: “One of the key elements essential for safe landing is the Hazard Detection and Avoidance (HDA) system. [It] comprises of several sensors... [which] provide information like lander’s horizontal velocity, vertical velocity, height above moon’s surface, relative position of the lander with respect to moon’s surface, and hazard/safe zone around the landing site.
“The HDA system processes the inputs from various sensors, compares the data collected with the information already stored in the lander and provides the required inputs to the navigation and guidance system in real time to correct the trajectory at the end of rough braking to enable a safe and soft landing.”
