21 July 2019
Chennai: The Indian space agency has done some hard work on the ground so that its moon lander -- Vikram -- can soft-land safely and rover -- Pragyaan -- can move around, said a senior retired official.
"The surface of the earth and that of the moon are entirely different. So we had to create an artificial moon surface and test our rover and lander," M. Annadurai, who retired as Director, U.R. Rao Satellite Centre (URSC), formerly ISRO Satellite Centre told IANS.
The moon's surface is covered with craters, rocks and dust and its soil is of different texture as compared to earths.
Annadurai said the legs of lander and wheels of the rover had to be tested before their flight.
While moon light dinners are known for the human beings, Indian space agency created moon light environment for testing its rover.
Importing lunar soil like substance from the US was a costly affair and ISRO looked for a local solution as its need was about 60/70 ton of soil.
Many geologists had told ISRO that near Salem in Tamil Nadu there were "anorthosite" rocks that would be similar to features of moon soil or regolith.
The ISRO finalised to take the "anorthosite" rocks from Sithampoondi and Kunnamalai villages in Tamil Nadu for moon soil.
According to Annadurai, the rocks were crushed to the required size and moved to Bengalure where its Lunar Terrain Test Facility was located and the test bed created.
"Initially a sum of Rs 25 crore was budgeted for the purpose but it came down drastically as the service providers did not charge us," Annadurai said.
Artificial lighting was also set up to resemble the Sun-lit environment on the moon.
"Initially the rover was of four wheel configuration. But we changed to six wheel configuration after experiments to give it more stability. Some changes were made to the wheel size as well," Annadurai recalled.
He said helium balloons were used to reduce the weight of the rover to simulate the moon's gravity which is less than that of the Earth's.
The communication compatibility of rover and the lander were checked at HAL's facility, Annadurai said.
In order to test the lander, a test bed was created at ISRO's facility at Challakere in Chitradurga district in Karnataka.
Artificial moon craters were created at Challakere, similar to the site where lander Vikram will set down its legs.
Prior to its soft-landing, Vikram's sensors would check whether the terrain is safe to land. Even after landing if the terrain is not suitable then the lander will go up and settle down at a nearby spot.
"We put the sensors in the small plane belonging to NRSC (National Remote Sensing Centre) and flew it over the test bed couple of times to check the sensors as to whether it can read the terrain properly," Annadurai said.
Other space agencies too, build such terrain facilities to test their equipments so that when the lander actually lands on the moon it does not topple.
According to him, the actuators of the lander were tested at ISRO centre at Mahendragiri and the thrusters were also tested.
The strength of the lander legs was also tested in two ways-falling when the engines were switched off and in a calibrated landing. The legs have to absorb the landing shock, he added.
The landing process is divided into "rough braking" and "fine braking".
The descent trajectory of Vikram has to take into account the variation in local gravity. Further the landing site, landscape features should not result in a communication shadow area.
The next challenge is the moon dust. The moon's surface is covered with craters, rocks and dust. Firing the onboard motors close to the surface will result in backward flow of hot gases and miniscule, hard/barbed/jagged dust.
Its negative charge makes it stick to most surfaces, causing a disruption in deployment mechanisms, solar panel and sensor performance.