Chennai, India

India's Chandrayaan-3 had packed a special element for all countrymen. As the rover traversed the lunar surface, it was expected that its rear wheels would leave the imprint of the Indian National emblem (Lion Capital of Ashoka at Sarnath) and the iconic ISRO logo on the talcum powder-like lunar soil. However, the rover's rear wheels have been unable to leave a clear imprint, as planned. Had this happened, a picture of the same would have been the Indian version of the iconic snap of Neil Armstrong's step that has been imprinted for eternity at the Apollo 11 landing site. 

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"We at ISRO had been working on lunar rovers since the year 2009, when the Chandrayaan-1 mission was underway. We began with four-wheeled rovers and gradually learnt our lessons, and then moved to a six-wheeled rover for Chandrayaan-2. During the initial phase of testing on a simulated lunar soil test bed, we realized that the rover wheels were sinking when they were commanded to turn by using a technique known as skid driving. During skid driving, the rover wheels on one side are rotated slowly and the wheels on the other side are rotated fast. While doing this, the rover turns towards the direction of the slower wheels. Therefore, we ensured that the treads on the wheels were modified to prevent sinking," Dr Mylswamy Annadurai told WION. Until 2018, he had served as the Director of ISRO's satellite building facility, UR Rao Satellite Centre, Bengaluru. 

He specifically recalls that the lunar soil test bed created by ISRO was modelled after the Apollo 11 landing site, which was the equatorial region of the moon. The Apollo series of NASA missions showed that the equatorial region had a talcum powder-like soft soil. Therefore, ISRO had also chosen to test their rover on soil from Earth that offered similar characteristics.

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"Even around 2012-13, when we were testing the Chandrayaan-2 rover (which is the same as the Chandrayaan-3 rover), we used to get the wheel imprints (national emblem and ISRO logo) on the soil," he adds.

Also read | India: ISRO extends wait, giving Pragyaan rover, Vikram lander 14 more days to awaken on lunar surface

Listing out some of the likely factors behind a clear imprint not being obtained, he says that the nature of the soil near the Chandrayaan-3 landing site could be different. "Just like there is sandy, clayey and other varieties of soil on earth, the same could be possible on different lunar regions," he explains. 

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ISRO's 6-wheeled rover Pragyan weighs 26 kg on Earth. However, given that the lunar gravity is just 1/6th that of the Earth's, Pragyan would weigh less than 4.5 kg on the moon. When WION queried if the low lunar mass of the rover could have prevented the clear imprints from appearing, Dr Annadurai said that the rover (when tested on Earth in simulated lunar gravity conditions using a helium balloon) had left behind imprints on the test bed. Perhaps a heavier rover could leave a better and much more noticeable imprint, he suggests. 

"The nature of lunar soil, how much the rover wheel treads can penetrate into the Lunar soil and even the mass of the rover (heavier the better) are factors that could play a role in successfully leaving imprints on the Lunar soil," he explains.

Maybe, when we check the highest resolution images of the rover's trail and zoom in adequately, we might find a faint trail, the veteran scientist who is popularly known as the Moon Man of India, suggests. 

Queried about what is being referred to as lumpy soil near the Chandrayaan-3 landing site, he said that it could indicate the presence of a volatile substance like water ice.

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