Chandrayaan-3: Will Pragyan rover wake up from its sleep mode again?

Pragyan rover has a small battery that helps it deploy its solar array

Untitled design - 1 Graphical representation of Vikram and Pragyan on moon (left) and an image of lunar surface from the Lander Horizontal Velocity Camera taken during the descent | ISRO

The Pragyan rover sent to the moon aboard the Indian spacecraft Chandrayaan-3 has now completed all its assignments, as per the Indian Space Research Organisation (ISRO). “It is now safely parked and set into sleep mode. APXS and LIBS payloads are turned off. Data from these payloads is transmitted to earth via the lander,” the ISRO posted on X on Saturday.

Interestingly the battery is now fully charged, with the rover’s solar panel geared to receive light as the sun is expected to rise again on September 22. The receiver is kept on. “Hoping for a successful awakening for another set of assignments! Else, it will forever stay there as India’s lunar ambassador,” ISRO had said.

No specific conclusion can still be given on whether Pragyan will ever work again or not. The Pragyan rover is a small, 26-kilogram robot that is about 36 inches long. It has a rectangular chassis with a solar array that can generate 50 watts of power. “The rover has a small battery that helps it deploy its solar array, but, after that, it runs solely on solar power. The rover’s electric motor makes maximum torque as soon as it starts to turn, which helps it move around on the lunar surface,” explained space expert Girish Linganna.

As per the space expert, lithium ion batteries are most commonly used for space applications because of their high energy density, a long cycle life and good performance at low temperatures. The capacity of the rover battery is 10 Ampere-hour (Ah). It is enough to meet the mission’s primary objective of carrying out scientific observations for one lunar day.

The solar array is a group of solar panels which can convert sunlight into 50 watts of electrical energy. “The Pragyan rover has a small battery that helps it deploy its solar array. This means that the battery provides the initial power needed to open the solar array. Once the solar array is deployed, it can generate its own power and the battery is no longer needed,” Linganna added.

Normally, a capacity of 20-40 Ah in the lithium ion batteries on earth would be sufficient to store the energy generated by a 50-watt solar panel. But in space, there are restrictions on battery capacity. However, compared to lead-acid batteries, lithium ion batteries have a lighter weight and a longer lifespan. The exact capacity of the battery needed will depend on the specific application. If a device is needed to be powered for a long period of time, it will need a larger battery. “If only powering the device for short periods of time is required, a smaller battery will be sufficient,” Linganna said.

Experts do point out that lithium ion batteries are prone to thermal runaway—a condition in which the battery’s temperature rapidly increases and can lead to a fire or explosion. This is a major safety concern in space, where there is no way to extinguish a fire or escape from the spacecraft. Also lithium ion batteries are sensitive to radiation, which can damage the battery’s internal components and reduce its lifespan. Space is a highly radioactive environment, so lithium ion batteries would need to be protected from radiation damage.

Low-temperature performance of lithium ion batteries is still limited by several factors, such as increased internal resistance, decreased electrolyte conductivity, reduced electrode kinetics and possible formation of lithium plating. Therefore, lithium ion batteries need to be specially designed and modified to work in the extremely cold environment of the moon, which can reach as low as -173°C at night.

Experts have time and again proposed or implemented methods to improve the low-temperature performance of lithium ion batteries such as using additives, solvents, or salts that can lower the freezing point and increase the conductivity of the electrolyte, applying surface or interface modifications to the electrodes to enhance the charge transfer and reduce the polarisation, incorporating heating elements or thermal insulation materials to maintain a suitable temperature range for battery operation and optimising the battery structure and configuration to reduce the internal resistance and improve the power output.

These methods can help lithium ion batteries work in sub-zero temperatures of the moon, but they also have some drawbacks, such as increasing the complexity and weight of the battery system. India currently lacks the technology to create electronic circuits and components capable of withstanding the extreme cold temperatures of the moon. Therefore, more research and development is needed to find the optimal balance between performance, safety and reliability for lunar missions.

“Very elaborate studies were done during the period 2010-12 itself to select proper components, wires, PCB materials, soldering procedure, solar panel construct, sleep wake up circuit and its survival ability in long cold, wheels lubrication, battery selection and power system configuration for system survival ability without battery etc. by a select committee in a campaign mode. Systems also underwent tests,” said Mylswamy Annadurai, popularly known as the ‘Moon Man of India’.

“So I have good confidence that rover could be able to wake up,” he said.