Explained | Sun has risen: India awaits possible re-awakening of Chandrayaan-3

After a 14-Earth-day long period of darkness and brutal cold (temperatures around minus 200 degrees Celsius), the Sun has risen over the Lunar south pole, near where India's Chandrayaan-3 lander and rover have remained in sleep mode since September 2. After performing a lunar soft landing on August 23, the solar-powered Chandrayaan-3 mission had completed all its planned science missions within a fortnight and signed off on a high by performing a surprise hop test at the final stage of the mission. All eyes are on the Indian space agency ISRO, as they expect to hear from Chandrayaan-3 by around September 22nd, Friday.

WION has learnt that the Sun has risen over the Shiv Shakti Point (which is the name that India has assigned to the Chandrayaan-3 landing site). However, the sunrise is not the real deal. The Sun must be at a certain angle, such that the solar panels of the Chandrayaan-3 Vikram Lander and Pragyan rover are able to receive sunlight and canharness the power of the Sunto warm up, charge their batteries and receive signals from Earth and then attempt a re-awakening.

"The solar panel is oriented to receive the light at the next sunrise expected on September 22, 2023.The receiver is kept on.Hoping for a successful awakening for another set of assignments! Otherwise, it will forever stay there as India's lunar ambassador," ISRO said on September 2.

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The parameters that determine the re-awakening

During the lunar day, temperatures are around 130 degrees Celsius (more than twice as hot as the hottest temperature recorded on Earth). By living and working throughout its planned operational life, the Chandrayaan-3 lander and rover demonstrated their total ability to function during the lunar day.

As the lunar night approached, ISRO fully charged the batteries of the Vikram Lander and Pragyan rovers and put them into sleep or hibernation mode. This is similar to what certain mammals (bears and rodents) do on Earthin preparation for the harsh and brutal winters when temperatures are freezing and food is scarce. They gorge on as much food as they can, slow down their metabolic activity and go into a dormant state for weeks or months.

Depending purely on solar power for its functioning, Chandrayaan-3 was put into hibernation mode, considering that there would be no sunlight (power) during the lunar night. For context, Russia's Luna-25 (which crash landed days before Chandrayaan-3 landing) was powered by a Radioisotope thermonuclear generator, a kind of nuclear battery that would help it function for at least one Earth-year on the moon. The power source used by the Russian lander would offer power supply and also help the lander keep warm during the lunar night. However, ISRO doesn't possess this technology yet and had to make do with the solar-powered lander-rover duo.

Lunar night temperatures in the Moon's equatorial regions are around minus 130 degrees Celsius. However, near the lunar South Pole, the temperatures could well be minus 200 degrees Celsius. In order for Chandrayaan-3 to survive, the circuits, sensors, electronics, hardware and other components of the Vikram Lander and Pragyan rover would have to live through this cold.

"The (lander-rover) systems are likely to work longer... But, because we don't have power for the next 14 days (lunar night), the mission is unlikely to survive beyond, as the battery cannot support for very long....We are hoping that after 14 Earth days of lunar night, it will come back, but it is not assured because all systems will go to minus 150 degrees centigrade, which is far beyond the survival limit of many materials used on the lander...By design, it (mission life) is 14 days, if we are lucky we may get longer," MSankaran, Director of ISRO's UR Rao Satellite Centre had earlier told WION.

Even after sunrise, the lunar temperature would have to stabilize to a daytime temperature, from the brutally chilly night temperature, which is a shift of more than 300 degrees Celsius.

Why the survival of Vikram is crucial?

ISRO's Vikram lander serves as the mother craft and the Pragyan rover is its baby. Until the time of landing and rover deployment, the 26kg Pragyan was in the belly of the Vikram lander.

While Pragyan does have its own cameras, solar panels and science payloads, its capabilities are limited and entirely dependent on the mother craft. For context, Pragyan only traversed distances on the moon that were within the visual range of the lander cameras, just like how a mother wouldn't let a baby venture out of sight. When ISRO mission operations teams issue commands to the Pragyan rover, they are not directly issuing commands from the Earth-based antennae to the rover. Instead, the commands are sent from Earth-based antennae to the Lander and then the lander cascades it to the rover. Likewise, the rover's own data is first sent to the lander and then to Earth.

As Vikram is the crucial communications link in this mission, its survival is of utmost importance, without which there would not be any meaning in the survival of Pragyan.

The backup communication link

If the Vikram lander is the only one to survive this brutal lunar night and ISRO has trouble communicating directly with the lander, there is a backup communication channel that ISRO has kept ready.

The Chandrayaan-2 orbiter, which has been circling the moon since September 2019 is still active and operational, according to ISRO. Days prior to the soft-landing of Chandrayaan-3, ISRO had established a communication link between the Chandrayaan-3 Vikram lander and the Chandrayaan-2 orbiter. Therefore, ISRO has a messenger in place around the moon, in case such a need arises.

A possible re-awakening is a big bonus

In addition to performing its planned mission flawlessly, Chandrayaan-3 also performed a hop test, a short-distance vertical take-off, hover and landing. The hop test data would provide crucial information for ISRO to plan more ambitious lunar missions (including sample-return missions), and work towards developing such technology.

As the science fraternity awaits a possible re-awakening, the copious amounts of data provided by the Chandrayaan-3 lander and rover are already being analysed by the principal investigators and the relevant teams behind the respective science payloads. It would take months or years for scientific inferences and theories to be derived from that data. That's what science missions are all about - the study of the mission data takes exponentially longer than the mission itself.

A possible re-awakening will be a pleasant surprise and a major validation of the extreme robustness of ISRO's lander-rover technology and offer significantly more confidence to ISRO for such extreme Indian science ventures on other celestial bodies.

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