So, Chandrayaan-3 has smoothly soft-landed on the Moon. Now what?

The scientists and engineers huddled at the Mission Operations Complex (MOX), ISTRAC, Bengaluru, for the past two days erupted in joy as the much-anticipated call came in: “I have made the touchdown.” With uncontrollable grins, and tears of joy rolling down their cheeks at the same time, they spontaneously rose from their seats and hugged each other. Millions glued to the television and online live transmission, who had their hearts in their mouth, heaved a sigh of relief.

Chandrayaan-3, India’s third Moon mission, successfully soft-landed on the Moon on Wednesday (August 23) evening, making India only the fourth country to achieve the feat after the former Soviet Union, the USA, and China. Coming close on the heels of the spectacular disaster of Luna 25, Russia’s mission to the Moon after a gap of 47 years, the soft-landing of Chandrayaan-3 was a great relief.

The Automatic Landing Sequence (ALS) programme code was uploaded around 12.44 pm, five hours before the planned landing time. The lander craft was primed for the landing manoeuvre with the commands uploaded and confirmed. At 5.44 pm, upon the arrival of the lander module at the appointed place above the lunar surface, about 750 km from the landing site, the AI-powered software initiated the sequence. The four throttleable engines were activated to commence the powered descent. One by one, the different phases of the tricky descent manoeuvre took place. The mission operations kept confirming the sequential execution of commands once the telemetry data was received from the craft.

The last few minutes were tense. Seventeen minutes into the manoeuvre, the craft reached 150 metres above the lunar surface and hovered for a few seconds to search for a safe place to land. At 18 minutes after the manoeuvre, the craft was on top of a spot, but it found it unsafe — with perhaps a big crater or a large stone in the way. It moved sideways to land at a safe spot.

The engineers and scientists were sitting on the edge of their chairs. Nineteen minutes after the landing manoeuvre, as the craft touched down, the touch sensor on the leg that first brushed the lunar surface was triggered, and the signal was immediately transmitted back to Earth. Traversing at the speed of light, the signal took about 1.25 seconds to reach the ISRO’s MOX. And finally, came the call: “I have made the touchdown.” There was a perceptible collective gasp. Moments later, uncontrollable joy spread throughout the Mission Control Complex.

Moon is messy

Landing on the Moon is a dusty nightmare — just one more reason why the ISRO’s feat is momentous. For billions of years, meteors have bombarded the Moon’s surface. Due to these repeated meteoritic impacts, the top layer of the lunar surface is covered with what is called the “lunar regolith” — fine particles less than 0.07 mm in diameter. For comparison, a grain of soil on Earth measures between 2.0 and 0.05 mm in diameter.

The high-velocity engine exhaust and the impact of the final touchdown must have kicked up quite a lot of fast-moving dust that can travel far and wide. The fast-moving exhaust gas hitting the surface can accelerate the lunar dirt to speeds as great as several kilometres per second and send them flying hundreds of kilometres away.

Propelled in a ballistic trajectory, these dust particles must will arch high into the lunar sky until lunar gravity draws them back to the surface. The gravity on the Moon is just one-sixth of the Earth’s and it lacks an atmosphere that can impede the speeding dust. Hence, unchecked, it is some hours before the dust particles will finally rain.

The Chandrayaan-3 module will wait for the dust rain to settle before taking the following steps. Its sliding door will unfold and reach the ground. The rover, Pragyan, will be lowered slowly down the slide using a pulley and rope. After rolling down, the tether will be cut, and the rover will stand on its six wheels. The solar panel will open up like a sail. Basking in the low-angle sun’s rays, the rover will come alive.

The rover will take an image of its mother ship, the lander, and in turn, the lander will take a snapshot of its child, the rover. The pair of images will be transmitted back to Earth.

The images will help identify whether the lander or the rover suffered structural damage during the complex descent on to the lunar surface.

We hope both mother and child will be in good health.

All science, no astrology

For the next 14 days, the lander and rover will explore the secrets of the Moon.

It is not astrology but science.

The appointed time for the landing of Chandrayaan-2 was September 2, 2019. The Moon was in the first quarter. That was suklapaksha sapthami. And so it is on August 23, 2023.

Why is this so? Again, it has nothing to do with astrology but only pure science.

A line called the terminator separates the sunlit and dark portions of the Moon. The idea is to land during the waxing phase and keep the landing spot just near the sunny side of the terminator line. As the waxing moon grows in size, the terminator line will move to the right. The landing spot will be well-lit. The waxing moon will soon grow into a full moon within seven to eight days.

Subsequently, the waning phase will begin. Now, the terminator line will creep from the left side of the Moon towards the right side. The nightfall will advance, and the landing site will enter the night side in the next seven days. That is, for 14 days, the landing site will have sunlight.

Sunlight is not only needed for powering the lander and rover, but also, during the night, the temperature will drop to minus 200 degrees. In this freezing cold, metals will crack, and the whole craft and instruments will decay.

We have to make the hay while the sun shines.

What next?

The scientific instruments aboard the Vikram lander and Pragyan rover will be calibrated and activated. With these instruments, the lander and rover will investigate various aspects of the lunar surface and interior for the next 14 days. There are two key instruments — the Alpha Particle X-ray Spectrometer (APXS) and the Laser Induced Breakdown Spectroscope (LIBS) — in the rover that can identify the elemental composition of the sample and the minerals present in them. Already, Chandrayaan-2 has made a mineral map of the Moon through remote sensing. The data from the rover will provide ground truths and increase our confidence in the remote sensing data.

The top layer of the Earth is broken into tectonic plates, which float on the semi-liquid mantle. The edges of the tectonic plates rub against each other or collide head on. That results in earthquakes and volcanoes. However, the Moon does not have tectonic plates. Yet, earlier missions have shown that the Moon, too, exhibits quakes similar to earthquakes. The Instrument for Lunar Seismic Activity (ILSA) fixed in the Vikram lander will investigate the mysterious lunar quakes.

We know that metals are good conductors of heat, but wood is a poor conductor. Chandra’s Surface Thermophysical Experiment (ChaSTE) will insert a heater and a heat sensor to study the head conducting properties of the lunar soil.

Solid, liquid, and gas are three well-known states of matter. However, there is another intriguing state of matter called plasma. When an atom loses all its electrons, it becomes plasma. Typically, the plasma state is not present on Earth naturally. A plasma state is produced momentarily on rare occasions, such as in a flash of lightning. However, due to intense solar radiation near the lunar surface, it is believed that a thin layer of plasma is formed. However, these fierce rays are filtered by the ionosphere above the Earth. The Radio Anatomy of Moon Bound Hypersensitive Ionosphere and (RAMBHA) will investigate the enigmatic plasma layer near the lunar surface.

In addition to these, at the request of NASA, a Laser Retroreflector Array (LRA) package has been affixed on the head of the lander. By reflecting a laser light onto the Retroreflector, we can precisely measure the distance of the Moon. Such precise measurements will help us understand the intricate dynamics of the Moon and help us understand the nature of gravity.