(Photo: Jayachandran/Mint)
(Photo: Jayachandran/Mint)

Opinion | Slow and steady, Vikram all set to land on Moon

Similar to Apollo 11, the final descent of Chandrayaan-2’s lander Vikram, a much lighter craft, will be slow so it doesn’t crash upon landing on the Moon

If you’re reading these words on Friday, 6 September, you should know — maybe you do already—that Vikram is about to land on the Moon. Just as Neil and Buzz and several others did, over three magical years half-a-century ago.

All right, let me admit: I don’t mean a human being called Vikram, but a landing craft that took off from Sriharikota on 22 July. It’s been orbiting the Moon for over two weeks now, first as part of the larger Chandrayaan-2 craft, the last few days on its own in lower orbits. Sometime after you finish this column, Vikram will leave its orbit to descend further and, eventually, attempt to land. That final approach will take about 15 minutes, a passage of time I described thus in an earlier column: “It will likely be the longest 15 minutes in the lives of all the members of the team at Isro (Indian Space Research Organisation)."

Long and nerve-wracking, and at the end, I fully expect that Vikram will be safely standing on the Moon’s surface. But until it happens, let’s take a look at what the whole process has involved.

In that earlier column, I tried to give you a sense of what it takes for Chandrayaan-2 to reach the Moon. To break free of Earth’s gravity, Chandrayaan-2 actually flew ever-more elongated elliptical orbits around the Earth, picking up ever-more speed with each pass around the planet. This eventually took the craft into what’s called an “Earth parking orbit", tracing an ellipse whose perigee (the point closest to the Earth) was 170km, but whose apogee (the point furthest from the Earth) was nearly 40,000km. From there, Chandrayaan-2 set out on a “lunar transfer trajectory"—in other words, a path to the Moon.

Or really, a path to the Moon’s sphere of influence. This means the point where Chandrayaan-2 got close enough to our satellite that its gravitational pull, rather than the Earth’s, was the major influence on its motion. When Chandrayaan-2 reached that point in its voyage, on 20 August, it went into orbit around the Moon. Again, this was an elliptical path, initially with a perigee of about 120km and an apogee of 4,500km from the Moon.

What then ensued was the opposite of what the craft did earlier. Whereas it had steadily elongated its orbits around the Earth, Chandrayaan-2 now steadily shrank its orbits around the Moon, simultaneously slowing its speed through space. In effect, it was “circularizing" its orbit, to a point when it was a nearly constant 100km above the Moon’s surface.

Why do this? Well, let’s say you are in a fast-moving train and have to get off at Lunavada in Gujarat. If you simply step off as it rushes through the station, you’ll agree that you’re likely to suffer some significant damage. Naturally, you’d like the train to slow down, preferably even stop, before you step onto Lunavada soil. As for shrinking its orbit, think of this other analogy I’ve used before. You’ve tied a stone to a long string and you’re whirling it rapidly around your head. If you’ve had enough and want to stop, you could just suddenly stop whirling. But then the string might wind itself around you, the stone’s momentum will cause it to move unpredictably, maybe hit you or someone nearby, again causing some significant damage. Much better to gradually shorten the string, controlling the whirling as you do, and stop only when you’re sure what the stone is going to do.

That should help understand why Chandrayaan-2 settled into a lower, slower orbit.

Having accomplished that, the craft was ready for the next move: Releasing Vikram to descend to the Moon. You may wonder why just this Moon lander will descend to the Moon—why not the whole Chandrayaan-2 craft? With previous manned Moon missions, there was a simple answer. Take Apollo 11: Michael Collins stayed in the command module, orbiting the Moon, while Buzz Aldrin and Neil Armstrong descended in the lunar module, did their Moon walk and returned. Then all three flew back to Earth in the command module.

But even with an unmanned craft like Chandrayaan, it made sense to have a separate lunar lander, because Vikram was then purpose-built just for landing and staying on the Moon. It carries only the instruments specific to its experimental purposes on the Moon, not the other instruments on Chandrayaan-2. All of which partly explains why Vikram is significantly lighter than Chandrayaan-2 itself—about 1,500kg compared to about 2,400kg. The lower weight itself makes the job of landing the craft safely easier. Think of dropping an object from the top of a building, and you don’t want it destroyed on impact. Which would you rather try this experiment with: A grand piano or a golf ball?

Vikram separated from Chandrayaan-2 last Monday, spinning slowly on its way down, initially establishing a lower orbit for itself. Let’s be clear here: It isn’t as if Chandrayaan-2 is dropping Vikram straight to the Moon’s surface; Isro’s scientists were tasked with finding a way to have it land without shattering.

Now, back in 1969, Nasa’s scientists had the same task with the Apollo lunar module. Though arguably, not shattering was then an even more critical concern, given that the module’s payload comprised, among other things, a certain Armstrong and a certain Aldrin. It seemed fragile, but it was designed tough enough to keep both men alive during the descent and the later ascent back to the command module.

Over an hour after separating from the command module and falling away towards the lunar surface, the lunar module fired its engine so its descent would slow. Not powerfully enough to send it back up, of course, but enough so that it wouldn’t drop like a stone, so that Armstrong could control its flight if and when necessary. Meanwhile, Aldrin used an on-board radar to keep close track of their speed and altitude. They fell towards the Moon with no major scare, heading for a spot that had been chosen for the landing. The time for landing had also been chosen carefully, early in the lunar day when the Sun, rising over the landing site, ensured that anything on the ground would cast long and thus visible shadows. Why this? Imagine looking from high above at objects directly below you on the ground. Absent shadows, how can you tell their size?

As it turned out, the shadows were useful indeed. For when the module got close enough for Armstrong and Aldrin to see the Moon’s surface clearly, they noticed that the site was dotted with large boulders. It was actually a crater (“West Crater"), about 100 metres across. With all the boulders, it would have been impossible to land there, upright and stable. So, Armstrong took control of the module so he could search for an alternate place to touch down.

He fired its engine, again, to slow the landing. He fired its tiny steering rockets to fly the module sideways. These manoeuvres, of course, used precious fuel on board the module. But West Crater’s boulders left Armstrong with no alternative. He flew the craft over the crater, then across a smaller one known as “Little West Crater". On a flat, featureless plain about 50 metres beyond this second crater, he put the lunar module down on the Moon.

It had just 30 seconds worth of fuel left. That’s how close Armstrong came to crashing the craft on the Moon. Once they landed safely and Armstrong stepped onto the surface of the Moon, he walked to Little West and took a famous panoramic photograph: Crater in the foreground, lunar module like a toy all the way on the left.

Except for sighting boulders and needing a manual takeover, Vikram’s final descent to the Moon will be broadly similar to this. It will land somewhere near the Moon’s south pole. Once it is on the ground, a small lunar rover with six wheels, Pragyan, will emerge from Vikram. Rolling along at a crisp 1 cm/second, it will roam the surrounds of the landing site for a full lunar day (about 14 days on Earth) and send data back to Isro. Eager scientists there are hoping that, among other things, it will detect water under the surface of the Moon. If it does, what a find that will be.

There you have it: The bare bones of a remarkable mission to the Moon. Not even my dry prose will, I trust, kill the wonder and romance of it all.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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