Monday, February 26, 2024

Above moon, a moment of stillness

You’d be pretty impressed, I’d say. Sure, you might run for cover so that it doesn’t fall on your head, but you would indeed be pretty impressed. How does this 1,700kg behemoth stay aloft?

Well, something akin to that small miracle is what happened on Wednesday evening. Ok, it wasn’t an SUV. Ok, it wasn’t above your head, nor even above the heads of anyone else on the planet. But it was the moon and it was the Chandrayaan-3 mission’s landing module, Vikram. And, for a few seconds on Wednesday evening, it was pretty much stationary above the moon’s surface. Hovering. Ready to drop those last few hundred metres.

There’s plenty to marvel at in the whole Chandrayaan mission, in space travel in general, really. But if you put a gun to my head and asked me to pick one heart-stopping moment, I would choose this one. For me, at any rate, it speaks of the singular triumph of science that this moon mission really stands for.

In that sense, it’s analogous to a plane taking off. Think of it: A little over a century ago, if you told a random human being that an enormous object weighing nearly 200,000 kg—your typical Boeing 747—could rise into the air and stay there, I think that random human being would have looked at you strangely and hurried to get away from you. Yet today, it’s just a routine part of life. It doesn’t even strike us that a plane’s take-off really embodies an enormous belief and trust in science.

Similarly with Vikram’s momentary pause in mid-descent to the moon. There may come a time when travel to the moon is as routine as planes taking off; when all of us know of and don’t pay attention to the occasional need for a landing module weighing two tonnes to hover above random moon dwellers’ heads. Yet being routine won’t change the essence—that same belief and trust in science.

And for me, that’s what’s truly worth celebrating in the Chandrayaan-3 mission.

Spend a few moments in thought about the numbers involved, again substituting an SUV for Vikram in your mind.

Our SUV’s final descent to the moon has been widely referred to as “15 minutes of terror”. That’s because it is effectively on autopilot, following preprogrammed instructions. Part of those instructions are to evaluate the moonscape below and make a decision to switch landing sites, if required.

But here’s the thing. At the start of those 15 minutes, the SUV is lying on its side, parallel to the surface, and some 30 km above, orbiting the moon with its wheels first. If you are on the moon, you may not even see the SUV that far above you, let alone be able to discern that it leads with its wheels. In the moon dawn, you might see it as a shining dot, moving swiftly across the moon sky. Swiftly is right: its speed is about 1.7 km per second.

The task, then, is this: slow the SUV down, get it to gradually descend, and turn it 90 degrees on its axis so its wheels point at the surface of the moon. We want it to land on those wheels, after all.

That’s just what the autopilot instructions set out to achieve. Over nearly 12 minutes, the SUV slows its forward speed by a factor of 5, to almost 350 m per second. In that time, it also moves forward about 715 km—about the distance between Mumbai and Hyderabad. Importantly, by then its landing site is only another 30 km or so ahead. By then, it has swivelled about 45 degrees on its axis; its wheels are about halfway to pointing where they need to, for landing. But also by then, the SUV has descended nearly 23 km. It continues to fall out of the moon sky at about 60m per second.

At its current forward speed, the SUV will appear over the landing site in about 90 seconds. But of course, we can’t have it still moving forward when it gets there. So, the SUV slows down some more, taking the next three minutes to arrive above the landing site—but with a forward speed of zero. But again importantly, at this point the SUV has swivelled a further 45 degrees, so that its wheels are now pointing directly at the ground. It’s close to 1 km off the moon’s surface. How did all this slowing down and swivelling happen? The SUV fired its on-board rocket engines judiciously.

Again, if you’re at the landing site and you look up, you’ll probably see the SUV there above you, about a kilometre away. Too far to read the licence plates, but close enough to realize with a shock that there’s a massive vehicle up there, about to fall on your head.

Yet that’s just the point. To your astonishment, the SUV doesn’t fall. Instead, it stays rock-steady there for a few seconds, as if contemplating its options. It does this by firing its engines, this time aiming them at the ground. Understand: The engines must generate exactly enough thrust to balance the SUV’s weight in the moon’s gravity, and keep the SUV in place.

This is the calculation that the Indian Space Research Organisation (Isro) made, that sees dramatic fruition right here. This was, for me, that heart-stopping moment. On the Isro broadcast of the landing, the continuously-reported downward vertical speed actually shrank to zero. So did the forward speed. In black-and-white there on the screen, we know Vikram is hovering above where it will land.

Sorry, our hypothetical SUV is hovering above. See those wheels, don’t you?

Things go quickly now. At close range, the site looks ok for a landing. Within a few dozen seconds, that’s just what happens. The SUV lands on its wheels.

Sorry, Vikram lands on its landing pads. It generates small clouds of moondust. It also generates rapturous applause in Isro, across a nation, around the world.

Yes, applaud the mathematics, the science, that has taken remarkable shape here. Isaac Newton once said of his work: “If I have seen further, it is by standing on the shoulders of giants.” Vikram stands today on the moon. It stands, too, on the shoulders of so many giants, Newton among them.

Somehow, that’s even more heart-stopping.

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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Updated: 24 Aug 2023, 11:47 PM IST

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