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By the time you read this, either the rover will be safely settled on the planet’s surface, or it will be destroyed. Indeed: The plan is that Nasa’s Perseverance rover will touch down on Mars at about 2.30am IST on Friday, 19 February. So yes, when you read this, you will likely know its fate.

With any touchdown on another celestial body—like our own Chandrayaan from 18 months ago—there’s substantial uncertainty in the operation. Will the craft withstand the heat on entry into the atmosphere? How will it decelerate from the speed it has cruised at for months? Can it land gently enough so that nothing is damaged?

For any mission through space, but especially an unmanned one, there will come a time when its designers simply cross their fingers and hope all goes well. Of course they have meticulously designed the craft, planned its journey, to answer the questions above. But there’s that design and planning process, and there’s the reality of the flight itself, and the twain don’t necessarily meet. This is why, sadly, Chandrayaan crashed onto the Moon’s surface. This is why only about 40% of the missions we humans have sent to Mars have been successful.

Mars presents challenges all its own. Partly, that’s because it is some 200 million km away. Any signal we send takes about 11 minutes to reach there. (To put that in perspective, imagine pressing the “lock" button on the key of a modern car, and the doors actually locking 11 minutes later). That delay makes any real-time control of Perseverance impossible, certainly in the relatively short time between its entry into the Mars atmosphere and touchdown.

No great secret: everything about that time has to be programmed earlier, as also the ability to detect and adapt to unforeseen situations or obstacles. This is why the descent of the Perseverance mission on Friday morning will be an agonizing time, ending only once we know it is safe on the surface.

Still, the plan for Perseverance’s touchdown is fascinating. Partly because it is so complex and needs to account for so many things, yes. But mostly because it ends with a moment of almost poetic imagery. Wait for it.

The Mars 2020 spacecraft launched on 30 July last year, carrying Perseverance: landing the rover and letting it explore is the point of the mission, of course. But the whole craft doesn’t need to reach Mars. So, shortly before entering the atmosphere, about 1,500km above the planet’s surface, it separates into two components.

The cruise stage, which carried solar panels and other equipment necessary for the long journey, is jettisoned. What’s left looks like a squat top. This module is really a container built to protect what’s inside—the descent stage and the rover itself—from the ravages of the imminent descent through the Martian atmosphere.

Now, the Martian atmosphere is much thinner than Earth’s. In effect, it is similar to a vacuum here. On the surface of Mars, the atmosphere is about as dense as Earth’s is at a height of 100,000 feet—very thin indeed. So you might wonder, what “ravages" are we talking about?

Well, even in such thin air, an object travelling at some 20,000km/hour—the speed of Mars 2020 on atmospheric entry—encounters friction that could burn it up, were it not for the protection the container offers.

So, before the entry, the container must orient itself so that its heat-resistant shield faces forward, in the direction of travel. It does this by firing one or more of its small thrusters until it is oriented right. Entering the atmosphere this way serves two purposes. One, it absorbs much of the heat of the entry. If the descent goes well, the shield will heat up to as much as 1,300°C, while allowing its precious contents to stay at a balmy 25-30°C.

But two, the atmospheric drag that is responsible for the heat also drastically slows the craft, which of course is just what we want. After all, if it is still zipping at 20,000kmh when it arrives at the surface, it’s goodbye Perseverance.

There is a specific target for the landing—the Jezero Crater—so it’s important to keep on course. As it enters the atmosphere and starts descending, the craft occasionally fires its thrusters, to make necessary small corrections in its path. This is why this stage of the approach is called a “guided entry".

Over a period of about 4 minutes, the heat shield slows the craft down from its rocketing 20,000kmh to about 1,500kmh, about 2-3 times faster than a typical plane at cruising altitude.

At this point, the craft is about 11km above Mars—and this is about when something happens that is so dramatic, so photogenic, that I wish I was there watching.

But I’ll have to imagine it. An enormous parachute, about 70 feet across, shoots out of the craft. The typical Army paratrooper uses a ‘chute that’s 35 feet across, so this is twice the diameter and holds eight times the volume of thin Martian air. If you have to slow a streaking spacecraft, you need a massive parachute like this one.

Another 20 seconds pass. The parachute has slowed the craft to about 600kmh, and it has descended another couple of km. Now, the heat shield—the “front" of the protective container - is jettisoned and falls away to the surface of Mars. For the first time since the mission left Earth, Perseverance itself is exposed to the elements. This is by design, because its cameras now start focusing on the ground below. It uses radar to precisely determine its altitude. It matches images of the land it “sees" below against a stored map of the area selected for the landing. It looks for hazards. It decides whether there’s a need to make further corrections in its path.

The parachute slows the craft some more, to about 300kmh. In the thin Martian air, it can do no more. At this stage, about 2km above the surface, the parachute and the rest of the container —the “backshell"—are cut free. What’s left of the mission is the Perseverance rover and a descent stage, and what’s left of the descent is only the last minute or so.

The descent stage has engines. They fire immediately, first to dodge away from the backshell and parachute and avoid any damage they might inflict, then downward to begin a “powered descent". The engines slow the descent stage and rover down to about 3kmh, at a height of just about 70 feet off the ground.

Now for that bit of poetry: what Nasa calls a “skycrane". Using 21 feet-long nylon cables, the descent stage now lowers Perseverance slowly. Meanwhile, Perseverance extends its landing gear—wheels and legs—for the final touchdown.

And I think of that scene from The Sound of Music. You know: Maria and the kids, their marionettes hanging from long strings just like Perseverance did for a few seconds today. That’s what it must look like, for those few seconds on Mars. Take a moment to imagine it.

The whole marionette mechanism descends gently. When the rover detects touchdown, it cuts itself free of the cables. The descent stage flies off, burns its remaining fuel and crashlands elsewhere. If everything has worked right, Perseverance is safe on ground on Mars, ready for its months of exploration.

Time since it entered the Martian atmosphere: seven minutes.

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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