Despite any touchdown on another celestial body like our own Chandrayaan from 18 months ago, there is considerable uncertainty in operation. Does the craft tolerate heat when entering the atmosphere? How does it degrade from the speed it has traveled for months? Can anything go down smoothly without being damaged?
For any mission through space, but especially unmanned, the time will come when its designers will go beyond their fingers and hope everything goes well. In fact they designed the craft nicely and planned its journey, to answer the above questions. But there is that design and planning process, and there is the reality of the flight, and the two do not necessarily meet. Sadly, Chandrayaan crashed on the surface of the moon. Thus only 40% of the missions we humans sent to Mars were successful.
Mars presents its own challenges. Partly, because it is 200 million kilometers away. Any signal we send takes 11 minutes to get there. (With this in mind, press the “Lock” button on the modern car key, and the door will actually lock after 11 minutes). That delay makes real-time control of perseverance impossible, certainly in the shortest time between entering Mars atmosphere and touchdown.
No great secret: everything about that time needs to be programmed in advance, as well as the ability to recognize and adapt to unforeseen circumstances or obstacles. So Friday morning will be the time of the desperate mission’s descent, ending only after we know it’s safe on the surface.
Still, planning for a touchdown of perseverance is fascinating. Partly because it is so complicated and so many things need to be calculated, yes. But mostly because it almost ends with a moment of poetic imagery. Stay tuned for it.
Launched on July 30 last year on the Mars 2020 spacecraft, Perseverance: The Rover’s Landing and Exploration Mission Point. But the entire craft does not need to reach Mars. So, shortly before entering the atmosphere, at a distance of 1,500 kilometers from the surface of the planet, it separates into two parts.
Cruise stage jettison carrying solar panels and other equipment needed for long journeys. The rest looks like a squat top. This module is actually a container built to protect the inward-descending phase and the rover from the imminent descent destruction by the Martian atmosphere.
Now, the Martian atmosphere is much thinner than Earth. As a result, it resembles a void here. On the surface of Mars, as high as 100,000 feet above the Earth, the atmosphere is dense — actually very thin. So you may be wondering, what “destruction” are we talking about?
Well, even in such thin air, an object traveling at a speed of 20,000 kilometers per hour — the speed of Mars 2020 upon atmospheric entry — would collide with it, if not for the protection provided by the container.
Therefore, before entry, the container should be in its direction so that its heat-resistant shield is in the direction of travel. It does this by firing one or more small thrusters. Entering the atmosphere in this way serves two purposes. One, it absorbs most of the heat of entry. If the descent is well done, the shield heats up to 1,300 C, while its valuable contents are 25-30. Allow to bulge at C.
But two, the atmospheric drag that causes the heat can also drastically reduce the craft, which is what we want. After all, if it still zips when it reaches the surface at a speed of 20,000 kilometers per hour, it is a farewell perseverance.
There is a specific goal for landing — the Zero Crater — so it is important to continue the course. As it enters the atmosphere and begins to descend, the craft occasionally fires its thrusters to make the necessary minor corrections in its path. Therefore the step of this process is called “guided entry”.
In about 4 minutes, the heat shield slows its rocket craft from 20,000 km to 1,500 km, which is 2-3 times faster than a normal plane flying at altitude.
At this point, the craft is 11 kilometers above Mars – and it is very dramatic and photogenic when anything happens, I just want to be there.
But I have to hold on to imagine. The enormous parachute, about 70 feet across, fires from the craft. The typical Army paratrooper uses a 35-foot horizontal ‘chute’, so it is twice the diameter and eight times the size of a thin Martian air. If you need to slow down the striking spaceship, you need a huge parachute like this.
Another 20 seconds pass. The parachute slowed the craft to 600 km and it was another two km. Now, the heat shield – the “front” of the protective container – is jettisoned and dropped to the surface of Mars. For the first time since the mission left Earth, perseverance is subject to the elements. This is due to the design, as its cameras now begin to focus on the ground below. It uses radar to accurately determine its altitude. It looks for dangers. It will determine if further corrections need to be made in its path.
The parachute will accelerate the craft for another 300 km. In the thin martini air, it no longer does. At this stage, about 2 km from the surface, the parachute and the rest of the container – the “backshell” – are cut off. The only thing left in the mission is the perseverance rover and the descent phase, and the only thing left in the descent is the last minute or so.
There are engines in the descending phase. They fire immediately, first to escape the backshell and parachute and then down to launch the “powerful descent” to avoid the damage they cause. The engines slow down the descent phase and rover at a speed of about 3 km / h, just 70 feet above the ground at altitude.
Now for that bit of poetry: NASA calls it “SkyCron”. Using 21-foot-long nylon cables, the descending phase now slowly reduces perseverance. Meanwhile, Perseverance has stretched its landing gear-wheels and legs for the final touchdown.
I was thinking about that scene from the Sound of Music. You know: Maria and the kids, their Marionettes are hanging from long strings as if they had persevered for a few seconds today. That’s how it should be for those few seconds on Mars. Think for a moment.
The whole Marionette approach descends slowly. When the rover detects the touchdown, it is without cables. The descending phase flies, burning the rest of its fuel and crash lands elsewhere. If all goes well, Perseverance will be safe on Mars, ready for its months of exploration.
This is the time Martin entered the atmosphere: seven minutes.
Dilip D’Souza, once a computer scientist, now lives in Mumbai and writes for his dinners. His Twitter handle is DeathEndSfun