September 27 2021 Some months ago, I wrote about a tiny helicopter's first flight ... on Mars. It's called Ingenuity, and the reason it was by no means certain it would lift off and fly on Mars is that the atmosphere on Mars is very thin.
But it did fly. Still, I wondered idly at the time, what if Ingenuity found itself in a spot on Mars where the air was even thinner? Would it still be able to fly? Well, that question became relevant some days ago, but not because the helicopter found its way to top of some massive Martian mountain. The air is subject to "seasonal" variations, and about now, it's getting measurably thinner. So yes, would Ingenuity be able to fly? That was the prompt for my last Mint column (Friday Sep 24), though when I started writing on it, I found myself exploring what humans, not just a helicopter, would be faced with in the Martian atmosphere. No more. Take a look: Breathe in the air, the higher you fly, https://www.livemint.com/opinion/columns/breathe-in-the-air-the-higher-you-fly-11632419745508.html cheers, dilip PS: Tell me if you catch the musical reference. --- Breathe in the air, the higher you fly It's a good bet you haven't paid a lot of attention to the density of air. At sea level, it's about 1.2 kg/cubic metre, meaning a cubic metre of air weighs about 1.2kg. You've probably seen references to how air gets "thinner", or less dense, at altitudes. This is why travellers to Ladakh, for example, are advised to take a day or two to "acclimatize" there. The thinner the air, the harder it is to take in enough oxygen to function normally. Of course, this is even more of a factor for climbers trying to summit Mt Everest. For at the peak of that mountain, a cubic metre of air weighs only about 400 grams - about a third of the sea-level figure. Not only must climbers take the time to acclimatize as they get ever higher up the mountain, they often carry oxygen along, to help them breathe at those heights. To understand all this, start with how much air you take in as you breathe: about eight litres a minute. At sea level, that much air weighs about 10 grams. 20% of that is oxygen, or about two grams. So just normal everyday functioning needs 2gm of oxygen per minute of normal breathing. At the top of Everest, you'll need three minutes, or much deeper breathing, to get that much oxygen into you. Hard work. No wonder most climbers carry tanks of oxygen as they near the top. Imagine, then, reaching a place where the air around you is so thin that a cubic metre weighs just 18 grams. Not over a kilogram, not 400gm, but a measly 18gm. If you're still on Earth, the eight litres you take in each minute will weigh just 0.15gm, of which 0.03gm is oxygen. How hard will you have to work to get your regular 2gm/minute dose of oxygen? No amount of acclimatization will help you survive in air that thin, even if the air was pure oxygen. You'd have to carry and breathe from oxygen tanks all the time. There is actually a place where the air density is this low, though it's nowhere on Earth. This is Mars, and in case the thinness of its air wasn't obstacle enough, there's an added complication. 20% of the Earth's air is oxygen, but only 0.16% of the air on Mars is. So for each minute of breathing on Mars, you will inhale only 0.00024gm - 0.24 microgram - of oxygen. The truth is that you'll be inhaling almost pure carbon dioxide, because that's what makes up 95% of Martian air. The real truth is that you'll quickly be dead. Unless you strap on those oxygen tanks you've lugged from Earth. Then again, there's an instrument called MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment) that travelled to Mars with NASA's Perseverance Rover that landed there earlier this year. The team that designed this toaster-sized device describe it as an "electrical tree", which is pretty accurate. Trees on earth absorb carbon dioxide and release oxygen, and that is exactly what MOXIE is designed to do. Remember that carbon dioxide is made up of carbon and oxygen (CO2). MOXIE "breathes in" carbon dioxide and breaks the molecules into those components, thus producing oxygen. But if that's just the theory, last April MOXIE went to work and actually produced about 10gm of oxygen in an hour. Just five minutes worth for your average human, sure, but real breathable oxygen all the same. And in doing so, it showed that future humans on Mars won't need to carry huge amounts of oxygen from Earth. That would be a nearly impossible job anyway. MOXIE, or really a MOXIE on a larger scale, can produce oxygen for them. Still, the main reason to produce oxygen there, at least with the first several arrivals on Mars and to allow any long-term human presence on the planet, is not so much because human visitors need to breathe. Even with 100 women and men on Mars, you'd need less than 300kg of oxygen per day for all their breathing needs. In contrast, to lift a rocket off Mars and carry some of those humans back to Earth - a voyage that will certainly happen over and over again - will need 25 tonnes of oxygen. That's 25,000kg for each liftoff. There's no way to carry that much oxygen to Mars. Instead, the scaled-up MOXIE will have to be transported there many months before the first humans arrive and put to work. For even if this hyper-MOXIE can generate a kilogram of oxygen an hour instead of 10gm, it will still take nearly three years to produce enough oxygen for one rocket liftoff. Then of course there's the need for facilities to store all that oxygen, something else that will need to transported to Mars. All this, just to give you a sense of what sending humans to Mars will entail. And yet, this column was really prompted not by humans' Martian needs, but by the problems thin air poses to something that's already on Mars. I'm referring, you may have guessed, to Ingenuity, a tiny helicopter that also travelled to Mars with Perseverance. Like MOXIE, this was really - as I wrote in a column on Ingenuity ( https://www.livemint.com/science/news/pisa-to-mars-ingenuity-all-the-way-11619116975602.html) - "just an experiment, designed to answer the question: is flight possible at all on Mars?" Why was this a question? Precisely because the air on Mars is so thin. Is there enough so that Ingenuity’s blades can lift the chopper off the surface? That was answered in April, when Ingenuity flew for half a minute. Since then, it has flown a dozen more times, spending nearly three minutes aloft on some of those flights. It has got as high as 12 metres off the ground, and on its ninth flight, in July, it flew over 600 metres horizontally. Yet its 14th flight, which will happen any day now, will be more in the nature of a test. This is because the air density on Mars, with the changing of "seasons" there, is falling. NASA estimates that "in the coming months, we may see densities as low as 0.012 kg per cubic metre during the afternoon hours that are preferable for flight" ( https://mars.nasa.gov/technology/helicopter/status/334/flying-on-mars-is-getting-harder-and-harder/) - meaning, a cubic metre of air will weigh just 12 grams. Not 18. Can Ingenuity handle that drop? The thinning air will need Ingenuity to spin its blades even faster than it has done till now: 2800 rpm compared to about 2500 rpm used so far. Will that be possible? Can the little helicopter sustain a rotor speed that high? There are various possible side effects to consider, some potentially damaging to Ingenuity's delicate mechanisms. Still, on 17 September NASA confirmed that Ingenuity did a rotor spin test at 2800 rpm ( https://twitter.com/NASAJPL/status/1438869634866761730), without taking flight. Thus we know the blades can spin that fast. So Ingenuity is ready to attempt flight again. Though when it happens, the 14th flight will operate at 2700 rpm, to leave a buffer in case it's needed at some point. It will be a brief flight - climb to 5 metres, fly a short distance sideways and land. That much will prove that Ingenuity remains able to fly, even as the air on Mars gets still thinner. But it's probably a good thing Ingenuity doesn't need any oxygen. -- My book with Joy Ma: "The Deoliwallahs" Twitter: @DeathEndsFun Death Ends Fun: http://dcubed.blogspot.com -- You received this message because you are subscribed to the Google Groups "Dilip's essays" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/dilips-essays/CAEiMe8qbprKGhKrpmsu9TPt%3DhNVFA3EMWesgrQPACuMzy23ZgQ%40mail.gmail.com.
