Home >Science >News >Pisa to Mars: Ingenuity all the way

A few years before you were born, Galileo Galilei conducted a famous experiment. One day in about 1590, he climbed to the top of the Leaning Tower of Pisa, carrying a heavy ball and a light one. He dropped them at the same time, from that height of about 55 metres. He measured how long it took for the balls to hit the ground.

Why did he do this? Because the generally-believed theory of gravity until then was Aristotle’s: that objects fall to the Earth at a speed determined by their weight. That is, heavy objects fall faster than light ones. After all, doesn’t a ball fall faster than a feather? But Galileo had come to believe Aristotle was wrong—that the speed of a falling object has nothing to do with its weight. This Pisa experiment was designed to confirm his thesis.

Sure enough, at Pisa that day the two balls touched down at the same time, proving Galileo right. I’m going to suggest that there’s a line of sorts that you can draw, all the way from Galileo at the Tower to a little helicopter that rose from the surface of Mars earlier this week. Bear with me.

Aristotle was indeed wrong. Galileo was correct in his observation and in his idea about how objects fall. A few decades later, Isaac Newton formulated his theory of gravity. Much later, Albert Einstein formulated a quite different theory—that the objects don’t actually “fall" in any sense, and it’s our particular frame of reference that suggests that they do. But for now, let’s stick to what Galileo observed at Pisa.

As you can imagine, there were still sceptics. After all, a ball still falls faster than a feather, a fact you can confirm right now, 400-plus years on. If that’s so, in what sense is the speed of its fall independent of the object’s weight?

No doubt you’re shaking your head at the naivete on display here. The feather drifts airily to the ground because it is falling through air, which offers enough resistance, by way of friction, to slow down the feather. It offers friction to the ball, too, of course. (This is why meteors that enter our atmosphere burn up—the friction heats them—and we see them as shooting stars). But the ball is heavy enough that the friction does not slow it materially. Of course, if the ball was falling through something much denser than air—thick honey or melted chocolate, or a soft cake—it too would slow down. Perhaps even stop: I’m yet to see a ball fall through a cake.

So ... aha! The weight of the falling object does indeed matter! So was Aristotle right then? Not quite. Because the real test would be to remove the medium through which our objects fall—air, honey, cake, whatever. Then what would happen?

You know the answer. Even so, it’s pretty dramatic to watch what the physicist and one-time rock musician Brian Cox made of it in a famous film for the BBC. In 2014, he visited NASA’s Space Power Facility in Ohio, where there is a massive chamber ordinarily used to test spacecraft. What’s relevant here is that this chamber can be almost entirely emptied of air. This is 30 tonnes of air we’re talking about, that’s pumped out of the chamber.

After setting up cameras to record the experiment, Cox winched a bowling ball and several large white feathers to near the top of the chamber, over 10 metres above the ground. Then he pressed a button that released ball and feathers simultaneously. No surprise, the ball plummeted to the ground. Or actually, so as not to damage the floor, it plummeted into a sort of sandbox on the ground. For their part, the feathers drifted about carelessly because of air resistance. Eventually, they made their way to the ground, too.

But next, the facility’s technicians removed the air from the chamber, all 30 tonnes. This left a vacuum inside the chamber. Once more, Cox winched feathers and ball to the top. Once more, he released them simultaneously. The result is just what you would expect, and it still takes your breath away.

Captured on the BBC’s cameras, feathers and ball plummet as if in lockstep, as if this was one bizarre object. Still in lockstep, they smash into the sandbox. This lockstep is because they are falling in a vacuum, facing no air resistance. Takes your breath away, because not a hair on the feather so much as flutters. No feathers you’ve known behave like this.

Now this is all dandy, but you probably are wondering what any of this has to do with helicopters and Mars. In a few words, this: the atmosphere on Mars is less like ours and much more like a vacuum.

Why is this important? Because here on Earth, a helicopter rises off the surface by, fundamentally, using its rotating blades to move air about. The blades are shaped so that they are curved on top and flatter below. When the blades rotate, the air flows faster over the top of the rotor than the bottom. By Bernoulli’s famous Principle, this means there is lower pressure above the blades than underneath. This produces a suction upwards—or a “lift" to the helicopter. Spin the blades fast enough and that upward force is enough to lift the helicopter into the air. That is, helicopters need air. Put one in that NASA Space Power Facility, pump all 30 tonnes of air from the chamber—and no matter how fast its rotor spins, the helicopter will not rise off the ground.

This was the challenge NASA’s engineers faced with their small robot helicopter, Ingenuity. They designed, built and sent it to Mars aboard the Perseverance rover that landed on Mars in February. While it is a serious flying machine, Ingenuity is really just an experiment, designed to answer the question: is flight possible at all on Mars?

On Earth, the answer is an emphatic yes, as years of flight have taught us; as in fact Ingenuity’s test flights here on Earth showed. But on the surface of Mars, the atmosphere is only about 1% as dense as the Earth’s. That’s not quite a vacuum like in the Ohio chamber, but it’s still very thin air indeed. Would there be enough air for Ingenuity’s blades to push down on, to create enough lift to get the little chopper off the surface of Mars?

At about 1am IST last Monday, Ingenuity’s two rotors fired up to about 2,500rpm, spinning in opposite directions. The delicate little machine, all 2kg of it, rose about 3 metres into the air, stayed there for half a minute and then returned to the surface of Mars.

117 years ago, the Wright brothers flew the first powered aircraft on any planet we know of. Ingenuity’s lineage goes back to that remarkable 12-second flight. But think also of Galileo on top of that tower in Pisa. The balls he took up there dropped to the ground in tandem. But that also explained that a ball falls faster than a feather because of the effect of air. Keep playing with that thought and all its implications and, eventually, you get flight.

You get a line of scientific inquiry and endeavour that stretches from Pisa to Kitty Hawk to a certain desolate plain on Mars.

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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