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Science
by Pat Murphy & Paul Doherty

A Lighter Look At Science

OVER THE years we've discovered that we can start an exploration of science and science fiction just about anywhere. Today we'll start with something that might seem trivial (and maybe not related to science fiction at all). We'll start with a balloon.

Our exploration of balloons will take us high in the sky (as you might expect) and deep in the ocean (which you might not expect). We'll talk about the adventures of a guy in a flying lawn chair and balloons on Mars. We'll explain why you might want to take a balloon full of lard on a submarine voyage. In the end, we'll leave you with a bang (not a whimper).

BALLOONS AT THE CUTTING EDGE

First, a nod in the direction of our sponsor (science fiction, that is): Balloons have long been the stuff of science fiction, starting when these lighter-than-air craft were at the cutting edge of technological advancement.

Back in 1844, Edgar Allan Poe wrote a newspaper article that purported to be a factual account of a three-day hot-air balloon voyage across the Atlantic Ocean by European Monck Mason. It ran with the headline: ASTOUNDING NEWS! BY EXPRESS VIA NORFOLK: THE ATLANTIC CROSSED IN THREE DAYS!

Poe was writing his article sixty-one years after the Montgolfiers took a short hot-air balloon flight. Extrapolating in the manner of skilled (and lying) science fiction writers everywhere, Poe provided plenty of details, including the method of propulsion, the precise dimensions of the air craft (length: thirteen feet six inches; height: six feet eight inches; volume: about 320 cubic feet of gas), and a description of the steering mechanism. Appearing in the New York Sun, Poe's story concluded with "This is unquestionably the most stupendous, the most interesting, and the most important undertaking ever accomplished or even attempted by man. What magnificent events may ensue, it would be useless now to think of determining."

Two days later, the Sun printed a retraction that began: "The mails from the South last Saturday night not having brought a confirmation of the arrival of the Balloon from England.… [W]e are inclined to believe that the intelligence is erroneous."

That's right—Poe made it all up. It wasn't until seventy-five years after Poe published his story (now known as the "Balloon Hoax") that the first human-carrying lighter-than-air craft (the British dirigible R-34) crossed the Atlantic in 108 hours and 12 minutes.

WHAT GOES UP…

We'll move from science fiction into science fact with a quick review of the basic science behind lighter-than-air flight. Consider, for a moment, a helium-filled toy balloon. Let go of the string, and the balloon rises into the sky.

Why? You could say it rises because the helium in the balloon weighs less than the same volume of air. You could also say helium is less dense than air. Just as an air-filled bubble rises in water because the air is less dense than water, a helium balloon rises because it's less dense than the surrounding air.

People usually say "a balloon rises," as if the balloon were causing this to happen. It's actually more accurate to say "the air pushes the balloon upward." The balloon shoots upward because the air around it is trying to squeeze into the space it occupies. Have you ever squeezed a slippery watermelon seed between your thumb and a finger? The seed usually goes shooting off.

The rising balloon is a bit like that watermelon seed. The great thumb and finger of air pressure squeeze on the balloon and it goes shooting in the direction of lessening air pressure—that is, it shoots upward. What makes something buoyant is a difference in pressure: the pressure on the bottom of the object, pushing up, is more than the pressure on the top of the object, pushing down. So air pressure pushes the balloon aloft.

As the balloon heads upward, the density (and pressure) of the surrounding air decreases. The balloon doesn't stop rising until the weight of the balloon and everything attached to it (the payload) equals the weight of the air it displaces.

LAWN CHAIRS IN FLIGHT

And all of this brings us to the guy in the lawn chair.

If you're like us, you've been wondering for years what happened to the version of the future where we all got to fly around on our personal jet packs. We are so ready to go flying. If not a jet pack, how about a personal balloon?

Back in July 1982, thirty-three-year-old Vietnam veteran Larry Walters decided to stop dreaming of personal flight and take action. He filled forty-five weather balloons with helium, arranging them in four tiers. He tied the balloons to an aluminum lawn chair that he'd bought at Sears. He equipped himself with a large bottle of soda, milk jugs filled with water (for ballast), a pellet gun (to blow out balloons when he wanted to come down), a CB radio, an altimeter, a parachute, and a camera.

When the tethers were cut, he rocketed into the sky above San Pedro, California, reaching an altitude of 16,000 feet. Startled airline pilots en route to the Long Beach Municipal Airport reported seeing him floating unprotected through the thin air some three miles above the ground.

Eventually, Larry shot out a few balloons and came back to earth, tangling in some high-voltage power lines on his way down. After that, he tangled with the Federal Aviation Administration, which just doesn't have much of a sense of humor about this sort of thing.

We have always been impressed by Larry's flight. We aren't tempted to duplicate it—but we are interested in considering some of the scientific principles it illustrates and in using those principles to answer questions that might have interested Larry and the other do-it-yourself balloonists who followed his lead.

How could Larry have stayed out of the air traffic lanes and perhaps avoided the FAA's attention? Now that you know the basics of buoyancy, you can probably answer that. Larry and his balloons rose until their combined weight equaled the weight of the air they displaced. Since the density of air decreases with altitude, it's possible to calculate about how high a given volume of balloons might carry you.

According to Paul's calculations (which we will admit, we have not confirmed with actual measurement), a balloon at sea level can lift one kilogram per cubic meter of helium (or about 0.062 pounds per cubic foot of helium). Since one is such a nice number to work with, we'll use metric in our calculations.

So to lift a person weighing 64 kg (about 140 pounds) you need a balloon that is 64 cubic meters. A balloon that size would fill a room that's four meters (or twelve feet square) and four meters (or twelve feet high).

That's enough to lift a person off the ground, but not very high. As altitude increases, air pressure (and the density of the air) decreases. At 5.6 kilometers or 18,000 feet, the density of air is half what it is at sea level. Since the lift of the balloon is cut in half, the balloon will only support 0.5 kg per cubic meter. So if I want to rise to 18,000 feet I need double the volume of the balloon or 128 cubic meters. Of course using a single balloon does not lend itself to slow descent when you shoot it with a gun!

You can do a similar calculation for any altitude you wish to reach using the exponential decrease in atmospheric density with altitude. However we suggest that you do not actually do this. At least one person has died trying to duplicate Larry's stunt.1

BALLOONS ON OTHER PLANETS

Of course, now that you understand the basics, you can extrapolate to other situations. Suppose you're on Mars and you have an urge to go for a balloon ride. You don't have to worry about the FAA, since they haven't reached Mars yet. But you have to figure out what gas will float your balloon in the Martian atmosphere.

The atmosphere on Mars is mostly carbon dioxide—compared with the air here on Earth, which is a mixture of seventy-eight percent nitrogen and twenty-one percent oxygen with argon, carbon dioxide, ozone and everything else making up only about one percent. ("But wait!" you say, being the astute sort of reader we expect here in the pages of Fantasy & Science Fiction. "What about the water vapor?" Well, we're giving the composition of dry air. Water vapor varies from nearly zero percent to nearly four percent depending on place and time.)

To float your balloon on Mars, you need a gas that's lighter than the carbon dioxide—and Avogadro's law will help you figure out what gases might qualify. Back in 1811, Amedeo Avogadro hypothesized that equal volumes of ideal gases, at the same temperature and pressure, contain the same number of particles, or molecules.

Real gases behave pretty much like ideal gases for our purposes. Since two identical volumes of two different gases at the same temperature and pressure contain the same number of molecules,you can figure out which one will be lighter by comparing the weights of the molecules. And you can do that easily enough by consulting the Periodic Table of the Elements and knowing just a little bit about gas molecules.

On the Periodic Table, you'll see that oxygen has an atomic weight of about sixteen (that's eight protons and eight neutrons). An oxygen molecule is made of two atoms, for a molecular weight of thirty-two. Carbon dioxide is two oxygen atoms and a carbon atom (which has an atomic weight of twelve) for a total molecular weight of forty-four.

Look on the table to see if you can find a lighter gas, and you'll see the same ones that work well on Earth. Helium has an atomic and a molecular weight of four. (It's an atomic gas—just one atom per molecule.) Hydrogen has an atomic weight of one, and it has two atoms per molecule, making it a molecular lightweight at two.

Since helium and hydrogen are both lighter than carbon dioxide, they would work fine. You might be reluctant to use hydrogen for lift on Earth (remembering the unfortunate fate of the Hindenburg), but hydrogen requires oxygen to burn. In the Martian carbon-dioxide atmosphere, you'd be fine.

Now suppose you decided to go ballooning on Jupiter. There, the atmosphere is mostly hydrogen and helium. A glance at the periodic table reveals that those gases are the lightest ones around.

So what do you do? You remember that Avogadro's law applies to identical volumes of gases at the same temperature and pressure. On Jupiter, we suggest you consider a hot-air balloon. When you heat a gas, the molecules bounce around faster, spreading out and taking up more space. The density of the gas drops—perfect for ballooning!

SUBMARINE BALLOONING

So far we've been considering balloons that are filled with air and float in air. In both science and science fiction, it's interesting to consider what happens if you take a commonplace situation and make just one change.

You can think of the atmosphere as an ocean of air and a balloon as a bubble floating upward in that ocean. Suppose you changed that ocean of air into an ocean of water. What would you put inside your balloon if you wanted to start at the bottom of the ocean and float to the top?

In science and science fiction, one change leads to other changes. Maybe you think you could fill your submarine balloon with air—after all, air is less dense than water.

But there's a problem. At the bottom of the sea, the pressure of the water would crush the gas. That's fine down to a certain depth. But when you reach the level where the volume of the gas has been reduced so much that your craft's weight exactly matches the weight of the water it displaces, the craft becomes neutrally buoyant. That means it doesn't rise and it doesn't sink.

What happens when you go below that point of neutral buoyancy? The water pressure goes up and the volume of the gas that serves as your balloon goes down. Now the craft weighs more than the water it displaces and you're sinking with no way to get back to the surface. Not a good situation.

So you need to fill your submarine balloon with something that doesn't compress under pressure. Marine mammals have solved this problem with a fatty layer of blubber. Not only does blubber insulate these animals from cold water, it also serves as an incompressible buoyancy device—sort of like a built-in life vest. Taking a tip from the whales, you could fill your balloon with fat. A big balloon full of lard would keep your craft buoyant.

Alas—as far as we know, no one has yet created a lard-based buoyancy control. The Bathyscaphe Trieste, a deep-diving research vessel with a crew of two people, solved the problem by filling its float chamber (an underwater balloon, if you will) with gasoline. Gasoline is less dense than water. Like other liquids, it doesn't compress significantly even at extreme pressures, making it as effective (though perhaps not as absurd) as a float chamber filled with lard.

You've probably heard of a bathysphere: a spherical deep-sea submersible that is lowered into the depths on a cable. A bathysphere isn't an undersea balloon. It's more like an undersea rock. It sinks and, when the adventurers aboard are done, it's hauled back to the surface. Unfortunately, using a cable to lower and raise a craft limits its range. The bathysphere's maximum was about 900 meters (or around 3,000 feet) down.

The bathyscaphe, on the other hand, is not limited by a connection to the surface. Because of its gasoline-filled float chamber, the Trieste could free dive in the water. In 1960, the bathyscaphe reached a depth of about 10,900 meters (35,761 feet), in the Mariana Trench, breaking every previous record and establishing a record that has not yet been matched.

WATER BALLOONS

Talking about underwater balloons got Pat thinking about water balloons.

Well, to tell the truth, Pat started out thinking about water balloons. We were working on this story during a rare heat wave here in San Francisco and she needed an excuse to spend an hour tossing water balloons out the window. ("It's science," she says. The neighbors wonder about that.)

She started thinking about water balloons when she took a look at a few of the many online videos that show a water balloon popping in slow motion. Just go to YouTube and search for "water balloon" and "slow motion." A slow-motion video lets you see aspects of an event that you never noticed before.

Our favorite videos show someone popping a water balloon with a pin or knife. The moment after a pin pricks the balloon, the rubber vanishes, speedily contracting to a fraction of its stretched size. For an instant, you'll see the water without the balloon, a beautiful crystal clear shape Then gravity takes over, pulling the water down down down.

That moment when you see the water without the balloon is an interesting one. As we mentioned in the discussion of the gasoline-filled submarine balloon, liquids don't compress much, even under pressure. So the water is contained but not compressed by the stretched balloon. Remove the balloon and the water stays put.

Equally amazing are the videos of water balloons that do NOT explode, but bounce. Paul likes to create such super strong water balloons by putting one balloon inside another to make a double-strength balloon. Watching a water balloon (or a tennis ball) deform when it hits the ground and bounces back is seeing conservation of energy in action. Energy, say the physicists, can neither be created nor destroyed.

Suppose you hold a water balloon a foot from your driveway. That balloon has potential energy. It has the potential to fall, pulled downward by gravity. As the balloon falls, the potential energy becomes the kinetic energy of motion. When the balloon hits the ground, it either breaks or bounces. If it breaks, its kinetic energy becomes the kinetic energy of water flying all over the place. (That's Pat's favorite part.)

But if it bounces, that kinetic energy goes into deforming the balloon—squashing it flat. The balloon membrane stretches, momentarily storing energy in the stretch. Some of the energy is lost to friction (which becomes heat), but most goes to restoring the balloon to its original shape and sending it springing back into the air.

When Pat was tossing water balloons and meditating on the conservation of energy, she noticed that balloons filled with air made a loud bang when she popped them. But when a water balloon popped, there wasn't much of a sound, other than the splash of the water and the yelps of the people who had been doused. Why, she asked Paul, didn't water balloons pop with a bang?

NOT WITH A WHIMPER, BUT A BANG

So why does popping a toy balloon make such a satisfying "BANG!"?

First, we'd better talk about what that noise is. The human ear and brain perceive sound when there is a pressure change outside the eardrum. A popping balloon creates a sudden pressure change.

Inside an air-filled balloon, air is trapped and squeezed. The air pressure inside the balloon is higher than the surrounding atmospheric pressure. When you prick the balloon with a pin, the stretchy latex of the balloon splits open. If you could somehow color the air inside the balloon, you would see a region of compressed colored air hanging where the balloon used to be, just like the water in the water balloon.

But unlike the water, the air is under pressure. Once it's released from its latex prison, the compressed air expands outward. The air in the center of the balloon pushes the air closest to the balloon's surface outward. The expanding air reaches maximum speed when the pressure of the air that was originally in the center of the balloon matches atmospheric pressure. The outwardly expanding air sends a compression wave spreading out at the speed of sound. When this compression wave passes your ears, the compression pushes on your eardrum and you hear a bang.

But wait—there's more! When the air that was inside the balloon reaches atmospheric pressure, it's still rushing outward. It overshoots, creating a lower pressure region where the balloon used to be. So then the air rushes back to fill that lower pressure area. This creates an expansion wave of low pressure, which follows the compression wave. This alternation of compression and expansion makes a sound with a given pitch.

Of course, it doesn't stop there. The air rushes back, overshoots, then rushes out, overshoots, and so on, getting a little closer to equilibrium each time. Depending on the size and initial pressure in the balloon, the oscillation between compression and expansion takes different times. That's why popping different balloons produces different pitches. Larger balloons produce lower pitched pops than smaller balloons with the same pressure.

GOING OUT WITH THE BIGGEST BANG

Once you have a loud explosive noise, it's only human nature to try to make that noise louder. We will be continuing our investigation of balloons with hands-on experiments into ways to get the biggest bang. Since the loudness of the pop increases with increased air pressure inside the balloon, we will be testing balloons to find the ones that maximize the internal pressure. We are also looking for an ideal balloon-popping environment. A large interior space with plenty of hard surfaces and no padding to absorb the sound (an empty gymnasium or a concrete stairwell) offers the best potential for echoes.

We invite you to join the investigation—in empty gymnasiums or concrete stairwells, with balloons of your choice. And if anyone complains about the noise, remember: you're not just popping balloons. It's science.


1 For people who prefer to experience a trip like Larry's more safely through the magic of fiction, we refer you to "The View from On High" by Steven R. Boyett from our August 2000 issue.

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The Exploratorium is San Francisco's museum of science, art, and human perception—where science and science fiction meet. Paul Doherty works there. Pat Murphy used to work there, but now she works at Klutz Press (www.klutz.com), a publisher of how-to books for kids. Pat's latest novel is The Wild Girls. To learn more about Pat Murphy's science fiction writing, visit her website at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.

===THE END===

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