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Pat Murphy & Paul Doherty
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by Pat Murphy & Paul Doherty

Under Pressure

It's 2001. Time to reflect on Stanley Kubrick's movie 2001: A Space Odyssey.

Remember the scene where HAL, the massively conflicted ship's computer, locks astronaut Dave Bowman out of the main spacecraft? Dave is in a small space pod and he needs to get back into the ship. But HAL won't open the pod bay doors---even when Dave says "please!"

Dave's wearing a space suit, but he doesn't have a helmet with him. To get back into the ship, Dave uses the manipulator arms of the pod to open the outer door to an airlock. Then he opens the hatch on the pod. The escaping air blasts him across the vacuum of space into the evacuated airlock. There Dave closes the outer door, repressurizes the airlock, and gets back into the ship to continue the movie.

In many other movies, people exposed to the vacuum of space swell up as their blood boils beneath their skin. So what's the deal? Did Arthur C. Clarke get it wrong? Is Dave's feat physically possible?

If you're interested in the answers to these and other strange questions (like why was Paul faced with a raw egg for breakfast on a trip to Ecuador?), read on. In this column, we explore the effects of changing air pressure--including boiling blood. We'll start on the surface of the earth and head up to Mars and beyond.

The Watched Pot

Before we can get to boiling blood, we need to talk about boiling in general. Imagine that a pot of water is heating on the stove. Hey, if you're home, go get a pot of water and put it on the stove. You're going to boil this water and notice what happens along the way. But first, consider the pressure on both you and that water.

The surface of the earth is at the bottom of an ocean of air. The earth's gravity pulls that air downward. Consequently, all the air that's up above you pushes down on you and on the water in the pot. How hard does it push? If you balanced a 2-meter-long piece of iron rebar on the palm of your hand, the pressure exerted by the bar would be about equal to atmospheric pressure at sea level. Pressure can be measured in units called atmospheres. At sea level, you experience one atmosphere of pressure.

Now suppose you turn up the heat under that pot of water. While the water starts to heat up, think about evaporation. At the surface of the water, molecules of water are making the transition from liquid to vapor in the process called evaporation.

As the water heats up, not much happens at first. Then you hear some popping and snapping noises. If you look at the bottom of the pot, you can see bubbles form. They start to rise but they collapse before reaching the surface. These collapsing bubbles make the sounds you are hearing.

As the water gets hotter, the bubbles rise further and further before collapsing. Finally, they reach the surface. When the bubbles start breaking at the surface, the sounds quiet down. At this point, all the water in the pot has reached at least 100 degrees Celsius, the boiling point of water at one atmosphere of pressure.

Water can evaporate at any temperature, but under one atmosphere of pressure, it doesn't boil until it reaches 100oC. What's the difference? In evaporation, the change from liquid water to water vapor happens at the surface. When water boils, that transition happens in the fluid below the surface. In boiling, bubbles form within the water.

A fluid's vapor pressure is a measure of its tendency to change into a vapor. Vapor pressure increases with temperature. A fluid boils when its vapor pressure equals the pressure of the surrounding gases.

The higher the pressure on the surface of a fluid, the harder it is to form bubbles of vapor inside the fluid. To make the bubbles form, you can heat up the fluid and increase its vapor pressure. On the other hand, you can also make a fluid boil by decreasing the surrounding pressure. Let's consider that option.

Going Up?

When Paul was in Quito, Ecuador on his way to climb a mountain called Cotopaxi, he ordered eggs for breakfast. When the waiter asked "quanto minutos?" Paul (proud of his few words of Spanish) answered "quatro minutos, por favor."

When his eggs arrived raw, Paul's physics professor brain clicked into action. (A little late, true.) In Mountain View, California, where Paul lives, water boils at 100 degrees Celsius. Eggs cook to Paul's liking in four minutes. But Quito was at an altitude of 10,000 feet. With the lower atmospheric pressure, water boils at a lower temperature---about 90oC.

Cooking time is very sensitive to temperature. When you cook an egg, you are denaturing the proteins---that is, breaking the weak chemical bonds that hold the protein molecules in a particular structure. The denaturing of proteins is an exponential function of temperature. Drop the temperature by 10 degrees Celsius, and you quadruple the cooking time.

At 20,000 feet, where the boiling point is 80oC, cooking food takes 16 times as long as it does at sea level. That's why Paul, like most mountaineers, brings precooked freeze dried food or foods that only need a few minutes in hot water to rehydrate. (Mountaineers who cook "real" food at altitude bring pressure cookers with them. Inside these pressure cookers, the pressure builds up to twice the atmospheric pressure at sea level, increasing the boiling temperature and reducing the cooking time.) Paul sticks with precooked foods, saying that the possibility of a pressure cooker accident in a tent is too messy to contemplate.

As a mountaineer, Paul has dealt with other consequences of high altitude. At sea level, dry air is 21 percent oxygen. At 15,000 feet, the air pressure---and the amount of oxygen in each breath---is half what it is at sea level. Rise quickly to 15,000 feet and Paul says you will know what it feels like to have one lung ripped out!

The next time you drive up into some mountains and reach 9,000 feet above sea level or higher, notice how you feel when you stop your car and get out. If you have not adapted to altitude you may feel weak and dizzy. If you go higher, the effects become more serious. Watching tourists step off a bus at 15,000 feet in Lauca National Park in Chile, Paul saw one man immediately pass out from lack of oxygen.

Given time, the body can gradually adapt to altitude. Within hours of moving to a higher altitude, the heart beats faster and breathing becomes deeper. Within weeks, the blood becomes thicker with oxygen-transporting red blood cells.

Mountaineers must learn how well and how quickly their bodies adapt to altitude. Paul has learned that he can go to 9000 feet his first night and then climb 1000 feet higher each day.

If you go high too fast, you can suffer the nausea and headaches of Acute Mountain Sickness (AMS). If you ignore the warning signs of AMS and stay too high, the consequences can be lethal as your lungs fill with fluid (pulmonary edema) or your head fills with fluid (cerebral edema).

Another thing that suffers with altitude is intelligence. Paul notes that mountaineers do better at altitude if the basic activities have been done so many times that they have become instinctive. That's one reason why experience is a great asset to mountaineers. A wise mountaineer gains experience in solving problems at low altitudes where he or she can think, before going to high altitudes. Then at high altitudes the mountaineer just remembers the solution to a given problem--rather than having to create an answer from scratch.

Mountaineers consistently deal with the effects of altitude, but you don't have to be an mountaineer to find yourself facing pressures lower than those on the summit of Mt. Everest in a few seconds!

Higher than Everest

Jet liners commonly fly at altitudes of 40,000 feet, significantly higher than Mt. Everest. The airplane cabin is pressurized to the equivalent of air pressure at 6000 or 8000 feet. You may have noticed that the bags of potato chips handed out by the flight attendant are extra puffy. Filled at altitudes lower than 6,000 feet, the air in the air-tight bags is at a higher pressure than the surrounding air.

In the comfort of a pressurized cabin, few people have difficulty with the pressure changes of flying---unless, of course, there's what flight attendants call a "loss of cabin pressure incident." (Other people call it "explosive decompression.") Since Paul has gone through explosive decompression training, he can tell the rest of us what to expect in such an incident.

Paul was with a bunch of fellow ROTC students sitting along the wall of a mockup of a military airplane cabin. Next to each participant was a sergeant wearing an oxygen mask. All participants had clipboards on which they were supposed to write their names over and over again. WHOOSH! Suddenly the air pressure in the cabin dropped to the pressure at 30,000 feet.

Air whooshed out of their mouths. In a situation like this, you should not try to hold your breath. Without the counterbalancing surrounding pressure, the pressure of the air inside your lungs could rupture your lungs and chest.

The cabin filled with mist as all of the high humidity air that came out of every orifice in their bodies expanded and cooled to form a cloud. When the cloud cleared, Paul noticed no discomfort. The human body determines the need to breathe by measuring the buildup of carbon dioxide in the blood, not by measuring the oxygen level. Paul wasn't breathing in enough oxygen to stay conscious, but he was breathing out carbon dioxide, so his body didn't register any problem. Without the slightest trace of discomfort and without any notice on his part, Paul slipped into unconsciousness.

Paul later learned that the lapsed time from explosive decompression to unconsciousness was fifteen seconds. Time enough to put on an oxygen mask if he had had one. When the pressure was restored, Paul woke up and was surprised to see that he had written his name several more times than he remembered. The memory of writing had not been stored due to the low oxygen level.

In the event of a "loss of cabin pressure incident," airplane pilots immediately put on their emergency oxygen masks and head for lower altitudes so that any passengers who do not get their oxygen masks on during the 15 second window of consciousness get to higher oxygen pressures quickly and live to wake up.

Boiling and Freezing

To reach pressures lower than those at 40,000 feet, we need to leave the planet earth. We haven't walked on Mars (except in our dreams), but an exhibit at the Exploratorium mimics Martian atmospheric pressure. In the Water Freezer exhibit, visitors can watch water boil until it freezes.

In the exhibit's air tight compartment, there's a puddle of water at room temperature. The visitor starts the vacuum pump, evacuating the chamber. At first nothing happens---the water just sits there. Then, as the pressure drops, bubbles appear in the water and it begins to boil.

Temperature relates to how fast molecules are jiggling around. In a glass of water at room temperature, the average speed of the water molecules is greater than in a glass of water at half that temperature.

When water boils, the water molecules moving fastest are the ones that become vapor first, escaping the liquid and leaving the slower, cooler molecules behind. Boiling is a cooling process. (That may seem strange, but remember that you sweat to keep cool. Evaporating water cools you off.)

In the Water Freezer exhibit, boiling cools the water to the point where it freezes. When the pressure is about 6 thousandths of an atmosphere, also known as 6 millibars, water boils at the same temperature that it freezes.

On Mars, at one level on the surface, the atmospheric pressure is near 6 millibars. At elevations below this level, the pressure is higher, and liquid water can exist in an open glass without boiling. Above this level, the water in the glass will boil away.

This atmospheric level was initially used to designate "sea level" on Mars-- the reference level for maps. Below sea level, liquid water could exist; above sea level, water would immediately boil away. Later investigation showed that the atmospheric pressure on Mars varies quite a bit over the course of the year. Every Martian winter, a portion of the atmosphere freezes solid as carbon dioxide snow at the Martian poles. So a reference level based on atmospheric pressure, however romantic and appealing, didn't prove to be the best choice. Newer maps of Mars use a more pedestrian reference level based on average radii of the planet.

But What About Dave?

We've been considering many things in our investigation of pressure and boiling, but we haven't forgotten Dave and his plight. When Dave goes out into the vacuum of space, he suffers explosive decompression. Having trained for this, he doesn't try to hold his breath; he allows the gas to burst from his lungs.

In cases where people have been accidentally exposed to vacuum, they lost consciousness after 6 to 9 seconds. (One person said that he noticed fizzy bubbles of cold boiling water on his tongue just before he passed out.) Anyway, Dave has 6 to 9 seconds to close the door and repressurize the air lock. If he has trained for such an eventuality so that he doesn't have to think too much, that's probably plenty of time.

But what about that problem of boiling blood? Well, it's not easy for a bubble to start to form inside the bubble-free fluids of a human body. Maybe you've noticed that when you pour a soda, most of the bubbles form on scratches and imperfections in the glass. Bubbles need a nucleation site--a place to gather. It's hard for a bubble to get started in the clean pure fluid.

If liquid reaches its boiling point but there are no sites on which bubbles can form, the liquid can become superheated--hot enough (or under low enough pressure) to boil, but temporarily free of bubbles. You may have encountered this problem when you heated water in a microwave oven. Sometimes, a cup of water can be superheated in a microwave. Such water is jiggled into an eruption of boiling when someone picks up the cup and jiggles hot water into contact with bubble nucleating sites along the walls of the cup.

The same thing happens in a human body subjected to a vacuum. Though the fluids under the skin are above the boiling temperature, it takes time for boiling to start. Seed bubbles have to form first. Accidents which have exposed people to vacuum revealed that it takes 15 seconds or more before the flesh starts to expand away from the underlying muscles. Eventually, the body inflates like a balloon until it doubles in size. If pressure is restored, the water vapor condenses quickly and the flesh will return to normal size.

So Arthur Clarke and Stanley Kubrick got it right. As long as Dave acts fast and repressurizes the airlock in less than 9 seconds, he'll be just fine.


If you want to read about Paul's adventures above 20,000 feet, check out his Web site at If you'd like to read about Pat's latest fictional adventures, visit her Web site at

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