A frequently asked question is: how realistic is the scene in 2001: A Space Odyssey where astronaut Bowman makes a space-walk without a helmet? How long could a human survive if exposed to vacuum? Would you explode? Would you survive? How long would you remain conscious?

The quick answers to these questions are: Clarke got it about right in 2001. You would survive about a ninety seconds, you wouldn't explode, you would remain conscious for about ten seconds.



Astronaut Bowman ejected into space without a helmet

Could You Survive?

The best data I have comes from the chapter on the effects of Barometric pressure in Bioastronautics Data Book, Second edition, NASA SP-3006. This chapter discusses animal studies of decompression to vacuum. It does not mention any human studies.

page 5, (following a general discussion of low pressures and ebullism), the author gives an account of what is to be the expected result of vacuum exposure:

"Some degree of consciousness will probably be retained for 9 to 11 seconds (see chapter 2 under Hypoxia). In rapid sequence thereafter, paralysis will be followed by generalized convulsions and paralysis once again. During this time, water vapor will form rapidly in the soft tissues and somewhat less rapidly in the venous blood. This evolution of water vapor will cause marked swelling of the body to perhaps twice its normal volume unless it is restrained by a pressure suit. (It has been demonstrated that a properly fitted elastic garment can entirely prevent ebullism at pressures as low as 15 mm Hg absolute [Webb, 1969, 1970].) Heart rate may rise initially, but will fall rapidly thereafter. Arterial blood pressure will also fall over a period of 30 to 60 seconds, while venous pressure rises due to distention of the venous system by gas and vapor. Venous pressure will meet or exceed arterial pressure within one minute. There will be virtually no effective circulation of blood. After an initial rush of gas from the lungs during decompression, gas and water vapor will continue to flow outward through the airways. This continual evaporation of water will cool the mouth and nose to near-freezing temperatures; the remainder of the body will also become cooled, but more slowly. "Cook and Bancroft (1966) reported occasional deaths of animals due to fibrillation of the heart during the first minute of exposure to near vacuum conditions. Ordinarily, however, survival was the rule if recompression occurred within about 90 seconds. ... Once heart action ceased, death was inevitable, despite attempts at resuscitation.... [on recompression] "Breathing usually began spontaneously... Neurological problems, including blindness and other defects in vision, were common after exposures (see problems due to evolved gas), but usually disappeared fairly rapidly. "It is very unlikely that a human suddenly exposed to a vacuum would have more than 5 to 10 seconds to help himself. If immediate help is at hand, although one's appearance and condition will be grave, it is reasonable to assume that recompression to a tolerable pressure (200 mm Hg, 3.8 psia) within 60 to 90 seconds could result in survival, and possibly in rather rapid recovery."

Note that this discussion covers the effect of vacuum exposure only. The decompression event itself can have disasterous effects if the person being decompressed makes the mistake of trying to hold his or her breath. This will result in rupturing of the lungs, with almost certainly fatal results. There is a good reason that it is called "explosive" decompression.

Will You Stay Conscious?

The Bioastronautics Data Book answers this question: "Some degree of consciousness will probably be retained for 9 to 11 seconds.... It is very unlikely that a human suddenly exposed to a vacuum would have more than 5 to 10 seconds to help himself."

A larger body of information about how long you would remain conscious comes from aviation medicine. Aviation medicine defines the "time of useful consciousness", that is, how long after a decompression incident pilots will be awake and be sufficiently aware to take active measures to save their lives. Above 50,000 feet (15 km), the time of useful consciousness is 9 to 12 seconds, as quoted by the FAA in table 1-1 in Advisory Circular 61-107(the shorter figure is for a person actively moving; the longer figure is for a person sitting quietly). The USAF Flight Surgeon's Guide figure 2-3 shows 12 seconds of useful consciousness above 60,000 ft (18 km); presumably the longer time listed is based on the assumption that Air Force pilots are well-trained in high-altitude procedures, and will be able to use their time effectively even when partially disfunctional from hypoxia. Linda Pendleton adds to this: "An explosive or rapid decompression will cut this time in half due to the startle factor and the accelerated rate at which an adrenaline-soaked body burns oxygen." Advisory Circular 61-107 says the time of useful consciousness above 50,000 ft will drop from 9 to 12 seconds down to 5 seconds in the case of rapid decompression (presumably due to the "startle" factor discussed by Pendleton).

A slightly more general interest book, Survival in Space by Richard Harding, echoes this conclusion:

"At altitudes greater than 45,000 feet (13,716 m), unconsciousness develops in fifteen to twenty seconds with death following four minutes or so later."

and later:

"monkeys and dogs have successfully recovered from brief (up to two minutes) unprotected exposures..."

Would Your Blood Boil?

No.

Your blood is at a higher pressure than the outside environment. A typical blood pressure might be 75/120. The "75" part of this means that between heartbeats, the blood is at a pressure of 75 Torr (equal to about 100 mbar) above the external pressure. If the external pressure drops to zero, at a blood pressure of 75 Torr the boiling point of water is 46 degrees Celsius (115 F). This is well above body temperature of 37 C (98.6 F). Blood won't boil, because the elastic pressure of the blood vessels keeps it it a pressure high enough that the body temperature is below the boiling point-- at least, until the heart stops beating (at which point you have other things to worry about!). (To be more pedantic, blood pressure varies depending on where in the body it is measured, so the above statement should be understood as a generalization. However, the effect of small pockets of localized vapor is to increase the pressure. In places where the blood pressure is lowest, the vapor pressure will rise until equilibrium is reached. The net result is the same.)

Would You Freeze?

No.

A few recent Hollywood films showed people instantly freezing solid when exposed to vacuum. In one of these, the scientist character mentioned that the temperature was "minus 273"-- that is, absolute zero.

But in a practical sense, space doesn't really have a temperature-- you can't measure a temperature on a vacuum, something that isn't there. The residual molecules that do exist aren't enough to have much of any effect. Space isn't "cold," it isn't "hot", it really isn't anything.

What space is, though, is a very good insulator. (In fact, vacuum is the secret behind thermos bottles.) Astronauts tend to have more problem with overheating than keeping warm.

If you were exposed to space without a spacesuit, your skin would most feel slightly cool, due to water evaporating off you skin, leading to a small amount of evaporative cooling. But you wouldn't freeze solid!

Has Anybody Ever Survived Vacuum Exposure in Real Life?

Human experience is discussed by Roth, in the NASA technical report Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects. Its focus is on decompression, rather than vacuum exposure per se, but it still has a lot of good information, including the results of decompression events involving humans.

There are several cases of humans surviving exposure to vacuum worth noting. In 1966 a technician at NASA Houston was decompressed to vacuum in a space-suit test accident. This case is discussed by Roth in the reference above. He lost consciousness in 12-15 seconds. When pressure was restored after about 30 seconds of exposure, he regained consciousness, with no apparent injury sustained.

A few further details are given here.

Before jumping to the conclusion that space exposure is harmless, however, it is worth noting that in the same report, Roth includes a report of the autopsy of the victim of a slightly longer explosive decompression incident:

"Immediately following rapid decompression, it was noted that he began to cough moderately. Very shortly after this he was seen to lose consciousness, and the picture described by the physicians on duty was that the patient remained deeply cyanotic, totally unresponsive and flaccid during the 2-3 minutes [to repressurise the altitude chamber] down to ground level.

... "Manual artificial respiration was begun immediately... The patient at no time breathed spontaneously; however, at the moment ground level was reached he was seen to give a few gasps. These were very irregular and only two or three in number.

...

"The conclusion of the [autopsy] report was as follows: "The major pathologic changes as outlined above are consistent with asphyxia. It is felt that the underlying cause of death in this case may be attributed to acute cardio-respiratory failure, secondary to bilateral pneumothorax..." "

Many other cases of death following decompression are noted in the aviation literature, including one spaceflight incident, the Soyuz-11 decompression accident, in 1971. A recent analysis of this accident can be found in D. J. Shayler, Disasters and Accidents in Manned Spaceflight .

On the subject of partial-body vacuum exposure, the results are not quite as serious. In 1960, during a high-altitude balloon parachute-jump, a partial-body vacuum exposure incident occurred when Joe Kittinger, Jr. lost pressurization in his right glove during an ascent to 103,000 ft (19.5 miles) in an unpressurized balloon gondola, Despite the depressurization, he continued the mission, and although the hand became painful and useless, after he returned to the ground, his hand returned to normal. Kittinger wrote in National Geographic (November 1960):

"At 43,000 feet I find out [what can go wrong]. My right hand does not feel normal. I examine the pressure glove; its air bladder is not inflating. The prospect of exposing the hand to the near-vacuum of peak altitude causes me some concern. From my previous experiences, I know that the hand will swell, lose most of its circulation, and cause extreme pain.... I decide to continue the ascent, without notifying ground control of my difficulty."

at 103,000 feet, he writes:

"Circulation has almost stopped in my unpressurized right hand, which feels stiff and painful."

But at the landing:

"Dick looks at the swollen hand with concern. Three hours later the swelling will have disappeared with no ill effect."

The decompression incident on Kittinger's balloon jump is discussed further in Shayler's Disasters and Accidents in Manned Spaceflight :

[When Kittinger reached his peak altitude] "his right hand was twice the normal size... He tried to release some of his equipment prior to landing, but was not able to as his right hand was still in great pain. He hit the ground 13 min. 45 sec. after leaving Excelsior. Three hours after landing his swollen hand and his circulation were back to normal."

See also from Leonard Gordon, Aviation Week, February 13th 1996.

Finally, posting to sci.space, Gregory Bennett discussed an actual space incident:

"Incidentally, we have had one experience with a suit puncture on the Shuttle flights. On STS-37, during one of my flight experiments, the palm restraint in one of the astronaut's gloves came loose and migrated until it punched a hole in the pressure bladder between his thumb and forefinger. It was not explosive decompression, just a little 1/8 inch hole, but it was exciting down here in the swamp because it was the first injury we've ever head from a suit incident. Amazingly, the astronaut in question didn't even know the puncture had occured; he was so hopped on adrenalin it wasn't until after he got back in that he even noticed there was a painful red mark on his hand. He figured his glove was chafing and didn't worry about it.... What happened: when the metal bar punctured the glove, the skin of the astronaut's hand partially sealed the opening. He bled into space, and at the same time his coagulating blood sealed the opening enough that the bar was retained inside the hole."

Explosive Decompression

The discussion here has focussed only on exposure to vacuum. However, in general the action of being exposed to vacuum will also involve a rapid decompression. This event is generally known as "explosive decompression," and apart from the simple effect of vacuum on the body, the explosive decompression event itself will be hazardous. As noted, explosive decompression will be particularly bad if the decompression subject attempts to hold his or her breath during decompression.

In The USAF Flight Surgeon's Guide, Fischer lists the following effects due to mechanical expansion of gases during decompression. Gastrointestinal Tract During Rapid Decompression.

One of the potential dangers during a rapid decompression is the expansion of gases within body cavities. The abdominal distress during rapid decompression is usually no more severe than that which might occur during slower decompression. Nevertheless, abdominal distention, when it does occur, may have several important effects. The diaphragm is displaced upward by the expansion of trapped gas in the stomach, which can retard respiratory movements. Distention of these abdominal organs may also stimulate the abdominal branches of the vagus nerve, resulting in cardiovascular depression, and if severe enough, cause a reduction in blood pressure, unconsciousness, and shock. Usually, abdominal distress can be relieved after a rapid decompression by the passage of excess gas. The Lungs During Rapid Decompression.

Because of the relatively large volume of air normally contained in the lungs, the delicate nature of the pulmonary tissue, and the intricate system of alveolar airways for ventilation, it is recognized that the lungs are potentially the most vulnerable part of the body during a rapid decompression. Whenever a rapid decompression is faster than the inherent capability of the lungs to decompress (vent), a transient positive pressure will temporarily build up in the lungs. If the escape of air from the lungs is blocked or seriously impeded during a sudden drop in the cabin pressure, it is possible for a dangerously high pressure to build up and to overdistend the lungs and thorax. No serious injuries have resulted from rapid decompressions with open airways, even while wearing an oxygen mask, but disastrous, or fatal, consequences can result if the pulmonary passages are blocked, such as forceful breath-holding with the lungs full of air. Under this condition, when none of the air in the lungs can escape during a decompression, the lungs and thorax becomes over-expanded by the excessively high intrapulmonic pressure, causing actual tearing and rupture of the lung tissues and capillaries. The trapped air is forced through the lungs into the thoracic cage, and air can be injected directly into the general circulation by way of the ruptured blood vessels, with massive air bubbles moving throughout the body and lodging in vital organs such as the heart and brain.

The movement of these air bubbles is similar to the air embolism that can occur in SCUBA diving and submarine escape when an individual ascends from underwater to the surface with breath-holding. Because of lung construction, momentary breath-holding, such as swallowing or yawning, will not cause sufficient pressure in the lungs to exceed their tensile strength. Decompression Sickness.

(also known as "Bends")

Because of the rapid ascent to relatively high altitudes, the risk of decompression sickness is increased. Recognition and treatment of this entity remain the same as discussed elsewhere in this publication. Hypoxia.

While the immediate mechanical effects of rapid decompression on occupants of a pressurized cabin will seldom be incapacitating, the menace of subsequent hypoxia becomes more formidable with increasing altitudes. The time of consciousness after loss of cabin pressure is reduced due to offgassing of oxygen from venous blood to the lungs. Hypoxia is the most immediate problem following a decompression. Physical Indications of a Rapid Decompression.

...

(a) Explosive Noise. When two different air masses make contact, there is an explosive noise. It is because of this explosive noise that some people use the term explosive decompression to describe a rapid decompression.

(b) Flying Debris. The rapid rush of air from an aircraft cabin on decompression has such force that items not secured to the aircraft structure will be extracted out of the ruptured hole in the pressurized compartment. Items such as maps, charts, flight logs, and magazines will be blow out. Dirt and dust will affect vision for several seconds.

(c) Fogging. Air at any temperature and pressure has the capability of holding just so much water vapor. Sudden changes in temperature or pressure, or both, change the amount of water vapor the air can hold. In a rapid decompression, temperature and pressure are reduced with a subsequent reduction in water vapor holding capacity. The water vapor that cannot be held by the air appears in the compartment as fog. This fog may dissipate rapidly, as in most fighters, or not so rapidly, as in larger aircraft.

(d) Temperature. Cabin temperature during flight is generally maintained at a comfortable level; however, the ambient temperature gets colder as the aircraft flies higher. If a decompression occurs, temperature will be reduced rapidly. Chilling and frostbite may occur if proper protective clothing is not worn or available.

(e) Pressure.



How Fast Will A Spaceship Decompress If It Gets Punctured?

The decompression time will depend on how big the hole is. For a fast estimate, you can assume that the air exiting through the hole will travel at the speed of sound. Since the atmosphere drops in pressure as it moves through the hole, the effective rate at which the atmosphere leaves is at about 60% of the speed of sound, or about 200 meters/second for room-temperaure air (see derivation by Higgins):

P = P o exp[-(A/V)t*(200m/s)]

The seminal paper on the subject is by Demetriades in 1954: "On the Decompression of a Punctured Pressurized Cabin in Vacuum Flight."

The decompression rate can be derived for laminar viscous flow (that is, near atmospheric pressure) using Prandtl's equation in the limit Po/P is zero, and assuming a simple aperture (a pipe of zero length). The gas flow conductance is C visc = 20 A liters/second (for A in square centimeters). As the pressure decreases the flow changes to molecular flow, and the depressurization rate decreases by about a factor of two. This is for air at 20 C; for the case of pure oxygen, the leak rate is about 10 percent slower.

For reference, when the pressure drops to about 50% of atmospheric, the subject will be entering the region of "critical hypoxia"; when the pressure drops to about 15% of atmospheric, the remaining time of useful consciousness will have decreased to the 9-12 seconds characteristic of vacuum.

Professor Andrew J. Higgins of McGill University had written a more detailed answer to the question of how fast a spacecraft will decompress through a given size hole; which I have reprinted with his permission here.



References

Andrew A. Pilmanis, and William J. Sears, "Physiological hazards of flight at high altitude," The Lancet, Volume 362, Supplement 1, December 2003, Pages s16-s17, doi:10.1016/S0140-6736(03)15059-3. Cited in Tubious, Nov. 2007.

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