Human Exposure to Vacuum
Geoffrey A. Landis
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.
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.
On 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.
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."
http://www.sff.net/people/Geoffrey.Landis/vacuum.html
May 15, 2008
April 10, 2008
Human Evolution
Is human evolution finally over?
Scientists are split over the theory that natural selection has come to a standstill in the West. Robin McKie reports
For those who dream of a better life, science has bad news: this is the best it is going to get. Our species has reached its biological pinnacle and is no longer capable of changing.
That is the stark, controversial view of a group of biologists who believe a Western lifestyle now protects humanity from the forces that used to shape Homo sapiens.
'If you want to know what Utopia is like, just look around - this is it,' said Professor Steve Jones, of University College London, who is to present his argument at a Royal Society Edinburgh debate, 'Is Evolution Over?', next week. 'Things have simply stopped getting better, or worse, for our species.'
This view is controversial, however. Other scientists argue that mankind is still being influenced by the evolutionary forces that created the myriad species which have inhabited Earth over the past three billion years.
'If you had looked at Stone Age people in Europe a mere 50,000 years ago, you would assume the trend was for people to get bigger and stronger all the time,' said Prof Chris Stringer, of the Natural History Museum, London. 'Then, quite abruptly, these people were replaced by light, tall, highly intelligent people who arrived from Africa and took over the world. You simply cannot predict evolutionary events like this. Who knows where we are headed?'
Some scientists believe humans are becoming less brainy and more neurotic; others see signs of growing intelligence and decreasing robustness, while some, like Jones, see evidence of us having reached a standstill. All base their arguments on the same tenets of natural selection.
According to Darwin's theory, individual animals best suited to their environments live longer and have more children, and so spread their genes through populations. This produces evolutionary changes. For example, hoofed animals with longer necks could reach the juiciest leaves on tall trees and therefore tended to eat well, live longer, and have more offspring. Eventually, they evolved into giraffes. Those with shorter necks died out.
Similar processes led to the evolution of mankind, but this has now stopped because virtually everybody's genes are making it to the next generation, not only those who are best adapted to their environments.
'Until recently, there were massive differences between individuals' lifespans and fecundity,' said Jones. 'In London, the death rate outstripped the birth rate for most of the city's history. If you look at graveyards from ancient to Victorian times, you can see that a half of all children died before adolescence, probably because they lacked genetic protection against disease. Now, children's chances of reaching the age of 25 have reached 98 per cent. Nothing is changing. We have reached stagnation.'
In addition, human populations are now being constantly mixed, again producing a blending that blocks evolutionary change. This increased mixing can be gauged by calculating the number of miles between a person's birthplace and his or her partner's, then between their parents' birthplaces, and finally, between their grandparents'.
In virtually every case, you will find that the number of miles drops dramatically the more that you head back into the past. Now people are going to universities and colleges where they meet and marry people from other continents. A generation ago, men and women rarely mated with anyone from a different town or city. Hence, the blending of our genes which will soon produce a uniformly brown-skinned population. Apart from that, there will be little change in the species.
However, such arguments affect only the Western world - where food, hygiene and medical advances are keeping virtually every member of society alive and able to pass on their genes. In the developing world, no such protection exists.
'Just consider Aids, and then look at chimpanzees,' says Jones. 'You find they all carry a version of HIV but are unaffected by it.
'But a few thousand years ago, when the first chimps became infected, things would have been very different. Millions of chimps probably died as the virus spread through them, and only a small number, which possessed genes that conferred immunity, survived to become the ancestors of all chimps today.
'Something very similar could soon happen to humans. In a thousand years, Africa will be populated only by the descendants of those few individuals who are currently immune to the Aids virus. They will carry the virus but will be unaffected by it. So yes, there will be change there all right - but only where the forces of evolution are not being suppressed.'
However, other scientists believe evolutionary pressures are still taking their toll on humanity, despite the protection afforded by Western life. For example, the biologist Christopher Wills, of the University of California, San Diego, argues that ideas are now driving our evolution. 'There is a premium on sharpness of mind and the ability to accumulate money. Such people tend to have more children and have a better chance of survival,' he says. In other words, intellect - the defining characteristic of our species - is still driving our evolution.
This view is countered by Peter Ward, of the University of Washington in Seattle. In his book, Future Evolution, recently published in the US by Henry Holt, Ward also argues that modern Western life protects people from the effects of evolution. 'I don't think we are going to see any changes - apart from ones we deliberately introduce ourselves, when we start to bio-engineer people, by introducing genes into their bodies, so they live longer or are stronger and healthier.'
If people start to live to 150, and are capable of producing children for more than 100 of those years, the effects could be dramatic, he says. 'People will start to produce dozens of children in their lifetimes, and that will certainly start to skew our evolution. These people will also have more chance to accumulate wealth as well. So we will have created a new race of fecund, productive individuals and that could have dramatic consequences.
'However, that will only come about when we directly intervene in our own evolution, using cloning and gene therapy. Without that, nothing will happen.'
Stringer disagrees, however. 'Evolution goes on all the time. You don't have to intervene. It is just that it is highly unpredictable. For example, brain size has decreased over the past 10,000 years. A similar reduction has also affected our physiques. We are punier and smaller-brained compared with our ancestors only a few millennia ago. So even though we might be influenced by evolution, that does not automatically mean an improvement in our lot.'
robin.mckie@observer.co.uk
February 03 2002
Assignment:
Create a post that contains a little SUMMARY and CRITIQUE of the article. SUBMIT TO ME BY 18/4
Scientists are split over the theory that natural selection has come to a standstill in the West. Robin McKie reports
For those who dream of a better life, science has bad news: this is the best it is going to get. Our species has reached its biological pinnacle and is no longer capable of changing.
That is the stark, controversial view of a group of biologists who believe a Western lifestyle now protects humanity from the forces that used to shape Homo sapiens.
'If you want to know what Utopia is like, just look around - this is it,' said Professor Steve Jones, of University College London, who is to present his argument at a Royal Society Edinburgh debate, 'Is Evolution Over?', next week. 'Things have simply stopped getting better, or worse, for our species.'
This view is controversial, however. Other scientists argue that mankind is still being influenced by the evolutionary forces that created the myriad species which have inhabited Earth over the past three billion years.
'If you had looked at Stone Age people in Europe a mere 50,000 years ago, you would assume the trend was for people to get bigger and stronger all the time,' said Prof Chris Stringer, of the Natural History Museum, London. 'Then, quite abruptly, these people were replaced by light, tall, highly intelligent people who arrived from Africa and took over the world. You simply cannot predict evolutionary events like this. Who knows where we are headed?'
Some scientists believe humans are becoming less brainy and more neurotic; others see signs of growing intelligence and decreasing robustness, while some, like Jones, see evidence of us having reached a standstill. All base their arguments on the same tenets of natural selection.
According to Darwin's theory, individual animals best suited to their environments live longer and have more children, and so spread their genes through populations. This produces evolutionary changes. For example, hoofed animals with longer necks could reach the juiciest leaves on tall trees and therefore tended to eat well, live longer, and have more offspring. Eventually, they evolved into giraffes. Those with shorter necks died out.
Similar processes led to the evolution of mankind, but this has now stopped because virtually everybody's genes are making it to the next generation, not only those who are best adapted to their environments.
'Until recently, there were massive differences between individuals' lifespans and fecundity,' said Jones. 'In London, the death rate outstripped the birth rate for most of the city's history. If you look at graveyards from ancient to Victorian times, you can see that a half of all children died before adolescence, probably because they lacked genetic protection against disease. Now, children's chances of reaching the age of 25 have reached 98 per cent. Nothing is changing. We have reached stagnation.'
In addition, human populations are now being constantly mixed, again producing a blending that blocks evolutionary change. This increased mixing can be gauged by calculating the number of miles between a person's birthplace and his or her partner's, then between their parents' birthplaces, and finally, between their grandparents'.
In virtually every case, you will find that the number of miles drops dramatically the more that you head back into the past. Now people are going to universities and colleges where they meet and marry people from other continents. A generation ago, men and women rarely mated with anyone from a different town or city. Hence, the blending of our genes which will soon produce a uniformly brown-skinned population. Apart from that, there will be little change in the species.
However, such arguments affect only the Western world - where food, hygiene and medical advances are keeping virtually every member of society alive and able to pass on their genes. In the developing world, no such protection exists.
'Just consider Aids, and then look at chimpanzees,' says Jones. 'You find they all carry a version of HIV but are unaffected by it.
'But a few thousand years ago, when the first chimps became infected, things would have been very different. Millions of chimps probably died as the virus spread through them, and only a small number, which possessed genes that conferred immunity, survived to become the ancestors of all chimps today.
'Something very similar could soon happen to humans. In a thousand years, Africa will be populated only by the descendants of those few individuals who are currently immune to the Aids virus. They will carry the virus but will be unaffected by it. So yes, there will be change there all right - but only where the forces of evolution are not being suppressed.'
However, other scientists believe evolutionary pressures are still taking their toll on humanity, despite the protection afforded by Western life. For example, the biologist Christopher Wills, of the University of California, San Diego, argues that ideas are now driving our evolution. 'There is a premium on sharpness of mind and the ability to accumulate money. Such people tend to have more children and have a better chance of survival,' he says. In other words, intellect - the defining characteristic of our species - is still driving our evolution.
This view is countered by Peter Ward, of the University of Washington in Seattle. In his book, Future Evolution, recently published in the US by Henry Holt, Ward also argues that modern Western life protects people from the effects of evolution. 'I don't think we are going to see any changes - apart from ones we deliberately introduce ourselves, when we start to bio-engineer people, by introducing genes into their bodies, so they live longer or are stronger and healthier.'
If people start to live to 150, and are capable of producing children for more than 100 of those years, the effects could be dramatic, he says. 'People will start to produce dozens of children in their lifetimes, and that will certainly start to skew our evolution. These people will also have more chance to accumulate wealth as well. So we will have created a new race of fecund, productive individuals and that could have dramatic consequences.
'However, that will only come about when we directly intervene in our own evolution, using cloning and gene therapy. Without that, nothing will happen.'
Stringer disagrees, however. 'Evolution goes on all the time. You don't have to intervene. It is just that it is highly unpredictable. For example, brain size has decreased over the past 10,000 years. A similar reduction has also affected our physiques. We are punier and smaller-brained compared with our ancestors only a few millennia ago. So even though we might be influenced by evolution, that does not automatically mean an improvement in our lot.'
robin.mckie@observer.co.uk
February 03 2002
Assignment:
Create a post that contains a little SUMMARY and CRITIQUE of the article. SUBMIT TO ME BY 18/4
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