Putting Humans in Stasis Is the Best Way of Getting Us to Mars

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Image from IEEE Spectrum

Human spaceflight to the moon took only 3 days one-way. A plan to send people to Mars would take much longer, 8-9 months under the best conditions where Earth and Mars are properly aligned. Sending anyone that far and back requires much more fuel, air, and food (including water) than a Moon mission, and the psychological stress of being so far away with so little to do until arrival (and return) can be a stressful experience. Keep in mind that all the astronauts would have to do is eat, sleep, and breathe, maybe do some exercising and take readings on the way. A minimum of 30 kg (66 pounds) of food and water is needed per astronaut per week, with critical systems recycling water and air along the way. But to conserve on supplies and minimize psychological stress (including boredom and loneliness), what if they could sleep most of the way, and how could that be done?

Bears hibernate, as do many other mammals. Can humans be forced to do that? A few scientific terms are important to consider. "Suspended animation" and "stasis" are not clear enough.

• Hibernation is a long condition, primarily in mammals, where metabolism and body temperature drop significantly. These changes allow survival through winter by conserving energy. Bears and hedgehogs hibernate; the only primate to do so is the fat-tailed dwarf lemur of Madagascar.
• Dormancy is the condition where a plant or animal has slowed down its bodily functions for a period of time, but it is not the same as hibernation (see table below). Some amphibians bury themselves during dry seasons where they remain dormant, for example. Seeds can be dormant until conditions are right for growth, and trees slow down metabolic activity in winter and reduce water content to avoid freezing damage. 
• A third term to remember is called torpor, which is an even shorter-term condition in animals with some metabolic changes less severe than in hibernation. It may last overnight, as in some hummingbirds, insects, and reptiles, or it may last a few weeks as with squirrels. The conditions that cause these body states and the length of time are all different (see below).

The Arctic ground squirrel hibernates 7-8 months of the year, because of the short summers in northern Canada, Alaska, and eastern Russia (Siberia). During hibernation, their normal body temperature of 37ºC (99ºF) can fall to below freezing -2.8ºC (27ºF). After a few weeks in torpor, they wake up and become active for a day to warm up again, then return to hibernation. 

Arctic ground squirrel (U.S. National Park Service)

In addition to the Arctic ground squirrel, whose body reaches temperatures below freezing, wood frogs in Alaska have been shown to survive even colder temperatures while they hibernate. Black bears hibernate for months at a time, but their body temperatures fall only slightly, from 38ºC (100ºF) to 31ºC (88ºF).

When we sleep, our metabolism naturally falls by around 15%, and our body temperatures also drop a few degrees. But we wake up hungry and need to get rid of built-up water as urine. To travel to Mars, it is estimated that a condition of torpor would be needed to reduce metabolism much more, about 75% (like bears). Here are some more specific reasons why this is necessary.

Since the 1980s, doctors have used targeted hypothermia (TH) to slow patients' metabolism with cold temperatures (32-34°C, 89-93°F) for a few hours for certain long surgeries or a few days (for certain brain trauma, for example). TH is done with cooling catheters, cooling blankets, IV with cold liquids, and applying ice around the body. 

Methods for inducing TH (Holzer, New England Journal of Medicine, 2010)

But patients get intravenous nutrients in the hospital, and their blood doesn't freeze. Blood flow is necessary to provide nutrients and remove waste. If blood freezes, that become a problem. The liquid part of blood is called plasma, and it is 92% water. Blood is made up of 55% plasma and 45% cells (which also contain about 60% water), so it is critical for an animal to control blood temperature during hibernation. Ground squirrels in North America regulate the content of their blood to avoid it freezing, especially around the heart, liver, and brain. Reawakening every 2 weeks simply causes enough physical activity to move blood around the other tissues. Because salty ions are taken out of the blood in torpor, the squirrels aren't thirsty.

Liquid and solid components of blood (BBC, The four components of the blood)

Muscle mass changes with the microgravity exposure. Researchers at the University of Marquette (Wisconsin) studied calf muscles of 5 astronauts and 4 cosmonauts on the International Space Station for 6 months in 2016. Some exercised on the treadmill for >200 minutes/week or less than 100 minutes/week. One example of a difference was in a calf muscle (soleus). Fibers deteriorated 3-8% in the longer running group but 27-29% in the shorter running group. Imagine if they were hibernating and not exercising at all!

Astronaut Steve Swanson using treadmill on the ISS. 

Bungee cords hold his body to the surface to counteract microgravity (NASA).

In 1990-1995, NASA joined forces with an American and Russian universities to study bone loss on 18 cosmonauts aboard the Russian Space Station Mir. The average bone loss rate was found to be 1-2% per month, even though the cosmonauts took part in two 1-1.5 hour sessions of treadmill or bicycle exercise for a total of 2-3 hours three days on and one day off. Again, hibernating individuals on their way to Mars will not exercise.

Science fiction stories have described putting humans into suspended animation, a form of artificial hibernation. The 1967 Star Trek episode "Space Seed" saw survivors in suspended animation chambers from 200 years before; the people were wrapped in a golden netted suit from neck to toes. The suit's purpose was not explained. A similar setup was used in Planet of the Apes (1968). Also in 1968, Arthur C. Clarke, had people in frozen storage chambers like a sarcophagus for an 18-month trip to Jupiter in 2001: A Space Odyssey. But in the 1984 sequel 2010: Odyssey Two, astronauts were on open beds and suited in a net-like uniform that presumably included sensors, food and waste tubing, and muscle massagers to prevent their bodies from deteriorating. People in the Alien movie series (beginning in 1979) wore minimal clothing in cold storage with sensor patches on their bodies. Greg Bear's 1985 novel Eon had people in frozen storage for 100 years or more; but their bodies were chemically treated before freezing to prevent cell damage in the freezing and thawing process, and nanotechnology helped repair any damage that might have occurred. But, just how feasible are those imaginative concepts now and in the near future?

With help from NASA, the aerospace company SpaceWorks Enterprises has been developing a module to keep humans in a torpor state on and off to simulate what squirrels do naturally. With the savings on food, air, and water, the SpaceWorks torpor module for 6 astronauts would be about 40 m³ in volume, compared to the original NASA design of 400 m³ which kept astronauts awake the whole trip. That's like the volume of 2 hollowed-out minivans (engine & seats included) vs. 20 of them.

Comparison of designs for astronaut habitats (NASA)

Depending on whether the torpor module has 4, 6, or 8 people housed inside, it's clear how much mass will be saved. The graph below shows the difference in consumables and machinery alone between the larger NASA module and the smaller SpaceWorks module.

Graph from NASA

How the bodies are arranged inside the module has not yet been settled (see 2 designs below right). But each person will need critical system monitoring. The theoretical care shown below represents two methods to feed the body either through the chest or thigh with Total Parenteral Nutrition (TPN): lipids, amino acids, dextrose, electrolytes, vitamins, and trace elements.

Images from NASA

Ways to regulate the temperature vary. A non-invasive method involves wrapping the body in a blanket, skull cap, or body pads that heats/cools, but they need to be changed every week. An invasive technique like the CoolGuard 3000R circulates a chilled/warmed saline solution through a catheter which is inserted into a large vein like a personal body radiator. Alternatively, RhinoChill is a small plastic tube placed in the nose where a spray of cooling mist evaporates near the brain, and the rest of the body is cooled as blood flows by. Finally, an entire warming/cooling platform like the KOALA System adjust the temperature of the entire enclosed habitat for each person.

From Schaffer et al., 2016

As shown earlier, laying out the astronauts can be done so that the torpor module can rotate to provide an artificial gravity. In addition to helping minimally with bone and muscle atrophy, gravity may also help the eyes, a serious concern for NASA. On Earth, gravity pulls blood and cerebrospinal fluids downward away from the head, but a lack of gravity redistributes the fluids evenly through the body. That means greater pressure inside the head and surrounding the back of the eyeball. It is enough to flatten the back of the eye and cause imperfect focusing on the retina. With 6 months or more in space, this can lead to serious vision problems.

Flattening of the eyeball after spaceflight (right) (Reilly et al., 2023)

What will be the problem on the brain itself if humans are placed in a cycle of torpor and waking? Ground squirrel EEG patterns are essentially flatline during torpor, so will that happen to people (and to what end)?  

A 13-stripe ground squirrel in torpor (Popular Science)

Is it safe or practical to put astronauts in a torpor state, to reawaken periodically, during an 8-month flight to and from Mars? Some sketches from SpaceWorks show possible robots to help make adjustments to the support systems attached to the astronauts. Alternatively, if a large crew were sent in several modules, a small crew of four might remain awake the whole time instead of using robots.

Two robots in the center of a hypothetical torpor module (NASA)

Brain damage is the number one concern for humans in any hibernation situation. Squirrel brain function as seen through EEG is nearly zero in the torpor state. But touching the animal or exposing it to a noise causes changes in the EEG, which suggests they are still aware of surroundings. Some even wake up in those experiments.

But to say "wake up" implies the animal or person is sleeping, and that's not technically true. Moreover, deep (REM) sleep is necessary for humans, and a lack of it causes problems: mood disorders, cognitive impairment, and difficulty learning new information. These are not issues that astronauts can afford to have so far from Earth! So, if humans can be put into torpor, they may likely not have REM because they are not really sleeping, and that's a concern.

Some studies have shown that as the brain cools down, specific areas shut off in a certain order. During torpor, the brain first reduces conscious awareness (neocortex), then arousal and sensory relay (reticular formation and thalamus), while keeping the hippocampus (converts short-term memories into long-term ones) active the longest. This could mean memory-related functions are preserved, even in deep torpor. More research is obviously needed, but this is a good start.

Mary Engle Pennington, the Ice Woman

The term "ice woman" often conjures up an image of a heartless female, but in the case of Mary Engle Pennington, nothing could be further from that. Born into a Quaker family on October 8, 1872, she grew up in Philadelphia, Pennsylvania. At 12, she became fascinated in medicinal chemistry from a library book, whereupon she asked local university professors to explain it to her. Despite being told to come back when she was older, she enrolled after high school in 1890 to the Towne Scientific School (formerly Department of Science) at the University of Pennsylvania to study biology, chemistry, and hygiene. The university did not grant bachelor's degrees to women, so she had to be satisfied with earning a certificate of proficiency instead.

Towne Building, University of Pennsylvania, 1906 (Penn Engineering)

Wanting to go further, she learned of an old college statute which gave the faculty power to grant certain degrees like a doctorate without the approval of the board of trustees (who had denied her bachelor's degree), and that's what she achieved in 1895. She studied under Edgar Fahs Smith, department chair who considered himself more of a historical chemist than a practical one. Smith had just completed advising Fanny R.M. Hitchcock, the first woman to get a doctorate in chemistry at that university (1894) and the first director of the women's graduate department in 1897. So, Mary's environment was now quite conducive to her later success. She probably gained more than chemistry training from Smith, since he was known to emphasize the humanistic side of science not a commercial approach to learning chemistry. This included moral aspects of their work, instead of just studying to be skilled technicians.

She stayed on for two more years as a doctoral fellow, then spent a year at Yale University working with Lafayette Mendel and Russell Henry Chittenden, the two founding fathers of the science of nutrition.

Photo from Yale University Library

Following that, she founded the Philadelphia Clinical Laboratory (1898-1906). She doubled as a bacteriologist with the Philadelphia Bureau of Health beginning in 1904, where she studied dairy samples for contamination. Pennington chose to teach farmers and ice cream salesmen about hygiene standards rather than just report the data. Basically, she just showed them what numbers of bacteria were in their samples under the microscope, and they immediately began boiling pots and ladles! Her standards for dairy quality became accepted nationwide.

In 1905, she added to her workload by accepting a position in the Department of Chemistry with the U.S. Department of Agriculture (USDA). One of her first projects was to investigate a claim that resturants had been served turkey meat that had been frozen for 10 years, but no customers had gotten sick. She showed that poultry could be kept frozen at -18ºC (0ºF) in good condition for one year. 

Her director Harvey Wiley (the "father of the FDA") there was so impressed with her work that he wanted her to lead the new division which became responsible for the 1906 Pure Food and Drug Act. To do so, she had to pass a Civil Service exam, not often given to women. She signed her application as "M.E. Pennington" to hide her gender and got the highest score on the test. When the Civil Service authorities told her director there was no precedent for hiring a woman, he countered by saying there wasn't a precendent against it, and she was hired.

Photo from Cowgirl Magazine

Two projects that she served on had immense importance to human health and earned her reputation as the "ice woman". First, she developed standards for the safe processing of chickens for human consumption. A simple but important finding was to keep fresh foods cold at a constant temperature. The 1906 Act didn't even refer to bacteria like Pasteur had shown half a century earlier in France. It just described avoiding "filthy, putrid,or decomposed" foods. Mary determined safe storage temperatures for many perishable food products like milk, eggs, and cheese. She also developed safer practices for handling raw poultry from slaughterhouse to market (Encyclopedia.com), as well as a new type of egg carton that reduced breakage during train shipping.

One example of her creativity is a patent she co-wrote in 1913 for a cooling rack for chickens and other meats. The rack could hold 180 chickens, ducks, or rabbits, 48-60 turkeys, or 72 geese in a design that maximized cooler space. Grading of the meat quality was easier and more accurate, too.

Pennington's patented cooling rack diagram and photo showing filled racks (USDA pat. 1,020,575)

Pennington's second achievement in refrigeration with the USDA concerned railroad refrigeration cars. Experimentation in shipping meats cross-country began in 1842. Just after the Civil War, cattle were shipped from Texas to processing centers around the country, but the animals lost weight or died in the transport. Railroad companies initially did not like the new cars for refrigeration because they were one third more expensive and might be used only on one-way trips. Some were filled at the ends with ice blocks, and the floors were insulated with flax or cattle hair. Allowing meat to directly make contact against ice resulted in discoloration and affected the taste. So, ventilation schemes ranged from simply opening the doors to open rooftop hatches to fans driven by the car’s axles. 

But the refrigerator was not a home appliance; people stocked cabinets called iceboxes with blocks of ice made artificially or harvested from frozen rivers and lakes. The first refrigerator was installed at a brewery in Brooklyn, New York, in 1870. The meatpacking industry followed with the first refrigerator introduced in Chicago in 1900, and it wasn't until 1913 that homes got their smaller models. Until such time as mechanical refrigerators were invented, ice sales were prominent. 

1867 refrigerated car with ice blocks on front and back 

Detroit, Michigan fish market owner William Davis devised a boxcar in 1868 with metal racks to hang the meat over a mixture of ice and salt. But the meat swayed and shifted the boxcar balance, sometimes causing derailments. Chicago meatpacking magnate Gustavus Swift then hired engineer Andrew Chase to make design improvements, and by 1881 he was sending 3,000 beef carcasses a week to Boston with almost 200 cars. Meats weren't the only food products shipped, and by the late 1890s, refrigerated shipping of all kinds of perishable foods including fruits and vegetables was done.

Rail car with ice compartment on top, 1877 (Wikipedia)

On October 3, 1908, Mary Pennington spoke to a Warehouseman’s Association in Washington, D. C. and explained the importance of cooling fruits immediately after being picked and not freezing them.  She showed that freezing food products caused chemical changes that significantly changed the composition of the product. Not only did she also show off a railroad refrigeration car there and explain its usefulness (something rather novel at the time), but she road with it to California where it was tested for use in hauling fruits from the orange and lemon groves and then shipped them to Florida.  Mary would travel with the car, check sensors, and carry out experiments along the way to improve their efficiency in transporting perishable foods. For example, she noted warm spots in the box cars and set standards for their construction to avoid such problems. Also, she found that the boxcar’s insulation was too thin and cracks could form in the exterior wall, exposing the scant insulation to the outside environment. She also discovered that meat should not touch and that boxes should have ventilation space between them.

Mary Pennington taking measurements on top of a box car, about 1910 (Ice Woman)

By 1930, her experiments revealed not only flaws in boxcar construction, but she also developed solutions to them.

• Boxcar walls should be made with several thicknesses of material
• Between each one should be insulating material of recognized efficiency.
• It should completely surround the boxcar, and especially protect joints, seams and corners.
• Wood or metal could be used for the outside of the box, but it should be attractive and easily cleaned.  
• The inside must be a material which moisture can't penetrate and which survives constant cleaning. 
• The inner wall should not have a finish to absorb odors or hold moisture, because it would permit mold growth.  
• Her examination found that only 3,000 out of 40,000 boxcars were certifiable.

She then turned her skills to households and food safety. In 1910, housewives in the U.S. refused to buy frozen foods, especially poultry because they thought they weren't fresh and because they caused illness. The reason was simple, though. After buying a frozen chicken, most of them put it in water or left it outside to thaw. Doing that contaminated the food. What they should have done was thaw it in the ice box. “No housewife can afford nowadays to remain in ignorance of what has happened to her chicken before she buys it,” she wrote in The Oregon Daily Journal on March 20, 1910.  Recognizing that women alone were not to blame, she ended the article as follows with a self-praising remark: “For some of its mischances, the housewife herself is responsible.  It is therefore fitting that, as women have done so much to afflict the modern supply of poultry, a woman [Pennington herself] should be the one to study out the remedies.”

Pennington in an undated photo (inventricity.com)

Six months after World War I started, Dr. Pennington attended the National Poultry, Butter and Egg Association conference in Chicago in 1917, where she spoke to encourage farmers to increase delivery of poultry, eggs, and fish. As you can see below, her words reflected not only the scientific statements of a scientists but also her compassion as a Quaker.

“A hungry man may rise to a moment of valor, but when a whole people are hungry, they become moral and physical weaklings.”

“The supply of beef is not enough to go around and the deficit must be made up with other food.”

“We must feed our men in the trenches and the men of our allies.  We must also feed the civilians of our own country and those of our allies.”

(Quotes from Wild Women of the West: Dr. Mary Pennington)

In 1919, Pennington resigned from the USDA and took a job director of American Balsa, which manufactured insulation used in refrigeration units. There, she created groundbreaking insulation techniques used on domestic refrigeration. A few years later, she started her own consulting company. The newly formed National Association of Ice Industries (NAII) contracted with her to be the head of their new Household Refrigeration Bureau. The NAII sought ways to increase ice sales in order to stock home iceboxes, but Pennington had other ideas to capitalize on her scientific credentials.

Her Bureau created pamphlets focused on the scientific basis of refrigerating foods and did not include any advertising of local ice dealers. This is important because the NAII had been formed in 1917 from 60 Chicago manufacturers and the publisher of the journal Ice and Refrigeration. She mailed from her New York City office only on request after home economics teachers, nutritionists, social workers, and other service professionals had seen them in circulars she sent out. Two notable titles were The Care of the Child's Food in the Home, Cold is the Absence of Heat, Journeys with Refrigerated Foods, and The Romance of Ice.

Pennington also contributed many articles to Ice and Refrigeration, such as the following:

• Standard Refrigerator Car Development (1919)
• Low Temperature in Transit (1924)
• The Construction of Household Refrigerators (1928)
• Fifty Years of Refrigeration In the Egg and Poultry Industry (1941)
• Refrigerated Warehousing of Tomorrow (1944)

She herself gave talks and a week-long Household Refrigeration School to teach ice company home service workers, mostly women, about the science behind spoiling of food. In 1927, she announced a "creed" for the Massachusetts Ice Dealers' Association; her Quaker roots caused her to write as one ideal of the creed: "There must be service, service, service, unselfish." Throughout the 1920s, she pushed ice manufacturers of the NAII to advertise for this service as a means to educate people (and as a side effect, it would help them to sell more ice). But by then, commercial refrigerators were on the market and had more advertising money to spend. When the stock market crashed in 1929, it was all over for the ice market, and Pennington stepped away from the Home Refrigeration Bureau.

Mary Pennington, 1940 (Wikipedia)

She maintained her consulting business until she died in 1952. By then, she had earned 5 patents and received the Notable Service Medal from President Herbert Hoover, as well as the Francis P. Garvan–John M. Olin Medal, which recognizes women chemists. Wikipedia sums up more accolades as follows: She was the first woman elected to the Poultry Historical Society Hall of Fame in 1959. She was inducted into the National Women's Hall of Fame in 2002, the American Society of Heating, Refrigerating and Air-Conditioning Engineers Hall of Fame in 2007, and the National Inventors Hall of Fame in 2018.

Dr. Mary Engle Pennington died on December 27, 1952, at the age of 80.