Putting Humans in Stasis Is the Best Way of Getting Us to Mars
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.
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).
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.
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!
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.
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.
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.
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.
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.
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)?
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.
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.
- 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.
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.
