A teenager joins a line of people boarding a spaceship. Once on board, she approaches a bed, crawls in, closes the lid and falls asleep. Her body is frozen for a trip to a planet several light-years from Earth. A few years later she wakes up, still the same age. This ability to put her life on pause while asleep is called “suspended animation.”
Scenes like this are a staple of science fiction. There’s plenty of other ways that suspended animation has touched our imagination, too. There’s Captain America, for instance, who survived nearly 70 years frozen in ice. And Han Solo was frozen in carbonite in Star Wars: The Empire Strikes Back. The Mandalorian’s main character brings in some of his bounties cold, too.
All of these stories have something in common. People enter an unconscious state in which they can survive for a long time.
Nothing like this is yet possible in the real world, at least for us humans. But some animals and birds have their own forms of suspended animation: They hibernate. This might hold some lessons for how to put astronauts of the future into hibernation for long space flights. But for really long journeys, a deep freeze might be the best option.
“I think this is realistic,” says Katharine Grabek. She’s a biologist who co-founded a company called Fauna Bio based in Emeryville, Calif. “I think it would be done by … making ourselves as similar as we can to a hibernator.”
Hibernation may look like a deep form of sleep, but it’s not sleep. As an animal hibernates, it chills its body and slows its heart rate and breathing. Metabolism also slows. To do this, an animal must turn on and off certain genes when they hibernate. Those genes do things like controlling whether an animal burns sugars or fats for fuel. Other genes are involved in keeping muscles strong.
Humans have many of these same genes. We don’t use them to hibernate. But turning some of these genes on or off might allow humans to do something similar to hibernation, Grabek says. Her company studies these genes and looks for drugs that can control them. Such drugs might allow people to hibernate without being really cold, she says.
Some animals’ body temperatures drop below freezing when they hibernate. Humans may not survive that chill, says John Bradford. He is the chief executive officer of SpaceWorks, a company in Atlanta, Ga. Bradford once proposed a space capsule where astronauts could hibernate. He thinks NASA could use such a capsule to send people to Mars.
Since a person probably wouldn’t survive their body temperature dropping below freezing, like a ground squirrel, Bradford suggests that people might hibernate like bears.
Black bears cut their metabolism by 75 percent when they hibernate. But their bodies stay somewhat warm. Normal body temperature for a black bear is 37.7° Celsius to 38.3 °C (100° Fahrenheit to 101 °F). During hibernation, their body temperature stays above 31 °C (88 °F).
Hibernating humans might have to lower their body temperature only a few degrees. “We can probably keep someone in this state very safely for about two weeks,” Bradford says.
If people are like bears, hibernation may help keep bones and muscles strong. That is important in space. Bones and muscles tend to break down in low gravity. Hibernation could cut the amount of food, water and oxygen that crews need. And it could save people from the inevitable boredom of long trips in space, Bradford says.
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The deep freeze
But hibernation may not be enough to get people through decades-long trips. That’s because even champion hibernators have to rouse sometimes. Most animals come out of hibernation after a few months, Grabek says.
Making people colder might slow their metabolism even more than regular hibernation. But what if you went really cold? Or even frozen? Wood frogs in the Arctic freeze solid for the winter. They thaw out again in spring. Could they be a model for humans wanting to travel the stars?
Shannon Tessier is a cryobiologist. That is a scientist who studies the effect of extremely low temperatures on living organisms. She is looking for a way to freeze human organs for transplants. She works at Massachusetts General Hospital and Harvard Medical School in Boston.
Freezing is usually bad for organs, she says. That’s because ice crystals can rip open cells. Wood frogs can stand freezing because they have ways of preventing ice crystals from forming.
Tessier and her colleagues, though, worked out a way to supercool human livers to freezing temperatures without ice crystals forming. Right now, most organs can only be kept on ice for about 12 hours. But the supercooled livers could be stored for 27 hours. The researchers reported the achievement in 2020 in Nature Protocols. But more research is still needed. Tessier doesn’t yet know if the thawed liver will work if transplanted into a person.
Plus, freezing may not be enough for long-term space travel, she says. Wood frogs can only stay frozen for a few months. Traveling to another solar system would take many years.
In true suspended animation, all metabolism in the body would stop. One way to make that happen is flash freezing to –140 °C (–220 °F). The ultralow temperature turns tissues to glass. That process is called vitrification.
Human embryos are stored this way by quickly freezing in liquid nitrogen. “We haven’t achieved that with a whole human organ,” Tessier notes. And you couldn’t dunk a whole person in a vat of liquid nitrogen. It would kill them.
Whole bodies would need to freeze from the inside out as fast as from the outside in, she says. And they would need to rewarm just as quickly. “We don’t have the science … to do that in a way that is not damaging,” she says.
Maybe someday humans on Earth will find our own carbonite. Then we might be able to travel as frozen cargo to a galaxy far, far away.