Long-term space travel is bad for your health. Very bad. Being in space exposes humans to dangerously high levels of radiation; extended exposure to microgravity can damage a range of organ systems, including muscles, bones and eyes. Living for months or years in tight quarters can have severe psychological effects.
The key to solving these problems could be a 250m-year-old physiological strategy that allows mammals, birds, fish and other animals to survive extreme scarcity by essentially going offline: hibernation. When they hibernate, animals almost completely switch off their bodily functions; they don’t eat, drink or move, and just as importantly, aren’t hungry, or thirsty and don’t seem to suffer from the cold. This remarkable ability could prove crucial in helping humans get to Mars and beyond – and could also help save lives on Earth.
It turns out that hibernation can protect against many of the hazards of extended spaceflight, including radiation exposure and bone and muscle loss. Moreover, putting travelers in a long-term unconscious state could help reduce the toll of living in a confined space for months or years. Hibernation could also significantly decrease the amount of food and water needed for the trip, reducing payload and enabling astronauts to reach their destination – and return – in less time.
The problem, of course, is that humans aren’t natural hibernators. Unlike squirrels, bears, bats and many other species, we haven’t evolved to radically decrease our metabolism when resources are scarce. To overcome this, a growing group of scientists around the world are working to develop techniques that can safely induce hibernation in humans.
These researchers, some of whom are being funded by the European Space Agency (Esa) and Nasa, are unraveling how hibernators switch themselves off – and then switch themselves on again – with no ill effects from months without food, water and exercise.
“This is a very promising area,” says Christiane Hahn, who oversees space biology research at Esa. “It could absolutely transform the future of space travel.”
The perils of space radiation
Radiation is a particular concern for long-haul spaceflight. On Earth, the atmosphere blocks most radioactive particles; in space, there’s no protection. Over a long space voyage, travelers would be continually exposed to dangerous levels of harmful ions. The particles can actually get trapped within the spacecraft, causing even more damage to those inside. “Protecting humans from radiation in space is very challenging,” Hahn says. “We haven’t yet found an effective shield.”
Research has found that hibernation defends against this harm. During hibernation, animals reduce their metabolic activity, use less oxygen, and tightly pack their DNA strands, all of which protect against radiation damage. Moreover, hibernators possess potent DNA repair mechanisms.
“It’s incredible what they can do,” says Yale University physiologist Elena Gracheva, who oversees a large colony of 13-lined ground squirrels (so named because they have that number of lines on their bodies), which are native to the midwest US and Canada. The creatures are kept in a hibernaculum, a specially designed facility that recreates their natural habitat.
“These animals are like us during the summer, but in winter they become completely different organisms,” she says. “Their heart rate drops to one beat every several minutes, and their body temperature goes to 4C [39F], which is the temperature in a refrigerator. Yet they’re still alive.”
Gracheva is studying how the animals can survive without water for up to eight months – while hibernating, the animals won’t drink, even if presented with water. She has identified a brain area, the subfornical organ (SFO), that seems to regulate this process, as well as a molecule that seems to abolish thirst when injected into the SFO. She notes that this brain area also exists in non-hibernating species, including humans.
Researchers are now exploring ways to hack human physiology so that we too can reap these benefits. They are experimenting with drugs, ultrasound and other strategies to enable humans to enter a state of synthetic torpor, as it’s known. (Although the two terms are often used interchangeably, scientists generally define torpor as a short-term state lasting between a few hours and a day, while hibernation lasts much longer, for weeks or months. Synthetic torpor tends to encompass both short- and long-term metabolic deactivation.)
“It is definitely feasible,” says biochemist Kelly Drew, a professor at the Institute of Arctic Biology at the University of Alaska. For more than two decades, Drew, who is funded by Nasa, has been studying arctic ground squirrels, which hibernate from August to May, dropping their body temperature from 37C (98.6F) to below freezing. Drew’s work has focused on how the animals protect their brains, hearts and muscles while at low temperature, a state that would normally kill living cells. She and her colleagues have found that during hibernation, myosin, a key muscle protein, radically changes how it uses energy, allowing it to survive cold temperatures without damage.
Identifying key mechanisms
In recent years, researchers have been able to induce synthetic torpor in a range of animals. Nearly all of these experiments have used invasive techniques, usually some kind of brain surgery. University of Bologna physiology professor Matteo Cerri, for example, has targeted cells in the raphe pallidus, a brain region that plays a key role in regulating temperature and energy use.
But while this work helps illuminate the mechanisms involved in the process, it wouldn’t be practical, or ethical, to open up space travelers’ skulls every time they had to enter or exit torpor. Since 2023, several groups, including scientists at Washington University in St Louis, have used ultrasound, a noninvasive technique that transmits sound waves, to trigger synthetic torpor in animals. Cerri and his colleagues, who receive funding from the Esa, are hoping to begin testing this approach soon in healthy human volunteers.
Hibernation is extremely complex – after all, it affects every cell in the body – and there are almost certainly multiple switches involved in the process. MIT neuroscience researcher Siniša Hrvatin has identified another brain region that seems to play a key role in the process. In a paper published earlier this year (but not yet peer-reviewed), he and his team targeted a region known as the preoptic area, which plays a key role in metabolism and temperature. By activating neurons in the preoptic area of hamsters, the researchers put them into torpor, lowering the animals’ body temperature to 15C.
Hrvatin notes that this preoptic neural circuit probably exists in a wide variety of animals, including some that don’t hibernate or enter torpor at all. To Hrvatin, this suggests that it would be possible to trigger a hibernation-like state in animals that don’t normally shut themselves down. “Key aspects of the circuit appear to be conserved across different animals,” he says. “I think we can use it to modify metabolism.” It’s not clear if this preoptic circuit exists in humans; no one has looked. Hrvatin is planning to explore this question soon.
Some scientists are already experimenting on people. In a study published last year, University of Pittsburgh researcher Clifton Callaway gave healthy humans a sedative called dexmedetomidine for five days; this caused a 20% drop in metabolic rate and a 30% decrease in overall calorie consumption. Compared with what a ground squirrel does, this is a small drop. But Callaway, whose work received funding from Nasa, says it might be enough to protect travelers from at least some of the hazards of spaceflight. And over a long voyage, even a relatively small metabolic decline would make a difference in efficiency.
“A trip to Mars is going to require something like 300kg of food per astronaut, there and back,” he says. “If you can reduce that by a quarter or more, that can add up.”
Saving lives on Earth, too
The promise of synthetic torpor extends far beyond making space travel safer. Scientists are studying it as a treatment for a wide range of diseases, including cancer and Alzheimer’s disease. Hibernation seems to trigger broad repair and regenerative capacities across many organs and cell types. And it seems to hinder the growth of cancer cells and makes them more vulnerable to treatment. Both Cerri and Hrvatin are exploring this area. “This has so much therapeutic potential,” says Cerri. “It’s just an incredibly exciting area.” Drew, the University of Alaska professor, and others think it could also be useful in obesity; by turning metabolism up rather than down, doctors could help people burn more calories.
A group of Dutch scientists has identified a hibernation-related molecule that they think has the potential to treat Parkinson’s disease, heart failure, asthma and other diseases. Rob Henning, Roelof Hut and Kees van der Graaf, researchers at the University of Groningen, isolated the molecule SUL-138 from Syrian hamsters, which enter torpor when temperatures drop below 18C. Henning and his colleagues have tested the compound in a range of non-hibernating animals and showed that it has broad protective and regenerative properties. They recently started a small human trial of the compound for patients with Parkinson’s.
“The sky is the limit,” Henning says. “When I talk to my medical colleagues, I always say: ‘What is your problem? I’ll solve it with hibernation.’”
Callaway, who is also an emergency room doctor, says synthetic torpor could be useful for all kinds of urgent medical situations, including heart attacks, strokes and brain injuries, when doctors want to quickly slow metabolism and reduce inflammation to give them time to treat the problem. Unlike patients in a medically induced coma, patients in synthetic torpor wouldn’t need to be on life support, because their brains would still be active. He says torpor has the potential to be an improved version of therapeutic hypothermia, a technique that has been used for decades.
While useful, hypothermia has a major flaw: the body fights back against the cold by shivering, increasing inflammation and boosting heart rate. This frantic effort to stay warm can limit the benefits of the treatment. By contrast, hibernating animals don’t respond as vigorously to cold; this difference may make synthetic torpor a much more effective tool in emergencies.
Most experts agree that the first human use of hibernation will probably be medical. Hrvatin sees organ transplantation as a likely first option: he says it would be relatively simple to activate some hibernation pathways to lengthen the survival time of an organ. Researchers are already experimenting with this, and have found that it can significantly increase organ longevity.
There are a range of views on when synthetic torpor will become a reality for humans. Cerri is among the more optimistic – he believes it will happen in the next 10 or 15 years. Most experts think it will take longer; Hahn, the Esa scientist, thinks it will take several decades.
She and others point out that before torpor can be used for space travel or for treatment, researchers must understand the process much better. Otherwise, she says, we run the risk of all kinds of nightmarish sci-fi scenarios. “Inducing torpor is fairly well understood,” she says. “Bringing someone out again is not. We need to make sure we get both parts right.”
