Astronauts Would Need to Be Obese for Space Hibernation, Study Suggests

It's one of science fiction's most common themes: a spaceship glides through the cosmos, its inhabitants eerily asleep in some frosty tube-shaped pod that allows them to pass long stretches of time in a state of hibernation.

Humans have yet to embark on any kind of long-distance space travel beyond the solar system or even the Earth-Moon system, but scientists have seriously questioned whether putting humans in some kind of preservative sleep state could be possible.

In a new study, Roberto Nespolo, an ecologist at the Austral University of Chile, and colleagues Carlos Mejias and Francisco Bozinovic, investigated the possibility of human hibernation whilst analyzing the benefits this sleep state confers to animals in nature.

The study imagined a future 120-year space voyage in which an astronaut is accidentally awakened by an onboard computer. The astronaut then faces a difficult decision: now that their metabolism is significantly higher than that of their sleeping crew-mates, will they condemn them to starvation by eating all the available food on board?

The study states that, based on energy reductions seen in small mammals that hibernate, the awake astronaut would consume the energy of 50 hibernating humans in a single day.

It's a nerve-wracking scenario, but there are other problems that preclude astronaut hibernation. Namely, that humans are simply too big to do it. A hypothetical 70 kilogram (154 pounds) hibernating human might hypothetically have an energy expenditure of 250 kilojoules per day, the study authors speculate.

At this rate, they would spend 6.3 grams of fat per day or 2.2 kilograms per year, and if a human wanted to hibernate for a voyage of 90 years, they'd need 204 kilograms (250 pounds) of fat first, according to the study—resulting in a BMI of over 50 at least for a six-foot astronaut. In other words, there's a natural fuel storage problem.

Speaking to Newsweek, Nespolo explained why some animals—which, except for bears, are all small—are able to hibernate through the winter and others cannot.

"In activity, small animals need to spend more energy to maintain body temperature constant, because at small sizes heat is dissipated at a higher rate because the surface-to-volume ratio is higher.

"But as we found, in torpor, the energy consumption per gram of animal is constant at any size. So that's why the benefits of turning off the switch of metabolism are greater in small animals and becomes approximately zero at the size of a small bear.

"Humans are simply too large, so the benefits of hibernation are little as in bears if we think just on energy savings."

It raises the question of why bears hibernate, which the study tackled. According to the study, there are other benefits to be gained—namely, nitrogen preservation during reproduction, as denning is coincidental with fetal and newborn growth of cubs.

Setting aside all this, the elephant in the room is how to get humans to hibernate in the first place. Nespolo said it has been possible to induce a "hibernation-like state" in lab mice by modifying genes. "Also, there is one primate, the Microcebus lemur of Madagascar, that expresses torpor naturally. So it is something present in many different mammals that can be expressed under the appropriate circumstances," he said.

How exactly to trigger this is unclear. A 2018 study by German researchers specifically investigating inducing hibernation in humans did not provide any concrete solutions but concluded: "Up to now no unique factors or genes or neuro-endocrine properties have been identified for this state, suggesting that also humans have the potential for this physiological state. Hypometabolism in mammalian fetuses at normothermic conditions may provide a model for torpor in humans."

For now, at least, astronauts on missions to Mars or elsewhere will have to make do with a good book and plenty of board games.

Nespolo and colleagues' study, Why bears hibernate? Redefining the scaling energetics of hibernation was published in the journal Proceedings of the Royal Society B on April 27.

Astronaut in space
A new study suggests human astronauts would have to be obese to survive a long hibernation during a long journey. A stock photo shows an illustration of an astronaut. Astronaut hibernation during long space journeys is a popular sci-fi theme. Михаил Руденко/Getty