Astronaut sleep mode may eventually prove impossible for long missions

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In a previous study, the European Space Agency (ESA) looked at astronaut hibernation as an opportunity for long missions. Induced hibernation, or “wintering” to use the exact term for the bear that inspired the method, would markedly reduce energy requirements. But is it really feasible? The Chilean researchers claim that the savings made so far have not been calculated correctly. Comparing the energy savings and expenditures of various mammals during hibernation, they suggest that, from a metabolic standpoint, humans are unlikely to survive a multi-year journey in space.

Placed in a state of torpor (hibernation) or hibernation, the body will see its metabolism decrease in order to achieve savings in oxygen, food and water. Thus, during a long flight, this could provide a significant advantage; especially since such a journey requires “about 30 kg per astronaut per day, and we have to take into account radiation, as well as mental and physiological problems,” Jennifer Ngo-Anh, coordinator of human and robotic research, explained in a press release last January. . research and payload at ESA.

While hibernation is a natural state experienced by many mammals and is interpreted as an adaptive energy saving strategy, Chilean researchers have studied in detail the relationship between body weight and energy expenditure in hibernating mammals. The calculation, they say, is useful for determining “the lifespan of hypothetical astronauts in space, or the size limit at which hibernation becomes ineffective.”

Only small mammals conserve energy during hibernation

The results of the study are surprising: on average, small hibernating mammals tend to save much more energy than large animals. For example, a tiny 45-gram marsupial retains 76% of its energy during hibernation compared to its normal active state. The brown bat and pygmy opossum can even drop their normal energy levels by up to 98% during hibernation! If we keep in mind the bear as the main hibernating mammal, then in fact it weighs an average of only 70 grams.

On the contrary, large bears do not save energy during hibernation, but, on the contrary, lose it. For example, a grizzly bear achieves a negative energy saving of 124%. Even a smaller bear (75 kg) does not receive any energy boost during hibernation compared to a normal sleeping state.

It should be noted that in the normal state of activity or sleep, small animals must burn more energy than large animals to maintain body temperature. But in a state of hibernation, everything seems to be equalized, regardless of the size of the body. “We evaluated the daily scale of hibernation energy expenditure and found that it is isometrically dependent on mass,” the researchers write in their study. “This means that one gram of a hibernating bat has a metabolism similar to that of one gram of a bear, 20,000 times greater.”

Therefore, it becomes a more important advantage for small living creatures and of little interest for larger ones, such as bears and people. Artificial astronaut hibernation in humans probably won’t save more energy than normal sleep, according to this study.

Thus, small mammals can easily take advantage of hibernation by accumulating energy stores in muscle and fat, whereas humans would not be able to do the same. And even if we took this into account, space travel could not exceed a few short years, because the fat loss would be significant. Hibernating in space requires 6.3g of fat per day, which is about 204kg for a 90-year journey, according to the study.

Proceedings of the Royal Society B.

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