Animals that hibernate naturally enter a phase of decline in core body temperature and metabolic rate during hibernation, a state known as torpor.
Since ancient times, human beings have never stopped thinking and exploring the universe. "The earth is the cradle of mankind, but mankind cannot live in the cradle forever," said Tziolkovsky, the founder of modern spaceflight and known as the father of spaceflight. In the 1960s and 1970s, though, man had already set foot on the moon. But until now, we were confined to cislunar space. That's because it would take 500 days to get to Mars, our nearest habitable planet, and back. The nearest star to our solar system, Proxima Centauri, is 4.22 light-years away. In addition to the long distance, a series of physical and psychological problems faced by astronauts in long-term space flights and the extremely limited resources on the spacecraft are also obstacles to our journey to deep space.
In science fiction and movies, astronauts often go into a "sleep state" to complete long distance flights. This is not a wild fantasy. As early as the 1960s, some scientists proposed the idea of "dormancy" by suppressing metabolism. In the 21st century, NASA and ESA have put forward the idea of inducing human "dormancy" and started to design and research. The first thing we need to understand about this cutting-edge technology is that "dormancy" is not freezing, but a variety of techniques to lower an astronaut's core body temperature and "blunt" his metabolism. "Dormancy" can reduce the consumption of supplies, reduce the need for astronauts to move space. These changes can allocate more space and resources to the power system in the design of spacecraft. More importantly, "hibernation" can not only reduce the physical impact of space environment on astronauts, but also eliminate stress and loneliness during the mission, reducing the threat to astronauts' mental health. The ESA therefore considers "controlled hibernation a game-changing technology for manned space flight." In the foreseeable future, low-temperature, low-metabolic human "dormancy" will be one of the key technologies for deep-space exploration missions such as landing on Mars.
Non-hibernating animals cannot enter hibernation spontaneously
Animals that hibernate naturally enter a phase of decline in core body temperature and metabolic rate during hibernation, a state known as torpor. Torpor is triggered by many factors, including environmental factors (e.g. cold, food shortage, circadian rhythm, etc.) and internal factors (hunger, sleep, etc.). During Torpor, animal body temperature can be as low as 3-5 ℃, basal metabolic rate can be as low as 1%-5%, and their behavior is inhibited, presenting a state of suspended animation. But this period did not last forever, interrupted by periodic wakeups of "Arousal" (2 to 30 days, with species differences). Metabolic levels and core body temperature rise rapidly after awakening, ensuring that some key physiological functions are not impaired. The cyclic torpol-arousal cycle formed the complete hibernation process. Because non-hibernating animals lack relevant biological mechanisms, they do not have the ability to spontaneously generate periodic state changes of Toror-arousal, so they need to be induced by some physical or pharmacological methods to enter a state of hypothermia and low metabolism.
The central nervous system is the core of modulation of Toror-arousal
Some anatomical and functional connections within the central nervous system play a key role in controlling the torpol-arousal transformation. Both dormancy and arousal are dependent on the regulation of specific neurons and neural circuits in the central nervous system. In June last year, Nature published two blockbuster articles on rodent induced Torpor. Similar new findings were reported December 11 in Nature Communications. Three studies have proposed and confirmed that specific neurons in the hypothalamus play an important role in dormancy induction in non-hibernating animals. These advances further suggest that the key to future human "dormancy" may be found in the hypothalamus and its associated circuitry.
A controlled and safe dormancy - wake transition is essential for dormancy in non-hibernating animals
So far, there has been some progress in the study of "dormancy" in non-hibernating animals. But when it comes to human dormancy, safety and control are the first issues the technology needs to address. During the transition from dormancy to awakening, there was no obvious organ and tissue damage in hibernating animals with the large fluctuation of core body temperature and metabolic rate. For non-hibernating animals, the absence of corresponding protective mechanisms is likely to cause significant cellular stress and functional damage. Especially for the largest energy consuming organ of the human body -- the brain, a long time of low temperature and low metabolism, the structure and function of the central nervous system will surely cause a profound impact. After a long period of dormancy, the neural connections and functions of hibernating animals can be restored to their pre-dormancy state. Therefore, revealing the neurobiological mechanism of natural hibernation is helpful to solve the key technical problem of human "dormancy".
The "dormant" future of humanity
The "sleep" of human beings cannot blindly pursue "time standing still". Depending on the type of task, the optimal balance between resource consumption and hibernation degree and mode can be found. Although the central nervous system plays an important role in regulating "dormancy", "dormancy" is an overall response of the body to environmental changes, involving almost all organs and tissues. Studies on "dormancy" of other tissues and organs need to be carried out at the same time. To sum up, human "dormancy" is a complex and multi-domain systematic engineering, which requires the integration of technologies from various disciplines including biology, medicine, materials and chemistry to realize this idea. It is worth mentioning that with the development of China's space station and extraterrestrial base construction, the main battlefield of "dormancy" research can be transferred to the space laboratory. This will be more conducive to the adjustment of astronaut selection and training ideas, and the early realization of safe and controllable human "dormancy" and the application of manned deep space exploration. In addition, the world's major space powers have launched plans to land on the moon or Mars, which will also accelerate the research on human hibernation.