Background. Human metabolic suppression is not a new concept, with 1950s scientific literature and movies demonstrating its potential use for deep space travel (Hock, 1960). An artificially induced state of metabolic suppression in the form of torpor would improve the amount of supplies required and therefore lessen weight and fuel required for missions to Mars and beyond (Choukèr et al., 2019). Transfer habitats for human stasis to Mars have been conceived (Bradford et al., 2018). Evidence suggests that animals, when hibernating, demonstrate relative radioprotection compared to their awake state. Experiments have also demonstrated relative radioprotection in conditions of hypothermia as well as during sleep (Bellesi et al., 2016 and Andersen et al., 2009). Circadian rhythm disrupted cells also appear to be more susceptible to radiation damage compared to those that are under a rhythmic control (Dakup et al., 2018).

An induced torpor state for astronauts on deep space missions may provide a biological radioprotective state due to a decreased metabolism and hypothermic conditions. A regular enforced circadian rhythm might further limit DNA damage from radiation.

The As Low As Reasonably Achievable (A.L.A.R.A.) radiation protection concept defines time, distance and shielding as ways to decrease radiation exposure. Whilst distance cannot be altered in space and shielding either passively or actively may be beneficial, time of exposure may be drastically decreased with improved propulsion systems. Whilst chemical propulsion systems have superior thrust to other systems, they lack high changes in velocity and fuel efficiency which can be achieved with nuclear or electric based propulsion systems.

Radiation toxicity could be limited by reduced transit times, combined with the radioprotective effects of enforced circadian rhythms during a state of torpor or hibernation.

Objectives. 1. Investigate how the circadian clock and body temperature may contribute to radioprotection during human torpor on deep space missions.

2. Estimate radiation dose received by astronauts during a transit to Mars with varying propulsion systems.

Methods. We simulated three types of conditions to investigate the potential radioprotective effect of the circadian clock and decreased temperature on cells being exposed to radiation such that may be the case during astronaut torpor. These conditions were:

- Circadian clock strength: strong vs weak.

- Light exposure: dark-dark vs light-dark cycle

- Body temperature: 37C vs hypothermia vs torpor.

We estimated transit times for a mission to Mars from Earth utilizing chemical, nuclear and electrical propulsion systems. Transit times were generated using the General Mission Analysis Tool (GMAT) and Matlab. These times were then input into the National Aeronautics and Space Administration (NASA) Online Tool for the Assessment of Radiation In Space (OLTARIS) computer simulator to estimate doses received by an astronaut for the three propulsion methods.

Results. Our simulation demonstrated an increase in radioprotection with decreasing temperature. The greatest degree of radioprotection was shown in cells that maintained a strong circadian clock during torpor. This was in contrast to relatively lower radioprotection in cells with a weak clock during normothermia. We were also able to demonstrate that if torpor weakened the circadian clock, a protective effect could be partially restored by an external drive such as lighting schedules to aid entrainment i.e.: Blue light exposure for periods of awake and no light for rest times

For the propulsion simulation, estimated transit times from Earth to Mars were 258 days for chemical propulsion with 165.9mSv received, 209 days for nuclear propulsion with 134.4mSv received and 80 days for electrical propulsion with 51.4mSv received.

Conclusion. A state of torpor for astronauts on deep space missions may not only improve weight, fuel and storage requirements but also provide a potential biological radiation protection strategy. Moreover, maintaining a controlled circadian rhythm during torpor conditions may aid radioprotection. In the not too distant future, propulsion techniques will be improved to limit transit time and hence decrease radiation dose to astronauts. Limiting exposure time and enhancing physiological radioprotection during transit could provide superior radioprotection benefits compared with active and passive radiation shielding strategies alone.

背景。抑制人体新陈代谢并不是一个新概念，1950年代的科学文献和电影证明了其在深空旅行中的潜在用途（Hock，1960年）。人工诱导的以玉米粥形式的代谢抑制状态将改善所需的补给量，从而减轻执行火星及以后任务所需的重量和燃料（Choukèr等人，2019）。已经设想了将人类静息的生境转移到火星的方法（Bradford et al。，2018）。有证据表明，与冬眠状态相比，冬眠时的动物表现出相对的放射防护作用。实验还证明了在体温过低以及睡眠期间的相对放射防护（Bellesi等，2016； Andersen等，2009）。

由于减少的新陈代谢和低温条件，航天员在深空飞行任务中诱导的Torpor状态可能提供生物辐射防护状态。正常的昼夜节律可能会进一步限制放射线对DNA的损害。

“尽可能合理地降低辐射（ALARA）”保护概念将时间，距离和屏蔽定义为减少辐射暴露的方法。虽然不能在空间上改变距离，并且被动或主动屏蔽可能是有益的，但改进的推进系统可以大大减少暴露时间。尽管化学推进系统具有比其他系统更高的推力，但它们缺乏速度和燃油效率的高变化，而基于核或电力的推进系统可以实现这种变化。

辐射毒性可以通过减少运输时间来限制，并结合在强迫或冬眠状态下强制性昼夜节律的辐射防护作用。

目标。1.研究昼夜节律和体温可能如何在进行深空飞行任务的人进行人为干扰时对辐射防护做出贡献。

2.估算宇航员在使用不同的推进系统前往火星期间获得的辐射剂量。

方法。我们模拟了三种类型的条件，以研究生物钟的潜在辐射防护作用和温度降低对暴露于辐射的细胞的影响，例如在宇航员进行Torpor时。这些条件是：

-昼夜节律强度：强弱。

-曝光：暗-暗与明-暗循环

-体温：37℃vs体温过低vs torpor。

我们利用化学，核能和电力推进系统估算了从地球到火星的任务的飞行时间。运输时间是使用通用任务分析工具（GMAT）和Matlab生成的。然后将这些时间输入到国家航空航天局（NASA）的在线辐射评估在线工具（OLTARIS）计算机模拟器中，以估算宇航员针对这三种推进方法所接收的剂量。

结果。我们的模拟表明，随着温度的降低，放射防护的能力也有所提高。最高程度的放射防护表现在在烘烤过程中维持强昼夜节律的细胞中。这与常温期间时钟较弱的细胞的放射防护相对较低相反。我们还能够证明，如果torpor削弱了生物钟，则可以通过外部驱动器（例如，照明时间表）部分恢复保护效果，以帮助诱捕，例如：处于清醒状态的蓝光暴露，休息时间无光

在推进模拟中，化学推进从地球到火星的传输时间估计为258天，接收到165.9mSv；核推进为209天，接收到134.4mSv，而电力推进到80天，接收到51.4mSv。

结论。在太空执行深空任务时，宇航员的工作状态不仅可以减轻重量，燃料和存储要求，而且还可以提供潜在的生物辐射防护策略。此外，在煎熬条件下保持受控的昼夜节律可能有助于放射防护。在不久的将来，将改进推进技术以限制飞行时间，从而减少对宇航员的辐射剂量。与单独使用主动和被动辐射屏蔽策略相比，限制在传输过程中的暴露时间并增强其生理辐射防护能力可以提供卓越的辐射防护优势。