Interplanetary travel has been the topic of many science fiction stories, and serious scientific inquiry for years. The technological breakthroughs of the past several decades have made the possibility deep-space travel, and in particular a manned mission to Mars, now more real than ever. NASA has declared its intention of taking humans to Mars by the 2030s, and several private companies seeking to deploy manned missions to the red planet, including the establishment of human colonies, have cropped up. Mars-One, for example, is a private not for profit organization that proposes to take a human mission on a one-way trip to Mars with a view to establishing an extraterrestrial colony, with trips starting as early as 2026.
But how close are these missions to reality? In preparation to interplanetary travel, scientists worldwide have been earnestly trying to solve the problems inherent to the survival of humans for extended periods of time outside of Earth. Researchers from the Wageningen University and Research Center in the Netherlands are conducting a study on the possibility of growing crops on Martian soil, a necessity for the establishment of a human colony. Using simulated Martian soil, they showed that not only is it possible to grow crops in this soil containing heavy metals, but that these crops, which include radishes, peas, rye and tomatoes are safe for human consumption.
But even as these hurdles are overcome, scientists are still concerned with the effects extended periods of time outside the Earth could have on the human mind and body. In particular, exposure to cosmic radiation is one of the biggest obstacles to deep-space travel.
Galactic cosmic radiation is made up of high-energy particles originating outside of our solar system. Although the precise source of the radiation is unknown, it is generally thought that they originate from supernova remnants. One component of cosmic rays, HZE particles, is made up of charged atoms of heavy elements. HZE particles are of special concern because, due to their high energy, they are able to traverse spaceship walls, and human tissues and cells.
When cosmic radiation particles hit an organism, they leave an ionized trail behind them, damaging the surrounding structures. With HZE particles, delta rays emanating from the original ionized trails further extend the damage zone. On Earth, we are protected from galactic cosmic radiation by our planet’s magnetosphere, but outside of this protective shield exposure to HZE particles is inevitable. Cosmic radiation has been linked to DNA damage and several adverse health effects, including cancer and alterations to the central nervous system (CNS).
In 2003, NASA partnered with the Brookhaven National Laboratory to create the NASA Space Radiation Laboratory (NSRL). Its objective is to study the effect of galactic cosmic radiation on living organisms, as well as on different materials and equipment that could be used in space travel. In this facility, cosmic radiation can be simulated by using a particle accelerator to produce beams of ions, which can be deflected using magnets to hit specific targets such as cells, animals, or materials. The samples are then sent to the scientists’ laboratories, where the effects of radiation on their properties can be studied.
Recently, Dr. Charles L. Limoli’s group, from the Department of Radiation Oncology in the University of California, published a study in Nature Scientific Reports on the effects of cosmic radiation exposure on the central nervous system after up to 6 months post exposure. Mice and rats were exposed to beams of ionized oxygen or titanium at the NSRL, and the effects on their cognitive abilities and brain cell structure was then analyzed after 12 and 24 weeks. The researchers found that irradiated animals failed to react to novelty, and also had reduced executive functions, which are the set of skills that allow us to focus on and complete tasks. These results suggest that cosmic radiation could result in significant cognitive reductions which could compromise the safety of a deep-space human mission.
The scientists used a set of behavioral tests to assess how the irradiated animals reacted to novelty. They then compared their behavior to that of normal, non-irradiated mice. In these tests, the mice are allowed to familiarize themselves with an enclosed area in which several objects are placed. After an initial acclimation, one of the objects is either switched for a new object, or the location of one of the familiar objects within the enclosure is changed. Normal-functioning rats and mice will spend a greater amount of time exploring the new object, or the new location.
In a different but related test, the mice are familiarized with two sets of objects, each 4 hours apart. When presented with both objects at the same time, non-irradiated mice show preference towards the first object that was introduced, instead of the more recent one. The time the mice spend exploring a novel object, or location, is measured and used to calculate a discrimination index (DI). In all of these tests, irradiated mice had significantly lower DI’s than control mice.
Furthermore, this study showed that mice exposed to radiation had a decreased fear extinction response, and increased anxiety. Fear extinction is what happens when a stimulus that is associated with a fear-inducing event is presented without the fear-inducing event. Over time, the stimulus by itself will stop eliciting the fear reaction. For example, people living in war zones may develop fear of loud noises because they are associated to bomb explosions. After returning to peaceful conditions, loud noises are no longer followed by explosions, and so the fear response associated to them would gradually decrease and eventually disappear.
Fear extinction failure may result in increased stress. In humans, a fear extinction deficit has been associated with anxiety disorders, including PTSD (post traumatic stress disorder). In the mice, anxiety was measured using an elevated plus maze. This maze consists of two arms, one which is walled and another which is open. The animals are allowed to go out on and explore the open arms, or remain in the closed, more protected section. Increased anxiety is manifested by less time being spent exploring the open arms, than in the closed ones.
“The inability to moderate reactions to certain unpleasant stimuli could elicit elevated stress, anxiety and otherwise disadvantageous responses in unexpected or emergency situations” write the authors in their discussion. “Such conditions could clearly be problematic for astronauts and their capability to efficiently operate over the course of a deep space mission, and impairments in executive function point to further potential complications in conducting complicated multifaceted tasks or in decision-making under stressful situations.”
The scientists further investigated the structure of the brain cells, or neurons, found in the regions of the brain implicated in the behavioral tests. This was facilitated by the use of mice that express a protein that makes their neurons look green. The authors found significant structural differences between normal and irradiated mice, including reduced numbers of dendrite branches and spines, and changes in the post-synaptic protein PSD-95. Moreover, they found that dendrite spine reduction as well as the changes in PSD-95 correlated significantly with the behavioral alterations observed previously. In other words, the lower the DI score of a mouse on the behavioral tests, the greater the structural changes in its neurons.
This study confirmed the research group’s previous findings and expands on them, showing that the cognitive and structural effects they had previously observed after 6 weeks persist even after 6 months. Exposure to cosmic radiation is clearly associated to adverse cognitive effects tied to neuronal damage, and the results from this study suggest that this damage is persistent, perhaps indefinitely.
“This is not positive news for astronauts deployed on a two-to-three-year round trip to Mars,” commented Dr. Limoli in a press release published earlier this month. “The space environment poses unique hazards to astronauts. Exposure to these particles can lead to a range of potential central nervous system complications that can occur during and persist long after actual space travel – such as various performance decrements, memory deficits, anxiety, depression and impaired decision-making. Many of these adverse consequences to cognition may continue and progress throughout life.”
Findings from studies such as this help paint a more accurate picture of the risks that will accompany deep space missions. This knowledge is critical for scientists working on the development of effective strategies to minimize them. Research being done regarding the reduction of possible adverse health effects caused by cosmic radiation includes the development of better shielding strategies, as well as drugs that can both protect against, and favor repair after, tissue damage.
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