Exposure to deep space poses many potential risks to the health of astronauts, but one of the biggest dangers is space radiation. Above Earth's protective shielding, astronauts are exposed to radiation from energetic charged particles that increases their risk of cancer, damage to the central nervous system and a host of other health problems.
A new study has now investigated how chronic, space-like irradiation impacts the brain function of mice. To learn more, I spoke with Ivan Soltesz, PhD, a senior author on the study and a professor of neurosurgery and neurosciences at Stanford.
Our basic question was 'what happens to your brain during a mission to Mars?' So far, only the Apollo astronauts have traveled far enough beyond the Earth's protective magnetic field to be exposed to similar galactic cosmic radiation levels, albeit only for short durations.
In previous rodent studies, my lab observed that neuronal function is disrupted by low levels of radiation, a fraction of the dose used for cancer therapy. However, technical constraints required us to deliver the entire radiation dose within minutes, rather than across several months as during a mission to Mars. In the current study, we are the first to investigate the impact of prolonged radiation exposures, at Mars-relevant doses and dose rates, on the neurological function. We used a new neutron irradiation facility at Colorado State University.
The hippocampus, which is critical for several important brain functions, including the formation of new memories and spatial navigation. And the medial prefrontal cortex, which is important for retrieving preexisting memories, making decisions and processing social information. Thus, deficits in either of these two brain regions could detrimentally impact the ability of astronauts to safely and successfully carry out a mission to Mars.
My lab at Stanford measured electrical properties of individual neurons from mice that were exposed to six months of chronic neutron radiation. We determined that after chronic radiation exposure, neurons in the hippocampus were less likely to respond to incoming stimuli and they received a reduced frequency of communication from neighboring neurons.
Our collaborators at University of California, Irvine found that chronic neutron radiation also caused neuronal circuits in both the hippocampus and medial prefrontal cortex to no longer show long-lasting strengthening of their responses to electrical stimulation, normally referred to as long-term potentiation. Long-term potentiation is a cellular mechanism that allows memory formation.
Our collaborators also conducted behavioral tests. The mice displayed lasting deficits in learning, memory, anxiety and social behavior -- even months after radiation exposure. Based on these results, our team predicts that nearly 1 in 5 astronauts would experience elevated anxiety behavior during a mission to Mars, while 1 in every 3 astronauts would struggle with memory recall.
By understanding radiation risks, future missions can plan practical changes -- such as locating astronaut sleeping spaces towards the center of the spacecraft where intervening material blocks more incoming radiation -- that may help to mitigate the risks associated with interplanetary travel.
However, my lab believes the best way to protect astronauts from the harmful effects of space radiation is to understand at a basic science level how neuronal activity is disrupted by chronic radiation exposures.
One promising sign is that radiation exposures that occur in space rarely cause neurons in the brain to die, but rather cause smaller scale cellular changes. Thus, we should be able to develop strategies to modulate neuronal activity to compensate for radiation-induced changes. Our team is already starting a new set of chronic space-radiation experiments to test a candidate countermeasure drug.
The radiation risks we discovered are mostly a concern for travel beyond low earth orbit, such as months-long missions to Mars. Shorter trips to the moon -- such as the Apollo missions -- or months spent in Earth orbit aboard the International Space Station appear to pose a much lower risk of radiation-induced cognitive deficits. I would definitely like to go into space for at least a few quick orbits.
I'm also confident that my lab and others will expand our understanding of how chronic radiation impacts the nervous system and to develop the effective countermeasures needed to enable safe missions towards the moon or Mars within the next decade. However, I'm not sure I'm ready to leave my lab unattended for two years while I take a sabbatical to Mars.
Photo by ColiN00B
