Humanity has big plans to go to the moon and beyond in the next two decades. But as our technology makes steady(ish) progress toward those massive goals, our bodies may present obstacles to long-distance space travel. A new paper in the journal ENeuro reveals some “unexpected potential problems” with mammal brains that may mean we’re not as ready to make the journey as we hoped.

Using a new “low-dose” radiation facility at Colorado State University, a team of scientists observed that when mice spent months exposed to radiation similar to that found in deep space, they started acting strangely. The mice in the study displayed “severe impairments” in learning and memory, and they became extremely anxious. These symptoms may sound unsurprising since the mice had just spent six months as part of an experiment, but the team also found physical changes in their brains that may explain the changes.

In the paper, co-authored by Charles Limoli, Ph.D., a professor of radiation oncology at UC Irvine School of Medicine, the team makes the case that its findings “would clearly impair the abilities of astronauts needing to respond quickly, appropriately and efficiently to unexpected situations that arise over the course of a mission to Mars.”

Going beyond low-Earth orbit might pose significant risks to future astronauts' brains. NASA

This study was done on mice, so we can’t say with certainty that the results will apply to humans. Still, it’s coming at a crucial time. Back in April the results of the NASA twin study showed that when astronaut Scott Kelly spent a year aboard the International Space Station, he sustained some small physiological changes compared to his Earth-bound twin, but nothing was life threatening. Those findings suggested that humans do well during extended space missions in low-earth orbit — which was a good starting point.

Low-Earth orbit is one thing, but going to Mars (and beyond) is quite another. Deep space missions will have to contend with galactic cosmic radiation (GCR), particles that are accelerating so fast that they’ve been stripped of their electrons, leaving only the nucleus behind. Those particles can “pass practically unimpeded through a typical spacecraft or the skin of an astronaut,” posing threats to human health, NASA has noted.

Limoli and his team at UC Irvine tried to mimic this inhospitable environment as best as they could by confining mice for six months in a low-dose radiation facility. It’s not perfect, but they argue that their experiment protocol “reasonably simulates exposures from GCR during a prolonged mission in deep space.”

Over time, they observed physical and behavioral changes that proved concerning. Specifically, they saw that the neurons in the hippocampus of the radiation-exposed mice were far less excitable than they were in the control mice. That effect created a reduction in signaling, which they say explains some behavioral changes when the mice performed memory and social interaction tests.

"Thus, in a crew of five astronauts traveling to Mars, we would expect at least one member to display severe deficits…”

Socially speaking, mice who were exposed to the consistent doses of radiation tended to spend more time actively avoiding interactions with other mice — though they seemed perfectly happy to interact with a new mouse. They also took longer to recognize new objects or understand when old objects were moved to a new location, which suggested that radiation had negative impacts on their memory.

But they also noted that the long-term potentiation of neurons in the hippocampus was also hampered. Long-term potentiation is a form of brain plasticity when two neurons “learn” to fire together, and form a strong connection that is believed to underpin learning and memory. Long-term exposure to radiation, these authors note, seems to disrupt that crucial process.

The authors note that translating a finding from a mouse brain into something that NASA can use to help inform human space travelers still “remains a challenge.” Space radiation may also affect different people in different ways, but based on their calculations in mice, the team estimates that a significant number of astronauts might struggle with cognitive function (like memory for example) once they’re out there in space:

“Thus, in a crew of five astronauts traveling to Mars, we would expect at least one member to display severe deficits in each of those cognitive functions by the time they return to Earth,” the teams writes.

Considering that space entrepreneurs are already planning to build full fledged civilizations on Mars, the idea that one in five people may struggle on the journey could pose a huge stumbling block. But it’s not insurmountable. Though the authors describe these specific risks as “unexpected,” NASA has been aware that deep space radiation may be a challenge for years, and is actively seeking solutions

For example, in 2003 NASA provided funding to the Brookhaven National Lab in New York to create the NASA Space Radiation Laboratory, which, like Limoli’s project, was at first intended to evaluate the risks of radiation similar to that found in deep space. The idea is that as we get a better handle on the risks, the research priority will “shift from risk assessment toward countermeasure development.”

We are on a bit of a tight timeline. Several estimates suggest that humans will get to Mars (and stay there) within the next two decades. But hopefully, that’s enough time to solve these problems before they slow us down too much.

Abstract:

As NASA prepares for a mission to Mars, concerns regarding the health risks associated with deep space radiation exposure have emerged. Until now, the impacts of such exposures have only been studied in animals after acute exposures, using dose rates approximately 1.5¯105 88 higher than those actually encountered in space. Using a new, low dose rate neutron irradiation facility, we have uncovered that realistic, low dose rate exposures produce serious neurocognitive complications associated with impaired neurotransmission. Chronic (6 month) low dose (18 cGy) and dose rate (1 mGy/day) exposures of mice to a mixed field of neutrons and photons result in diminished hippocampal neuronal excitability and disrupted hippocampal and cortical long-term potentiation. Furthermore, mice displayed severe impairments in learning and memory, and the emergence of distress behaviors. Behavioral analyses showed an alarming increase in risk associated with these realistic simulations, revealing for the first time, some unexpected potential problems associated with deep space travel on all levels of neurological function.

Correction 8/6/19: The low dose radiation facility is based at Colorado State University. A previous version of this article misstated the location.