As mankind prepares for its great migration away from Earth, new research suggests that astronauts on manned flights to distant planets could develop major cognitive impairments as a result of prolonged exposure to galactic cosmic rays. Not only might this affect their ability to safely operate their spacecraft and complete their missions, but it could also leave interplanetary voyagers with lifelong dementia.

The findings are published in the journal Scientific Reports.

The highly charged particles in galactic cosmic rays are, according to the study authors, capable of penetrating all types of brain cell. However, because space stations are protected from this radiation by Earth’s magnetosphere, no astronaut has ever been exposed to these particles for extended periods of time, which means we don’t know how harmful they actually are.

^{So far we have only sent rovers and landers to Mars. How well humans will fare remains to be seen. Triff/Shutterstock}

To investigate, researchers bombarded mice with highly charged particle radiation, before observing how this affected their performance when carrying out a number of cognitive tests. The team then imaged the animals’ brains six months after the experiment, in order to look for long-term changes in neuronal structure. 

At the six-month stage, mice continued to exhibit significantly reduced neuronal complexity in brain regions such as the hippocampus and prefrontal cortex, both of which are highly involved in memory, emotion, and general cognition. In particular, the researchers found that the neurons in these brain areas had fewer spines and dendrites – or connecting branches – while the integrity of the junctions between neurons, known as synapses, was also compromised. The upshot of this is less efficient transmission of signals, and reduced brain power.

As a result of these deficits, the mice performed worse on all cognition tests than they had done before being exposed to radiation. Of particular concern was their inability to complete the “fear extinction test”, whereby they first learn to associate a particular sound with a painful electric shock, and are then required to “forget” this association. Fear extinction deficits are linked with anxiety, which is clearly not an ideal characteristic for an astronaut taking on the highly stressful challenge of interplanetary travel.

“This is not positive news for astronauts deployed on a two-to-three-year round trip to Mars,” said study co-author Charles Limoli.

“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.”

If this cognitive impairment develops to full-blown dementia, early colonizers of the Red Planet may even be left unable to remember their heroic Martian adventures.

It is worth noting, however, that not all laboratory experiments provide a true reflection of real-life scenarios, so whether or not the negative effects experienced by these mice will also apply to Mars astronauts is not a foregone conclusion. Prominent aerospace engineer Robert Zubrin, for example, has rubbished Limoli's study, claiming that the radiation he subjected his lab mice to is unlike that actually encountered by astronauts, and insisting that travelling to Mars won't cause brain damage.

^{Researchers cannot say for certain how astronauts will be affected by cosmic ray radiation when they are far from home. Vadim Sadovski/Shutterstock}