If getting to Mars isn't hard enough, scientists at the UC Irvine say that cosmic radiation could cause astronauts on deep space missions to develop symptoms of dementia. Rodent tests indicate that exposure to high-energy particles produce cases of "space brain" marked by long-term neurological damage, cognitive impairment, and diminished judgment.

Radiation has long been recognized as a constant and very real threat to space travelers, which is the reason why crews on the International Space Station (ISS) are legally classed as radiation workers. Prolonged exposure to cosmic rays can result in an increased chance of cancer, impaired immune systems, and even affect the brain and nervous system. The latter is of particular concern because it's already known that patients undergoing radiation therapy to treat brain tumors can suffer severe neurological symptoms, such as problems with cognition and memory.

For astronauts on the ISS, radiation mainly curtails how long and how many visits crews can make in a lifetime. But outside of the protection of Earth's magnetic field, which means any deep space mission, it's another matter. On long missions, including to Mars, galactic cosmic rays become a major hazard. These immensely high energy charged particles that originate outside the Solar System can shoot through spacecraft hulls and passengers as if they aren't even there. However, when the heavier of these particles, such as the nuclei of oxygen and carbon atoms, strike, they can directly or indirectly cause major damage to living tissue.

As part of NASA's Human Research Program, a UCI team under professor of radiation oncology Charles Limoli used the NASA Space Radiation Laboratory at New York's Brookhaven National Laboratory to approximate cosmic rays using an earthbound particle accelerator. Since real cosmic rays are hard to create if one doesn't have a quasar or exploding galaxy handy, the team used fully ionized oxygen and titanium nuclei to approximate the impact of lighter particles of much higher energies.

The radiation generated was used to expose male Wistar rats at dose rates between 0.05 and 0.25 Gy/min. This is equivalent to the doses a human would suffer on a deep-space mission, where 400 to 900 mSv would be absorbed on a typical Mars mission. Since the purpose of the study was to assess damage, no attempt at shielding the animals was made.

The rats were then returned to Limoli's UCI lab, where they were observed over 24 weeks and put through medical and behavioral tests to determine what effects the radiation had. They found that not only was there damage, but it persisted even six months after relatively low exposures.

Physically, the rats suffered from damaged neurons, reduced dendritic complexity causing disrupted nerve transmissions, changes in synaptic protein levels, and elevated neuroinflammation of the brain. This was reflected by the rats developing problems in learning new tasks and impaired memory.

The rats also showed problems with "fear extinction," which is the ability of the brain to handle unpleasant and stressful memories. In other words, it's what helps you get back on the horse again after a fall.

"Deficits in fear extinction could make you prone to anxiety," says Limoli, "which could become problematic over the course of a three-year trip to and from Mars."

Limoli says that the symptoms produced would be like dementia and would manifest themselves within months of leaving Earth. The astronauts would have problems with anxiety, impaired memory, reduced ability to multitask, and poor decision making. Worse, the condition would continue indefinitely.

According to Limoli, these findings are supported by a 2015 study, where he noticed similar effects in brain cancer patients undergoing chemotherapy and cranial irradiation.

Currently, solutions are being sought for the cosmic ray problems, but the amount of shielding needed to protect Mars crews would make any mission too expensive to mount, and the highly energetic particles would still penetrate the hull or cause radiation cascades like a shell turning armor plate into shrapnel. With this in mind, the UCI team is looking at a medical solution using compounds that could scavenge free radicals and protect neurotransmission.

"This is not positive news for astronauts deployed on a two-to-three-year round trip to Mars," says 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."

The research is published in the Nature Research Journal Scientific Reports.