Bad for Astronauts: Extended Exposure to Cosmic Rays may Cause Dementia

What happens to an astronaut's brain during a mission to Mars? Nothing good. It's besieged by destructive particles that can forever impair cognition, according to a UC Irvine radiation oncology study appearing in the May 1 edition of Science Advances.

Charles Limoli and colleagues found that exposure to highly energetic charged particles - much like those found in the galactic cosmic rays that bombard astronauts during extended spaceflights - cause significant damage to the central nervous system, resulting in cognitive impairments.

"This is not positive news for astronauts deployed on a two- to three-year round trip to Mars," said Limoli, a professor of radiation oncology in UCI's School of Medicine. "Performance decrements, memory deficits, and loss of awareness and focus during spaceflight may affect mission-critical activities, and exposure to these particles may have long-term adverse consequences to cognition throughout life."

For the study, rodents were subjected to charged particle irradiation (fully ionized oxygen and titanium) at the NASA Space Radiation Laboratory at the Brookhaven National Laboratory before being sent back to Limoli's Irvine lab.

The researchers found that exposure to these particles resulted in brain inflammation, which disrupted the transmission of signals among neurons. Imaging revealed how the brain's communication network was impaired through reductions in the structure of nerve cells called dendrites and spines. Additional synaptic alterations in combination with the structural changes interfered with the capability of nerve cells to efficiently transmit electrochemical signals. Furthermore, these differences were parallel to decreased performance on behavioral tasks designed to test learning and memory.

Similar types of more severe cognitive dysfunction are common in brain cancer patients who have received various photon-based radiation treatments at much higher doses. In other research, Limoli studies the impact of chemotherapy and cranial irradiation on cognition.

While cognitive deficits in astronauts would take months to manifest, Limoli said, the time required for a mission to Mars is sufficient for such deficits to develop. People working for extended periods on the International Space Station do not face the same level of bombardment with galactic cosmic rays, as they are still within the protective magnetosphere of the Earth.

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Now there is a nice natural control study: examine communities at high altitude.  Is 'cognitive impairment' at old age more prevalent than those who have lived their lives at sea level? Make sure you eliminate those who fly frequently from either study.  We know high altitude communities have a higher rate of brain cancer.  How about dementia and whatnot?  If not, perhaps this ought to be reconsidered.

Background Neutron Radiation Correlation With Mutation Rate

When cosmic rays hit the upper atmosphere, they send high-energy particles, such as neutrons, showering down towards the surface. So an interesting question is how these neutrons influence things on the ground.

Researchers have long known that high-energy neutrons can smash into atomic nuclei causing all kinds of damage to the structure of materials. This is one cause of errors in computer memories.

Indeed, back in 2004, a group of researchers from IBM measured the flux of neutrons from cosmic ray collisions and used their results to predict the error rate in computer memories another electronic logic devices. Their predictions closely matched the observed rate of error, suggesting that neutrons are indeed an important source of problems in computing.

That finding has captured the imagination of Augusto González at the Institute of Cybernetics, Mathematics and Physics in Havana, Cuba. If background neutron radiation can cause errors in computer circuitry, then it ought to have a similarly destructive effect on another much more common information processing system–life.

Evolutionary biologists have long known that spontaneous mutations occur at a rate that has a crucial influence on the nature of evolution. But exactly what causes spontaneous mutation has never been properly understood.

Now González says that same approach that the IBM researchers used to predict errors in computer memory also explains the rate of spontaneous mutations in living things.

González bases his work on a fascinating experiment carried out by Richard Lenski and his team at Michigan State University that has been ongoing since 1988. These guys have been growing the bacteria, E. coli, since then and monitoring the mutations that occur between generations.

Every day, a small amount of bacteria is taken from one culture and allowed to grow in a new dish of glucose over the course of the following day. The bacteria reproduce until the glucose runs out, usually within eight hours or so. A small amount of bacteria is taken from this dish and allowed to grow in a new one and so on.

Since 1988, the team has observed how the bacteria have evolved over 60,000 generations. And they’ve discovered that the number of point mutations in the bacteria after 20,000 generations is about 300 million. That’s a rate of about 1 per second.

The question that González addresses is whether this mutation rate can be explained by the background neutron radiation. He does this by creating a mathematical model of the environment in which bacteria grow, which is essentially water.

He calculates that high-energy neutron would enter the water on a bacterial sample about once every 125 seconds. This high-energy neutron would then transfer its energy to water molecules creating a relatively short track of ions. He says that a single neutron would generate some 300 ions over a track length of about 100 nanometers and about 30 ions at a distance of 0.1 mm

“The bacteria touched by this ion shower could be destroyed or experience a permanent damage, especially in their DNA, which can be later inherited by the descendants,” he says.

The question then is how often this happens. And he calculates that this rate is consistent with the frequency of deleterious mutations measured in Lenski’s experiments. “In this way, we are indicating the probable origin of a class of “spontaneous” mutations,” he says.

Of course, González is pointing out a correlation between background neutron radiation and the observed rate of deleterious mutations in E. coli. That’s only part of what is required for scientific confirmation.

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