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The Dangers of Galactic Cosmic Rays

The Dangers of Galactic Cosmic Rays

Eager to travel to Mars? Think twice!

Explorations of Mars with probes and spacecraft are revealing intriguing features of the Red Planet. The most recent discovery by the NASA spacecraft Mars Reconnaissance Orbiter, that liquid water very likely flows on Mars, has stirred enthusiasm among scientists who have been looking for signs of “life-as-we-know-it” on the planet for the last twenty years.

One of the major future goals of both the European and American space agencies, ESA and NASA, is to send human explorers to Mars to carry out investigations that cannot be performed by robots.

But aside from the technical challenges, how safe is it for the human body to travel in the cosmos under a shower of galactic cosmic rays?

Galactic Cosmic Rays

Galactic cosmic rays are composed of highly energetic, charged particles that originate outside of the solar system. When reaching the Earth and colliding with molecules in its atmosphere, they transform into other particles (the so-called secondary rays) and are in part deflected by the Earth’s magnetic field. For these reasons, even if we are constantly hit by secondary cosmic rays, we are only exposed to very low doses of radiation, which is not dangerous.

The story is different for astronauts when they leave the Earth’s atmosphere. No spacecraft can assure sufficient protection from the highly ionizing primary cosmic rays that easily breach the spacecraft’s hull, showering its occupants. Cosmic rays thoroughly penetrate the human body and tissues causing severe damage which the normal repair mechanisms of our cells cannot easily fix.

So, could cosmic rays harm astronauts on a mission to the extent of compromising their ability to accomplish their tasks? What type of damage can cosmic rays inflict on the human brain, our highest operational center, and with what consequences?

In order to better understand the impact of cosmic rays on the brain, Parihar and their colleagues from the University of California, Irvine performed a series of experiments in mice, examining their behavior after exposure to low doses of charged particles corresponding to those found in space and hence mimicking cosmic rays.

Through specific behavioral tests, the scientists found that irradiated mice partially lost their ability to discriminate between new and familiar objects placed in their cages, as well as their ability to remember the location of the objects.

These deficits in both episodic and spatial memory were associated with structural changes in mice brains, in particular in areas of the medial prefrontal cortex (mPFC), which, by connecting to the hippocampus, is essential for memory function. In fact, the mPFC showed a significant decrease in the number and density of dendrites and spines, indicating reduced connections to the hippocampus and possibly to other brain regions. Dendrites and spines are protrusions that extend from neurons and physically and functionally link different brain areas, thus supporting signal transmission.

The scientists also found that the extent of the structural changes in the mPFC correlated with the degree of memory deficits suffered by the mice, strongly suggesting that the damages induced by cosmic-ray-like charged particles directly caused cognitive problems.

What could happen to astronauts during spaceflight?

Astronauts on a long mission, such as a Mars exploration, will inevitably be exposed to cosmic rays that will impact their bodies.

Though the effects of cosmic rays on mouse brains may not completely overlap with those on humans, charged particles will certainly penetrate the astronauts’ brains and very likely affect the neurons’ structure and function. As a consequence, astronauts might suffer from cognitive problems that could significantly impair their judgmental abilities. This represents a serious hazard because astronauts might become incapable of taking complex decisions involved with the difficult and unforeseen challenges that space missions could present.

In addition, even if cosmic rays’ effects turn out to be manageable during space missions, the astronauts could suffer from radiation exposure later on after their return. Damage to their brain may become permanent and accelerate illnesses linked to neuronal degeneration.

The search for solutions that would assure adequate radiation protection for the astronauts is proceeding at full power. Simply increasing spacecraft shielding by building a thicker hull is not possible, because this would make the spacecraft significantly heavier and much more difficult to be sent into space. It will be necessary to develop new techniques, and maybe materials, in order to sufficiently block cosmic rays while keeping the spacecraft apt for space travel.

Much work has to be done to fully understand and reduce the risks that space missions pose to astronauts’ health. For this reason, also, Mars will have to wait quite a bit for the first human visitors.

References:
Parihar, V., Allen, B., Tran, K., Macaraeg, T., Chu, E., Kwok, S., Chmielewski, N., Craver, B., Baulch, J., Acharya, M., Cucinotta, F., & Limoli, C. (2015). What happens to your brain on the way to Mars Science Advances, 1 (4) DOI: 10.1126/sciadv.1400256
NASA – NASA Confirms Evidence That Liquid Water Flows on Today’s Mars
NASA – The Mars Exploration Program
R.A. Mewaldt – Cosmic Rays

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Agnese Mariotti
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