Beyond the protection of Earth’s
magnetic field, charged atomic nuclei whiz through space at nearly the speed of
light. This radiation is one of NASA’s biggest concerns as it considers manned
missions to Mars.
And a new study on mice finds that
exposure to the equivalent of a few weeks of space radiation made them perform
poorly on tests of learning and memory, and visibly damaged their brain cells.
Radiation
in Deep Space
Cosmic rays are a form of ionizing
radiation. This means that they have enough energy to ionize, or electrically
charge, atoms of any matter they pass through. In the tissues of the body,
ionization leaves a trail of damage along the cosmic ray’s path. The ionization
also sends energetic electrons shooting out into the tissue around the original
path, causing more damage.
Both Earth’s atmosphere and its much
larger magnetic field protect life on Earth from cosmic rays and other
high-energy radiation. The magnetic field deflects many cosmic rays before they
hit Earth’s atmosphere, and the atmosphere shields the planet’s surface from
the rest. The magnetic field extends about 60,000 miles into space, so though
astronauts in low-Earth orbit on the International Space Station (249 miles
high) are exposed to cosmic rays, it’s much less than they would face on more
distant missions. And though the astronauts who traveled to the Moon during the
Apollo program ventured outside Earth’s magnetic field, they spent just a
handful of days on the trip. A manned flight to Mars would require much more
time in deep space, which could mean dangerous exposure to cosmic rays.
New
Experiments
Researchers were especially
interested in the brain’s prefrontal cortex and hippocampus, both areas
involved in memory. Earlier experiments with proton radiation, the type used to
treat some brain tumors, showed it can damage neurons in mice’s hippocampi.
That matched well with doctors’ observation that human patients who receive
radiation treatment for brain cancer often develop cognitive deficits, such as
becoming more easily confused.
In the latest experiment researchers
tested the same kinds of charged particles that make up cosmic rays: 16O
(oxygen nuclei with a positive charge of 16) and 48Ti (titanium
nuclei with a positive charge of 48). Researcher Charles Limoli says that
the mice received about as much radiation as they would have in about ten to
thirty days in deep space.
Damaged
Brain Cells
Six weeks after their radiation
exposure, the mice were given tests that assessed their memory and their
ability to react to novel objects and situations. Compared to mice which hadn’t
undergone radiation, the mice in the test groups showed less curiosity. They
also showed poorer spatial memory and episodic memory, the memory of specific
events, times, and places.
What’s more, when researchers
examined the mice’s brains under a microscope, they found that the radiation
had snapped off crucial parts of the neurons. In treated mice the dendrites —
branching, tree-like structures that receive information and transmit it back
to the nucleus of a neuron — were shorter, with fewer branches. Loss of
dendritic branches is linked to cognitive decline in Alzheimer’s and other
diseases.
Preparing
for the Future
Limoli says it’s difficult to
determine exactly how long a human would need to spend in deep space before
suffering from cognitive problems, but he told Discover, “Three to six
months would be a good estimate for the start of any potentially adverse
cognitive side effects.” Using current technology NASA estimates a one-way trip
to Mars would take six months.
Future missions in deep space would
need to protect astronauts from cosmic rays. It’s theoretically possible to
shield a spacecraft’s hull, but it would be expensive and might make the
spacecraft too heavy to launch. Limoli suggests that future strategies might
include new shielding materials, shielding only certain areas of the
spacecraft, or even shielded helmets. He also said that medications might help
mitigate the effects of radiation.
For now, more research is needed to
better understand how ionizing radiation damages brain cells and how long its
effects might last. Space-based studies are planned, but Limoli says they will
be limited, since they’ll still take place inside Earth’s magnetic field.
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