When astronauts first travel to Mars, or anywhere else in deep space, they will have to contend with two problems that our planet itself protects us from on its surface - radiation and weightlessness. Scientists already know both can have a profound impact on the human body, but just how much of one, and how long that impact can last, remain a mystery. A new paper by researchers at the Medical College of Wisconsin, and published in PLoS One, tackles those questions, and unfortunately comes up with some less than ideal answers in terms of the long term implications of deep space exposure.
As of yet, humanity hasn’t yet built a deep space research lab where they can run experiments in the actual environment of deep space itself. So researchers have to make due with what they have on the ground, in most cases. And since running these types of long-term exposure experiments on humans is both impractical and unethical, the researchers turned to a cornerstone of biological testing - rats.
To simulate weightlessness in Earth-bound rats, NASA developed a protocol called hindlimb unloading (HLU). In this particular experiment, middle-aged rats, chosen to mimic the typical age of career astronauts, had their back legs lifted off the ground at a 30 degree angle by a specialized harness connected to their tail for thirty days. This tilt shifts much of the body’s fluids to the head, and takes the weight off their lower bodies at least, which is about as close as the researchers can get to mimicking what happens to astronauts when they are floating in microgravity.
Fraser talks about one of the ways we can protect astronauts from radiation - with a magnetosphere.
Recreating radiation is harder, and requires some specialized equipment. Luckily, NASA has developed just such equipment. At Brookhaven National Laboratory, the NASA Space Radiation Laboratory is capable of simulating the kind of radiation environment that is constantly bombarding the solar system outside the protective shell of Earth’s atmosphere. For this particular experiment, scientists used a five-ion, six-beam cosmic ray simulator dubbed “simGCRsim”. It blasts a target with high speed protons and heavy ions like silicon and iron, that would potentially make its way through the hull shielding of a ship going to Mars.
The experimental protocol described in the paper had the rats “unloaded” for five days, bombarded for a few minutes at extreme high radiation levels meant to mimic months of exposure, and then continued to have their hind legs suspended for another 25 days. After that they were allowed to live out the rest of their lives, with the monitoring period of about a year equivalent to around 25 years of human life.
There were three primary findings described in the paper, but the one that was originally the researcher’s focal point concerned the heart. With a dose of 1.5 Gray (a measure of absorbed radiation dose), the rats began to develop perivascular fibrosis - a type of scar tissue that wrapped around the coronary vessels that feed blood to the heart. 270 days after the radiation exposure, their scar tissue size had doubled, squeezing the blood vessels to a point where a heart attack became much more likely. Interestingly, the weightlessness simulation had essentially no effect on this outcome - it appeared to be entirely caused by the radiation.
Discussion about how cosmic rays are potentially dangerous. Credit - Discovery UK YouTube Channel
When the researchers exposed the rats to a lower dose that was more akin to the total amount of radiation exposure astronauts would receive over the course of a three year mission to Mars, they noted that the heart scarring wasn’t visible right away. But a year after the exposure, 83% of rats had developed it. Which means that the effects of radiation might take literal decades of a human lifespan to present themselves.
Another concerning finding was the protocol’s impact on the rat’s immune systems. Months after their exposure, the rat’s spleens had still lost two thirds of its Natural Killer (NK) cells. These cells wipe out virus-infected cells, and, critically, cancerous ones, so a lasting dip in their population leaves the entire immune system compromised. Microgravity and radiation worked in tandem here to create these effects - rats that were exposed only to microgravity eventually recovered back to normal cellular levels. But if they were also hit with radiation, the recovery was muted - it appeared that the cosmic rays stunted the bone marrow’s ability to respond to physiological stress.
Still more bad news came when they sequenced the genomes of the rat’s white blood cells. 235 days after exposure to the radiation, the researchers noted a series of permanent mutations in a number of genes, some of which are directly tied to Clonal Hematopoiesis of Indeterminate Potential (CHIP). On Earth, CHIP starts to appear as we age, when a single mutated blood cell starts outcompeting its neighbors. That might not sound too bad, but this particular mutated blood cell carries with it an increased risk of leukemia as well as quadrupling the risk of heart attacks and strokes, since they cause extra inflammation to arterial walls. Exposure to radiation seemingly fast-forwaded the clock on those changes, severely compromising the cardiovascular system much more quickly than is typical for a rodent lifespan.
As the researchers themselves note, there are some limitations with this experiment. Crucially, the dose of radiation was delivered all in one go rather than continual exposure over months or years. It's unclear whether cellular repair mechanisms would be able to keep up with their efforts to combat the effects noticed in the rats, or whether the long-term effects would accrue no matter what their repair mechanisms would do. Also, the fact that the experiment was run only on male rats means a large proportion of the potential astronaut population might respond differently.
But for now, none of this is particularly good news for the future of long-term space flight. But it’s also likely not very surprising to the biologists who study these effects. We knew making our way outside of the protective shell of the Earth was going to be hard. This study just helps quantify the types of problems that we will eventually have to deal with.
Learn More:
M. Lenarczyk et al - Multiple simulated spaceflight stressors impact cardiac fibrosis, calcium dynamics, immune function, cytokines and gene variants in rat, Rattus norvegicus
UT - Cancer Rates Rise and Fall with Cosmic Rays
UT - Cosmic Rays: They Aren't What We Thought They Were
UT - Scientists Investigate the Biological Effects of Spaceflight Using Worms