The Radiation Hazards of Space Travel

  radiation-miceIt is well known that ionizing radiation can cause cancer. Less known are the effects on heart health. Some surprising results.

A recent article in JAMA (Journal of the American Medical Association), here, explored the possible effects of extended exposure to space radiation on endothelial health. The idea was triggered by earlier research on mice, which was aimed at testing the effects of long term radiation, as would be encountered on a trip to Mars. The mice sustained significant endothelial damage—blood vessels—which made them susceptible to atherosclerosis—heart disease.

This is a somewhat unexpected result. Increased death due to cancer would be the expected one, radiation being a known cause of that.

The JAMA article decided to study the 24 astronauts that have made the round trip to the moon. The radiation experienced was five to ten times higher than on earth, but the trips were short, 8-12 days.

Other astronauts spend considerable time in the orbiting space station. They receive a higher dose as well. As it turns out, one year in the space station is about equal to a round trip to the moon. There are still other astronauts that are exposed to neither.

Of the 24 moon travelers, seven have died, and three from cardiovascular causes. Now this is not enough of a sample size to make any conclusions, but it does raise eyebrows. Astronauts are always physically fit and this would normally be a formula for heart health. The other, earth-bound astronauts have a very low rate of cardio death; a rate that is about third that of the general population, and about a fourth that of their fellow moon travelers. The astronauts that have spent time in the orbiting space station have a slightly higher rate, but it doesn’t stand out, and is a lot lower than the general population.

So what is going on? The space station inhabitants are receiving a radiation dose about equal to a trip to the moon but are having far fewer heart issues.

Now three out of seven is a really low sample size, and we would be inclined to throw such data out were it not for those mice. The higher doses for a shorter period of time, seem to have additional effects. The usual model is that it is the accumulated exposure that counts. This makes sense, as any particle of ionizing radiation (about 30 pass right through our bodies every second) has some small chance of hitting some DNA just right and causing damage. However, can such a model explain the heart issues with the moon astronauts or with the mice? Seems not. If the mice results are valid, it would surely mean that short periods of higher radiation are significantly more dangerous than the equivalent amount spread over a longer period.

What is endothelial damage?


Endothelial cells line the blood vessels and are the first line of defense against atherosclerosis. The cells are paper thin (compared to other cells) in order to facilitate their main job: letting nutrients get to the hungry cells on the other side of the blood vessels, while at the same time keeping everything else out. Anytime an endothelial cell is damaged, stuff can leak behind the cell wall. The more damage, the more stuff that leaks, and the more atherosclerosis. Exercise tends to keep the endothelial cells happy, likely one of its principle benefits.

The mouse results would seem to indicate that endothelial cells may be particularly susceptible to radiation damage. Would ours be as well? Probably so. There is no particular reason why men or mice should have evolved a tolerance to radiation. This is obviously speculation. There are a lot of unknowns.

What is ionizing radiation?

We are awash in a sea of subatomic particles. Some of them are so elusive that they pass right through the earth unimpeded. Since they don’t get tangled up in us, they do no harm, which is a good thing, as tens of thousands zoom right through us every second. Radiation that can do harm is called ionizing, meaning that can it knock electrons off our cellular DNA (turning it into a reactive ion), and thereby compromising it. The types normally discussed are alpha, beta, and gamma rays. Alpha rays are simply helium nuclei. They are generally wimps. A piece of paper (or our own skin) will stop almost all of them. The radium on a watch dial is mostly alpha radiation. The beta particles are simply electrons. They are small, and harder to stop. However, a few millimeters of metal will do the job. They do penetrate our skin, though, by an inch or so.

Gamma particles, or equivalently, gamma rays, are a type electromagnetic radiation, or, again equivalently, photons. Electromagnetic radiation come in a variety of energies from radio waves (low) up through light, ultraviolet, x-rays and beyond that, gamma rays. The energy is proportional to the frequency. The low energy ones, (radio, television, cell phone, wifi) don’t penetrate our skin and aren’t thought to be dangerous. Going up the frequency bands, we are ok through infrared, red, yellow, and blue light. These don’t significantly penetrate the skin either. At least they don’t penetrate to the lower layers where they could cause problems. A little higher, and we get into the realm of ultraviolet. Here the photons do penetrate, and can damage the stem cells that generate new skin. This is where squamous and basal cells skin cancers develop. These cancers are easily and effectively treated. (The deadly skin cancer, malignant melanoma, is probably not initiated this way.) As we go on up, we get to higher and higher energy particles, which become more and more dangerous. Long term exposure to X-rays is known to be carcinogenic. Gamma rays are generally those with frequencies above X-rays. These can penetrate several inches of lead, so shielding here is going to be tricky. Since they do not have a charge, a magnetic field isn’t going to help either.

What About Mars?


To travel to Mars, we are either going to have to have a spaceship with a pretty thick lead shield, or just learn to live with the consequences. Speed would certainly help. At the speed of light, we could get there in around 15 minutes (assuming I did the math correctly). If we were willing to take a day, we would only have to go 1% of the speed of light—a far more leisurely pace.

Cool, and at that speed we all want to go!


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