Radiation - What Can We Expect on the Trip?

We've spent almost all of the previous posts discussing potential opportunities for getting to Mars. Can we do it, how fast can we get there, how comfortable can we make the trip? Now it's time to talk about what's out there in the open ocean when we minnows leave the relative safety of the mangroves. And it turns out to be radiation, from a number of sources. This might end up being more than one post. Holy cow, is there ever a lot to talk about!

To begin, most of the radiation we encounter every day is our friend. You can read this post because radiation bounces off your screen and into your eyeballs, where it excites individual pixels (rods and cones). That data is sent to your brain, which translates it into what passes for reality around here. All of the food you had for breakfast this morning was stored radiation from the sun, plants have little chemical factories where they convert radiation from the sun into chemical energy and then use that to make sugars, starches, etc. And then there's the 'man-made' stuff; where would we be without microwave ovens and CAT scans and X-ray machines and radar and Wi-Fi? We bathe in an ocean of radiation, and we have evolved over millions of years to use it to our benefit.

But then there's the radiation that ISN'T our friend. That radiation ocean we're swimming in doesn't include much of the harmful stuff, because several filters shield our tender pink bodies from the cold, uncaring universe. So when we leave Earth behind, we will be like little fishies moving out of the mangroves and into the open ocean; there are many dangers out there and many strategies that might help us survive and prosper, but eventually we have to fish up and swim out there. Can't live in the mangroves forever.

What can we expect to encounter out there?

The Van Allen Belts. There are two of them, they are part of Earth's magnetosphere, and they are highly radioactive. Eat your liver radioactive. They are our primary protectors from harmful space radiation, but they also accumulate highly energetic particles (alpha and beta radiation). The inner belt (2,000-12,000 km) is less energetic and therefore less dangerous, but the outer belt (20,000-40,000 km) is not a nice place for pale squishy humans to be in for very long. The Apollo astronauts minimized the dangers by avoiding the lower belt (highly inclined orbits) and by quickly traversing thinner parts of the upper belt. I'm going to assume that we can do at least as well as they did fifty-five years ago.

Galactic cosmic rays. We don't even know where they come from. We think they are most probably created during supernovae, but how can supernovae accelerate these particles to at or near the speed of light? According to recent studies, the particles are 90% Hydrogen nuclei, 9% Helium nuclei, and 1% 'other'. The 'other' includes various heavy metals, and through the magic of science, researchers have dated the particles as approximately 6,000 years old. Knowing what direction they came from, they have tentatively identified their origin as a large cluster of stellar giants and super-giants, which is of course fertile ground for supernovae! So we're closing in on the mystery, but the reality remains that they can scorch through your spacecraft and your spleen and keep going like nothing ever happened. That isn't a big deal, one lousy particle, but they interact with (read 'beat the crap out of') nuclei in our bodies, which interact with others, which... resulting in a shower of energetic particles which can do real damage to organs and tissue. The radiation shielding can sometimes actually do more harm than good because of the shower of secondary particles! So this is serious stuff that we'll discuss later under health effects and counter-measures, but it's not catastrophic damage that will fry us in our beds. It's a chronic problem that we can manage.

The Sun. Our major culprit. Down here on Earth Surface, our magnetosphere and atmosphere shield us from harmful radiation while allowing us to bask in beneficial radiation. Or, to put it another way, we have evolved to make use of the radiation that gets here, but we have no defenses against the radiation that doesn't get here because why would we? Then along comes space travel. Rats! Crank up the evolution machine! But in the meantime, we all still have to survive somehow, so what does this mean for space travelers? We will need shielding against energetic sunlight like high ultraviolet, x-rays, etc that our planet normally screens out. Happily, that's fairly straightforward, perhaps ten centimeters of polyethylene or a similar hydrogen-rich material and Bob's Your Uncle. Think of a layer of insulation in the walls of our home.

The Sun also emits cosmic rays, but they are much less energetic than galactic cosmic rays; think billiard ball versus RPG round. But as we all know, a crisply thrown billiard ball to the forehead can spoil one's entire afternoon, so we'll need to take measures to protect ourselves from them. Again, something we'll discuss later under health effects and counter-measures, but it's not catastrophic damage that will kill us on the trip to Mars.

But then there's the real killer, the solar storm. That's a pretty innocuous term for 'the sun gets pissed off for no apparent reason and shoots out enormously powerful death rays that will kill us in minutes, followed by huge volumes of highly charged particles that will fry us in hours'. OK, this is serious stuff. The sun looks so happy and friendly and, I don't know, normal. It isn't. It's a gigantic ball of explody thermonuclear nastiness, and there's no control switch, it just does whatever it wants to do whenever it wants to do it. It gets brighter and dimmer (we're not sure why). It has sunspots the size of our planet (we're not sure why, but we have a few guesses), that appear and disappear in a cycle of nine to fourteen years (we don't know why there is a cycle, or why the cycle varies). And periodically, at or near the height of the cycle, the sun blasts out a solar flare containing extremely energetic radiation accompanied by highly charged particles traveling near light speed. You can't see these babies coming, there's no warning. And behind it, on a bad day, the Sun burps out an enormous volume of highly ionized and magnetized particles, called a Coronal Mass Ejection, which sometimes accelerates particles all by itself, creating a Solar Particle Event. If we are traveling to Mars and are unlucky enough to be in the path of a Coronal Mass Ejection, it will show up anywhere from a day to a week after the initial solar flare. So we're going to need protection in place to deal with the solar flare because it shows up without warning and it can kill us in minutes. The Coronal Mass Ejection is equally dangerous, but we'll have some warning before it gets to us, so we'll have time to prepare. We'll discuss how to prepare a bit later on.

So, it's a rough neighborhood. We'll need to keep our doors locked and windows rolled up, keep a beady eye out for any trouble on the road ahead, and have a plan for what we will do if trouble arises. But how do we quantify those dangers? What does all of this radiation stuff mean in terms of our short and long-term survivability, not showing up dead on Mars, not having children with three heads, etc?

First, the long-term stuff. The Sievert is a unit of measurement developed to calculate the health effects of low-dose radiation. It's particularly useful in that we can calculate the total energy from various types of radiation (for example, particle energy times the number of particles), add them all up, and we have an estimate of the total radiation energy entering our body. The official SI value is 1 joule per kilogram of body weight. The maximum permitted doses for NASA astronauts are 1 Sievert per year, 1 to 4 Sieverts lifetime depending on age and sex. NASA believes that these maximum doses represent a 3% additional risk of cancer during an astronaut's lifetime, and everyone is comfortable with that minor additional risk as the price of living and working in orbit. NASA has, of course, done a lot of work evaluating the Earth-Mars radiation climate, and their latest estimate is that for a one-year return trip to Mars, an unprotected traveler in an unprotected ship would receive 0.66 Sieverts, roughly what an astronaut receives during their 6-month residency at the ISS.

Despite all the media and popular science articles and blogs that constantly try to frighten us with the dangers of space radiation, those numbers don't look bad - certainly not when compared to all the other risks that Mars colonists will face! And keep in mind this is for completely unprotected travelers. It's also important to keep in mind that NASA is a very cautious organization, (cautious is good in the space business) and that the actual risk of contracting cancer from this level of exposure is likely much lower than 3% because it's based on the Linear No-Threshold Model. I'll quickly point out three examples that support a lower risk, and you can follow it up for yourselves on the interweb if it piques your curiosity.

Taiwan 1982. Someone goofed at the recycling plant and mixed a batch of radioactive Cobalt60 into the scrap metal, which was subsequently made into rebar, which was used to build 2,000 apartments that housed 10,000 residents. Significant oopsie. Widespread long-term radiation exposure. Discovering the mix-up ten years later, the authorities conducted several studies to determine the human cost. The increase in cancer occurrence and cancer mortality was - negative. The median estimate was a 40% reduction; those low levels of radiation were almost certainly beneficial.

Guarapari, Brazil. This beautiful seaside resort town has beaches that are derived from Monazite, a Thorium ore, and are significantly radioactive. Dose rates per hour (it's a beach, people are transient) vary from 40 to 600 times the average background dose in the US. No adverse health effects have ever been reported.

Rimsar, Iran. Roughly 2,000 people live in the most radioactive continuously-occupied area on the planet. Hot springs bring radioactive water to the surface where it forms travertine, which is used as a building stone for the local homes. The walls, floors, ceilings - all radioactive. Further, the gardens are irrigated with radioactive water and the locals eat the plants. These people have a lifetime exposure of 1-10 Sieverts, and this has been going on for a very long time. The increase in cancer occurrence and mortality is - zero to negative. Studies are ongoing.

Inhabited Areas with Very High Background Radiation

There are numerous other examples. The point is the Linear No-Threshold Model - which assumes that any level of radiation is harmful no matter how low - is almost certainly bogus. And when we look for the scientific justification for the model, there is none. Seriously. Someone just drew a straight-line relationship years ago, and no one ever questioned or challenged it because what's the harm in being a little too safe?

I'm going to cut this off here as we're already a bit over-long and that was a lot of information to absorb in a single post. The next post will cover the DANGEROUS radiation out there, and what we can do to protect ourselves and our ships while en route.

Thanks for reading along!