How much radiation is too much?

The most dangerous kind of radiation in space comes from cosmic rays. These are ultra-fast, ultra-heavy particles shot from across the cosmos. The good news is that they’re relatively rare. The bad news is that they’re highly dangerous.

Another concern is radiation from the Sun. While more numerous, these are simple protons; it’s like a BB bullet compared to a cosmic ray cannonball. This radiation can damage cells and genetic material, but they’re much easier to shield against, and only become truly dangerous during solar storms. And with technology already available today, solar scientists can see these events coming far enough in advance to give at least a few minutes of warning to take shelter for space farers on the Moon, Mars, or in a space station.

Yet for any type of radiation, it’s still difficult for scientists to predict what makes a truly dangerous dose. Horrific real-world events like Hiroshima, Nagasaki, and various nuclear power accidents have proven that high radiation doses in a short period are deadly. But what if that same dose is spread out over a year? Three years? Five? A lifetime? Scientists still don’t know. The simple tests that would tell them are impossible to perform for ethical reasons, and difficult to synthesize using tissue or cell samples, since radiation can affect different biological systems – the brain, cardiovascular, or reproductive organs – in different ways.

“The problem with radiation,” says Marco Durante, director of the Biophysics Department at Germany’s GSI, “is we don’t know the risk very well.” The tests they run are based on cell or tissue samples, or, at best, animals like mice. They also usually bombard their test subjects with lots of high-energy radiation all at once. “First you have to extrapolate to humans, and then low dose rate, and then to space,” Durante says. “And there are not very effective countermeasures. There’s no magic drug that can save you from radiation effects.”

So even though researchers can perform biology tests with particle accelerators, the results are tricky to interpret and apply to humans in situations that are likely to be realized within the next decade.

Durante wants future experiments to at least test the effects of high-energy, low-dose radiation on samples by keeping them in the room, for example, but not directly in the beam of the particle accelerator. But the labs, built for cutting edge physics research, aren’t set up that way, and those experiments haven’t happened yet.

What is testable is how to block radiation. And this is where the research can provide more straightforward conclusions.

Homemade cosmic rays

Durante says that materials with lots of hydrogen prove to be good shields. Polyethylene, a simple and ubiquitous type of plastic, shields well, as does a substance called lithium hydride. Both are also extremely lightweight, a necessity in space travel, where lifting something above Earth’s atmosphere is expensive. The problem is that both of these compounds are flammable, a big no-no in space travel. Aluminum, while much heavier, is still preferred because it won’t catch fire.

To build the next generation of better-shielded, lightweight spacecraft, engineers will have to figure out how to safely enclose the shielding material inside something airtight and inflammable.

Once on solid ground again, like the Moon or Mars, more options become available. Tunneling underground provides a cheap and easy shield underneath the regolith. Or, Durante points out, if Mars has more substantial stores of water, an igloo would be the perfect shield. Water is excellent at blocking radiation.

Until scientists can find a way to test the safe limits of long-term radiation exposure on humans, shielding may be our only option.