How space radiation hurts astronauts

NASA image of the Crab Nebula, a remnant of a supernova. Scientists think that Galactic Cosmic Radiation comes from places like this.

Space is full of radiation. It's impossible to escape. Imagine standing in the middle of a dust storm, with bits of gravel constantly swirling around you, whizzing by, pinging against your skin. That's what radiation is like in space. The problem is that, unlike a pebble or a speck of dirt, ionizing radiation doesn't bounce off human flesh. It goes right through, like a cannonball through the side of the building, leaving damage behind.

Last week, researchers at the University of Rochester Medical Center published a study that suggests long exposures to galactic cosmic radiation — like the kind astronauts might experience on a trip to Mars — could increase the risk of developing Alzheimer's disease.

Reading stories about that paper made me curious. We've now been sending people into space for more than 50 years. We've been able to track a generation of astronauts as they aged and died and we're constantly monitoring the people who travel in space today. Research like what was done at the University of Rochester is conducted on lab animals, mice and rats. It's meant to help us prepare for the future. But what do we know about the past? How has radiation affected the people who have already been to space? How is it affecting the people who are there now?

There is one key difference between the astronauts of today and those of the future. That difference is the Earth, itself.

Galactic cosmic radiation — also called galactic cosmic rays — is the kind of radiation that researchers are most worried about. It's made up particles, bits and pieces of atoms that were probably flung off from the aftermath of supernovas. The majority of this radiation, roughly 90%, is made up protons ripped from atoms of hydrogen. These particles travel around the galaxy at almost the speed of light.

And then they hit the Earth. This planet has a couple of defense mechanisms that protect us here on the ground from the impact of galactic cosmic radiation. First, Earth's magnetic field both pushes away some of the particles and blocks others completely. Then, the particles that make it through that barrier start to encounter the atoms that make up our atmosphere.

If you drop a big tower made of Legos down the stairs it will break apart, losing more pieces every time it hits a new step. That's a lot like what happens to galactic cosmic radiation in our atmosphere. The particles collide with atoms and break apart, forming new particles. Those new particles hit something else and also break apart. At each step, the particles lose energy. They get a little slower, a little weaker. By the time they "come to a stop" at the ground, they aren't the galactic powerhouses they once were. It's still radiation. But it's much less dangerous radiation. Just like it would hurt a lot less to be hit with one Lego block, than with a whole tower of them.

All of the astronauts we've sent into space so far have, at least partially, benefited from Earth's protective barriers, Francis Cucinotta told me. He's the director of the NASA Space Radiobiology Program, the go-to guy for finding out how radiation hurts astronauts. He says, with the exception of Apollo flights to the Moon, the human presence in space has happened within the Earth's magnetic field. The International Space Station, for instance, is above the atmosphere, but still well inside the first line of defense. Our astronauts aren't exposed to the full force of galactic cosmic radiation.

They're also exposed to it for a relatively limited amount of time. The longest spaceflight ever lasted a little over a year. And that matters, because the damage from radiation is cumulative. You simply can't rack up as much risk on a six month jaunt to the ISS as you could, theoretically, on a multi-year excursion to Mars.

But what's interesting, and concerning, is that even with those protections we do see signs of radiation damage to astronauts, Cucinotta told me.

The big thing is cataracts — changes in the lens of the eye that make it more opaque. With less light able to get into their eyes, people with cataracts lose some of their ability to see. In 2001, Cucinotta and his colleagues looked at data from the ongoing Longitudinal Study of Astronaut Health, and found that astronauts who had been exposed to higher doses of radiation (because they'd flown more missions in space, or because of the specifics of the missions they'd been on*) were more likely to develop cataracts than those who had been exposed to lower doses.

There's also probably an increased risk of cancer, though it's difficult to estimate how much, exactly. That's because we don't have human epidemiological data about the kind of radiation astronauts are exposed to. We know the rates of cancer for survivors of the nuclear bombs dropped on Hiroshima and Nagasaki, but that radiation isn't really comparable to the stuff in Galactic Cosmic Radiation. In particular, Cucinotta is concerned about particles known as HZE ions.

These particles are very heavy and very fast and we don't experience them here on the ground. They're the kind of things that get filtered out and broken down by Earth's defense systems. But HZE ions can cause more damage, and different kinds of damage, than the radiation scientists are really familiar with. We know this because scientists actually compare samples of astronauts' blood before and after a spaceflight.

Cucinotta calls this pre-flight calibration. Scientists take a blood sample from an astronaut before the launch. While the astronaut is in space, the scientists divide that blood sample up and expose it to various levels of gamma rays — the kind of damaging radiation we're used to dealing with on Earth. Then, when the astronaut comes back, they compare those gamma ray-affected samples to what has actually happened to the astronaut while in space. "You see about a two-to-three fold difference across the population of astronauts," Cucinotta told me.

One example of how HZE ions are different: They seem to be able to affect cells they don't even touch. In non-human trials, these non-targeted effects can happen in cells up to a millimeter away from the cells that have actually been irradiated and we don't really know what that means yet. But it definitely changes the way we think about radiation risks, which is a model based on the assumption of a direct, linear connection between dose and risk. With HZE ions, that might not be true.

All of this explains why studies like the one published last week are going on. It's not that we're seeing horrible effects in astronauts who've been to space in the last half-century. Instead, there are two things those astronauts have shown us. First, there are genetic changes and damage happening even within the relatively safe confines we've traveled thus far. Second, there is a hell of a lot we don't know about how radiation exposure and risk works in outer space. It's almost like we can smell gas in our house, but we don't yet know whether there's a serious leak, or we just left a stove burner on for a couple minutes.

If our future really does lie in the stars, then this is a mystery we're going to have to figure out.

*The astronauts who flew on Skylab and the NASA-Mir missions were exposed to much higher doses of radiation than those on Mercury, Gemini, Apollo, or the Space Shuttle. The average dose to the eyes for those astronauts was around 90 mSv. None of the other missions had an average lens dose higher than 15 mSv. This probably reflects the longer amount of time spent in space on the Skylab and Mir missions, and possibly the construction and orientation of Skylab and Mir.


• The new paper on Galactic Cosmic Radiation and Alzheimer's disease

• An introduction to the space radiation environment

• NASA primer on cosmic rays

• A 2006 essay in The Lancet, written by Francis Cucinotta, about cancer risk and Galactic Cosmic Rays

• Cucinotta's 2001 paper on cataracts in astronauts

• A 2004 NASA Science News piece that also explores cataracts in astronauts