This article on radiation hazards in space is part of the Science in Sci-fi, Fact in Fantasy blog series. Each week, we tackle medical or technical aspects of science fiction or a historical / world-building topic of fantasy with input from an expert. Please join the mailing list to be notified every time new content is posted.
The Expert: Steven Fritz
Steven Fritz has a Ph.D. in Radiation Biophysics and spent two decades as a medical school faculty member in Radiology Physics before going on to be a senior university administrator. For five years he operated a seed stage venture fund financed by the State of Maryland before spending another five years as an avionics entrepreneur. He also had a second career in the U. S. Naval Reserve as a Naval Aviator, which explains his continuing fascination with space travel.
Steven is also a science fiction writer. He has published three novels, Asteroid Gambit, Terraformed Earth—New Millennium, and Terraformed Earth—Discovery. His stories have appeared in Amazing Stories, AntipodeanSF, Flash in a Flash, and other magazines. His short story “Museum at the End of Time” was published in the inaugural issue of Z-Sky magazine.
Radiation Hazards in Space
Radiation is widely misunderstood in both science fiction and popular culture. The word is used to refer to radiation emanating from many things that have little in common: power lines, cell phones, microwave ovens, UV lights, X-ray machines, radioactive isotopes, and nuclear reactors among others to name a few. But the real hazard in space comes from specific types of ionizing radiation. Ionizing radiation, as the name implies, is radiation with enough energy to ionize atoms as it passes through material.
Here on spaceship Earth, we are shielded from most of it by the atmosphere. At the surface we are exposed to a Dose Equivalent, a way of expressing whole body radiation risk, of roughly 0.4 millisieverts (mSv). This has no measurable health effects, although radiation risk models suggest a very small risk of cancer. The International Space Station in Low Earth Orbit is also shielded by the Earth’s magnetic field. However, as humans venture deeper into space, they will lose the protective shields of Earth’s atmosphere and magnetic field. Then they will face one of the major hazards of space travel: ionizing radiation.
Radiation and DNA Damage
Ionizing radiation is dangerous to living things in particular because it can damage DNA. Most cells keep their DNA in tightly packaged bundles, which helps protect it from damage. However, rapidly dividing cells — like bone marrow stem cells — frequently unpack their DNA as part of the cell division process, and thus are especially vulnerable to things that can cause damage, such as radiation or chemical mutagens.
Most of the time, DNA damage in a cell leads to cell death. Kill enough rapidly dividing cells, and the organism can die. Even at lower doses, radiation can cause mutations in DNA that may increase the risk of developing cancer years later. If the DNA of sperm or ova are damaged, those mutations maybe passed on to future generations. Thus, understanding the sources of radiation and developing ways to mitigate the hazard are crucial to human space travel.
Sources of Radiation in Space
Radiation in space comes from three primary sources: solar wind, solar energetic particle (SEP) ejections, and galactic cosmic radiation (GCR). The primary danger from all of these sources comes from fully ionized nuclei of all the atoms in the periodic table. In all three sources, 90% or more of the particles are hydrogen nuclei(protons). The number of other nuclei fall off rapidly with atomic number, with only trace amounts of heavy nuclei like uranium.
The main differences between the three sources are fluence (number of particles per unit area) and energy spectrum. The solar wind proton fluence at an energy of 1 MeV (million electron volts) is eight orders of magnitude greater than the fluence in GCR and SEP. However, the solar wind fluence drops exponentially with increasing energy below 1 MeV, while the GCR fluence peaks at about 400 MeV. Both have to be considered for radiation protection of astronauts.
Regolith on the asteroid Eros captured by Near Earth Asteroid Rendezvous – Shoemaker during descent to land. The area is about twelve meters across (Credit: Wikipedia).
One last concern is gamma radiation, highly energetic electromagnetic radiation. Gamma radiation fluence is far less than charged particle fluence, but its properties make it a real concern for shielding.
Radiation in Science Fiction
Most science fiction deals with radiation in space by neglect— they don’t mention it. When they do, it’s rarely realistic.
In one of my favorite sci fi books as a preteen, Rip Foster Rides the Gray Planet, the characters ride on an asteroid made of pure thorium (an error for another day) for months and then suddenly realize they’re in danger from radiation from the asteroid. No mention of the solar wind and GCR streaming through them every day.
In 1931, in A Daring Trip To Mars by Max Valier, published by Wonder Stories, the author wrote:
“My greatest concern is whether the insulating stratum of ozone which is between the double walls of our cabin in order to check the deadly short-wave gamma rays of space, would prove effective.”
At least he recognized the hazard, but his solution is pure fantasy.
In a more modern novel, Ringworld, Larry Niven solved the problem of space radiation with two types of shields. He protected his ships with a “General Products hull” that was transparent to visible light and opaque to electromagnetic energy and any kind of matter. He also postulated a Slaver stasis field, a device that creates an area in which no time passes, even as outside the field time passes normally. Anything in a Slaver stasis field is impervious to change of any sort. Needless to say, both are pure handwavium.
In The Martian (2011) protagonist Mark Watney uses a Plutonium 238 Radioisotope Thermal Generator (RTG) inside his rover to provide heat. This is actually not wrong, since most of the energy Pu238 emits is in the form of alpha particles (Helium nuclei) which cannot penetrate its box. However, the author ignores the radiation hazard from solar wind and GCR while Mark spends time on the regolith in the open.
A popular trope in movies is using hazmat suits to protect against radiation. These are intended to protect against airborne radioactive material. They provide no shielding whatever from X-rays, gamma rays, or neutron fluence.
Overall, few science fiction stories have paid any attention to radiation hazards.
Getting Radiation Right
So how do we get it right as SF authors? You’re designing a spacecraft for a trip to Mars or out to the Asteroid Belt. What are the radiation hazards? What kind of protection can you provide to your astronauts?
The first rule is to recognize the hazard as it really exists. You don’t have to do a lot of science exposition. If you recognize solar wind and GCR as threats, even by as simple a mechanism as having a character check for cumulative exposure to GCR once or twice, or tap into NOAA’s space weather prediction center, you’ve acknowledged the glowing beast in the room.
What about protection?
The good news is that, unlike gamma radiation, charged particles lose energy rapidly as they penetrate solid matter. At 100 MeV, a proton’s range in water is 7.8 cm, about three inches. For 350 Mev protons, the peak of the GCR energy curve, the range is about seven times greater. So a shield made of water two feet thick will provide adequate shielding for protons in both solar wind and GCR. The vast majority of the energy deposited will be in the outer layers of the shield.
How A Water Shield Would Work
On an interplanetary trip the astronauts will need water and water recycling. If the ship is built with wraparound water tanks just beneath the outer layer of the hull, most of the ambient radiation will be absorbed harmlessly. Make the tanks out of a plastic like polyethylene, with its low atomic number and high hydrogen density, and you get even more protection.
What about lead? After all, Superman comics pretty well established that lead stops X-rays. Period. Well, maybe in the imagination of a couple of teenage authors during the Depression. In reality, x- and gamma radiation are attenuated exponentially. That doesn’t mean really fast, it means that no matter how much thickness of shielding you have, there is still a diminishing fraction that gets through.
A useful concept for dealing with this is half-value layer (HVL). It’s the thickness of shielding that reduces intensity by half. For gamma rays at 500 keV, the half value layer in water is 7 cm. HVL for lead at this energy is 0.4 cm. HVL is less at both higher and lower energies, so his makes a worst case point for evaluation. Our postulated ten inch (25,4 cm) water shield would reduce gamma ray intensity to between an eighth and a tenth of its initial exposure rate. Given that it’s relatively low in the first place, that’s good enough.
The gamma ray hazard from external sources isn’t the real problem. As protons, helium ions, and heavier nuclei pass through dense material they can undergo Bremsstrahlung interactions with atoms in the shielding that create high energy photons. The probability of such a reaction depends heavily (no pun intended) on the atomic number of the material it’s penetrating. Lead, atomic number 79, has a much higher potential for Bremsstrahlung than water. But it can happen even in water. Furthermore, if Bremsstrahlung creates very high energy photons, they can interact with the shield and create lower energy electrons in a recursive process called a cascade.
The solution is to put lead shielding inside the water shield so that the Bremsstrahlung fluence passes through the lead. That way, few nuclei make it to the lead to create high energy photons, but the photons created in overlying water are absorbed well by the lead inner shield.
A solution that NASA has already used is to build a refuge in a spacecraft that incorporates additional shielding. Given warning, astronauts can take shelter there and wait out the storm.
One last issue is radiation protection on the Moon or Mars (or an asteroid). Regolith is the loosely compacted material found on the surface. Figure 1 shows the regolith on the asteroid Eros. It’s possible to build a shelter on the surface of a solid body (many asteroids are not solid, so be careful) and pile regolith on top of it to increase shielding. A better solution might be to build shelters underground, either in an existing structure like an empty lava tube or a freshly dug cavity beneath the surface. This is easier on the Moon or Mars than on your average asteroid.
Okay, you’re got it. You understand the radiation environment in space, the kind of hazard it represents, and measures that can be taken to mitigate it. Go forth, do more research, write amazing science fiction about humanity’s future in space.
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