Space radiation comes in two principle forms and several variations. We will discuss each in turn.
Solar Flares (Photons)
When a solar flare occurs, some of the trapped radiation crashes back into the sun's surface. This can release strong bursts of x-rays and gamma rays. Since these are light radiation, they can reach Earth's orbit in eight minutes. Since we can see the flare erupt before the trapped particles crash back into the sun's surface, we may have up to five minute's warning of a radiation burst of this type.
Shielding from x-rays and gamma rays can be achieved by the same methods as protection from Solar Particle Events. That said, the radiation would arrive with far less if any warning. The hull of the spacecraft would be adequate protection in all but the most extreme cases. Also, light decreases in intensity with the square of distance. Therefore a flare would have 40 percent as much energy by the time it reached the orbit of Mars versus the orbit of Earth.
The sun emits a strong solar wind that produces a relatively steady flow of protons and helium nuclei (alpha particles) and electrons. Since these are positively or negatively charged, the magnetic field of the Earth deflects or traps these particles into the Van Allen radiation belts. For crews orbiting below the belts, say on the International Space Station, they are protected from this radiation. The radiation belts are intense for humans, but they are also at the same altitude as our geosynchronous communications satellites. Therefore, we have decades of experience in engineering computers, solar panels, and other systems that can operate in the radiation belts for extended periods.
Any neutrons emitted by the sun never reach beyond the orbit of Venus. When a neutron is isolated from an atomic nucleus, it decays into an electron and a proton within an hour, with a half-life of 10.5 minutes. Neutrons may be emitted from the surfaces of worlds without atmospheres, because cosmic rays can knock those neutrons off the surface atoms. This is how several lunar and asteroid probes examine the chemical composition of the surface of the worlds they orbit – by examining these neutrons as they hit detectors on the spacecraft.
Solar Particle Events (SPEs)
These are more commonly known as Coronal Mass Ejections, solar storms, or solar flares. All three terms are related. A solar flare occurs when the magnetic fields on the surface of the sun twist like a rubber band until they break. The flare can release bursts of ultraviolet and X-ray radiation, which reach Earth in eight minutes and can disrupt the Earth’s ionosphere. Any particles trapped in that field are thrown out at such incredible speed that they cross the 93 million miles to Earth in a day and a half.
The particle bursts themselves are called Coronal Mass Ejections. When they hit a planet or vehicle, they are called SPEs or solar storms. These bursts are deflected by the Earth’s magnetic field to the poles, where they collide with the atmosphere and produce aurora. Similar aurorae are also visible on other planets in the solar system with magnetic fields and atmospheres, such as Jupiter. While these magnetic fields protect planets, there are events so intense that they can overwhelm these magnetic and atmospheric protections and cause power blackouts and other electronic disruptions.
In deep space – starting in high Earth orbit and going to interplanetary space, there is no magnetic field to block these events. Fortunately, the types of radiation and intensity are relatively easy to block with 6-12 centimeters of shielding in the form of water, food, or plastic. Small deep space vehicles (such as the Apollo command module) can simply turn the heat shield and service module toward the sun to block the radiation from the crew. The lunar module had no protection from flares, so crews would have been vulnerable on the surface of the Moon.
Galactic Cosmic Rays (GCR)
Gaps in Science
As recently as ten years ago, we had little understanding of the cosmic ray environment. Our margins of error were as much as 600 percent. That has largely been remedied since then due to the AMS on the space station, and radiation instruments on lunar and Mars missions.
We have yet to launch crews into near-lunar space to do animal research on the full deep space environment. We need to do animal research to assess the impact across generations. It also needs to be done with a spinning vehicle to simulate gravity. Some impacts on health are microgravity, others radiation, and others both. For example, the tendency to not grow more bone may be some combination of microgravity making the bones less stressed and radiation killing off the cells that make bone.
A future Space Settlement Lab could solve for gravity and radiation at various levels of each. This is not currently planned by NASA or New Space companies, but is something I would recommend.
A Few More Interesting Items
Phobos: The inner moon of Mars, Phobos, orbits very close to the surface. A base on the side that permanently faces Mars would have Mars fill most of the sky. Since the asteroid moon itself would block the other half, cosmic radiation would only come in from near the horizon. A large crater is on the side facing Mars, and the walls of the crater would block most of that. In other words, a base in this crater would be completely exposed to space, with no magnetic field or other protection, and yet have an extremely low radiation environment.
Polar Craters on the Moon and Mercury The poles of the Moon and Mercury have almost no axial tilt, and permanently shadowed craters. While exposed to cosmic rays, any solar activity would be moot in these locations.
Jupiter's Radiation Belts The radiation belts of Jupiter are the most deadly in the solar system. The intense magnetic field has captured enough radiation to make crewed missions to Io, Europa, and Ganymede highly unlikely in the next generation or two. Callisto orbits outside these belts. The radiation environments of Saturn, Uranus, and Neptune are far less dangerous. Despite some beautiful science fiction, do not anticipate crewed missions to Io or Europa anytime soon. The radiation environment is so intense that engineers are deeply challenged to think of ways to make robotic missions to Europa with a lifespan of more than a few months.