On Space.com’s web site, I read an article today suggesting that radiation exposure for astronauts might be too much for long duration space missions. Though radiation exposure to astronauts is a real concern, it isn’t really news. Radiation exposure is something that has been a concern since the very first days of spaceflight. In fact, one reason for sending animals into space before humans was to study the effect that radiation has on the body. Even America’s first satellite, Explorer 1, has a Geiger counter aboard to measure the radiation above the Earth’s atmosphere.
Physicists have known for many years that radiation is coming to Earth form space. The discovery of these cosmic rays is generally attributed to Victor Hess, who in 1912 took an electroscope aloft in a balloon. An electroscope is a device (now practically obsolete) used to measure electric charge. Electric charge would charge the electroscope, causing gold foil leaves to separate due to electrostatic repulsion. But, the electroscopes would always gradually discharge themselves. In the presence of the newly discovered radioactive materials, electroscopes would discharge more quickly. In fact, that is one way to define ionizing radiation: it is something that can ionize air. But, it can also ionize other things, including the atoms in your body. Ionizing atoms in your body changes their behavior, and that can be a bad thing if the atoms are part of DNA. The standard warning symbol for ionizing radiation is the trefoil, seen above. But, even when there was no known radioactive source nearby, electroscopes would discharge. This was taken to be due, in part, to background radiation resulting from the presence of radioactive atoms in the surroundings. Of course, some of the discharge was due to leakage of charge from the atmosphere itself. To differentiate between the atmospheric effects and the background radiation effects, Hess decided to take an electroscope high aloft in a balloon to get it away from background radiation from the ground. To his surprise, he found that there was more radiation at higher altitudes! Later measurements of radiation from mountaintops confirmed his findings. Not only do we have to contend with background radiation from the Earth, but we also have radiation bombarding us from above. This radiation from above is called cosmic rays.
People living at high altitudes, therefore, receive more radiation exposure than people living at lower altitudes. A couple of years ago, I wrote a rather long post about cosmic rays and radiation exposure for airlines. I was surprised in my research to find that airline crew members can receive more annual radiation exposure than many people working with radioactive materials. And, almost nobody wants to talk about that!
Astronauts already receive a lot of radiation exposure. However, that radiation exposure is going to be even greater for missions to the Moon or to Mars. Some cosmic rays are charged particles from the Sun. Others, the higher energy ones, are typically from deep space, originating far beyond our solar system. Many of these galactic cosmic rays are also charged particles. Charged particles are influenced by magnetic fields. The Earth’s magnetic field protects us from the direct effects of the solar cosmic rays, and some of the galactic cosmic rays, too.
The earliest space flights were all within the Earth’s magnetosphere (the region of space where Earth’s magnetic field dominates). Even the International Space Station and the Space Shuttle are within Earth’s magnetosphere. But, the Apollo missions to the Moon took astronauts beyond the shielding of the Earth’s magnetic field. Cosmic rays are pretty much a constant threat in space. But, worse are solar storms, in which the Sun can eject massive amounts of material into space. Astronauts, and even aircraft flying at high altitudes, can experience very large doses of radiation during such storms, and that is within the Earth’s magnetosphere. Astronauts exposed to such storms outside of Earth’s magnetosphere would receive very dangerous levels of radiation. Radiation exposure at such levels would most certainly be injurious to astronauts. And that, of course, is what this recent news story is about.
It isn’t really news, though. As I said, we’ve known about this for a long time. But we don’t really know just how much radiation exposure astronauts might get outside of Earth’s magnetosphere. A National Research Council report (again, not new) suggests that this radiation exposure is too high to safely conduct missions to Mars or extended missions to the Moon without serious radiation mitigation. New Scientist has a similar report several years ago, but as important an issue as this is, in needs to be continually in the minds of mission planners. The problem is that shielding against high energy radiation is tough, and the shielding is heavy. Estimates of the cost of sending a round trip spacecraft to the Moon or Mars are perhaps as high as $200,000 per pound (most of that is the cost of the rocket and fuel). So, heavy shielding could prove cost prohibitive. There are suggestions for lighter weight shielding, such as plastics, but more study is needed. It has also been suggested in numerous plans to use the on board water supply needed by the astronauts as shielding. Water makes a good shield for many forms of radiation. For lunar missions, it has long been suggested that burying a lunar colony might shield it from radiation. Another idea is to use electromagnetic shielding. After all, that is how Earth does it, right? So, perhaps it might be possible to build an electromagnetic shield that could deflect most of the harmful radiation. If so, then that would be the lightest and most cost effective way of shielding astronauts from hazardous cosmic radiation.
Radiation exposure is something that needs to be addressed before we can truly be serious about a mission to Mars or an extended stay on the Moon. But, at least there are people thinking about it.
Magnetosphere drawing courtesy NASA