Cosmic Rays
Published on Oct 18, 2009 at 4:01 pm.
4 Comments.
Filed under astronomy, space radiation.
Part 3 of my series on space radiation is about the radiation itself. The term cosmic rays is generally used to describe this radiation. Unfortunately, as often happens, the term evolved before the nature of the phenomenon being studied was known, so it may not be the best term to use. Nonetheless, we are stuck with it. In fact, the term has historically been used to describe more than one phenomenon.
About a century ago, when physicists were first seriously studying radiation and seeking to know the nature of this phenomenon, these researchers noticed that even when all known sources of radiation were removed from the vicinity of a radiation detection device that device still recorded a low level of radioactivity. Eventually, researchers began to realize that radioactive elements and isotopes were all around them. Even the materials used to shield against radiation, such as concrete blocks or lead plates, contained some radioactive isotopes. Some researches then began to wonder if air itself might contain a very small level of radioactivity. To isolate the radiation of just the air and not terrestrial surroundings, researchers put radiation detectors in balloons. As expected, the background radiation level dropped in the balloon flights at low altitudes. However, as the balloons went higher, the radiation level began to increase with altitude. Victor Hess won the 1936 Nobel Prize in physics for his work in radiation studies, including his explanation for this mysterious increase in radiation with altitude: cosmic radiation. In short, Hess realized that the source of some of the background radiation was from above, having its origins in space. At first, the nature of this cosmic radiation was unknown, only that it originated from beyond the Earth. Today, we know a lot more about cosmic radiation.
As it turns out, much of what Hess and others in his day were measuring was not the radiation that astronauts would experience. In fact, they did not even measure what we now think of as cosmic rays. What they measured were secondary cosmic radiation or particle showers created by the space radiation’s impact with the upper atmosphere. This is the radiation that airline passengers and crew are exposed to. I’ve written about that before. What I want to concentrate on this time is the radiation in space itself.
Remember, from my previous posting, the term radiation actually applies to a wide range of phenomena. All of these different uses of the term are at work in space, so there are multiple sources of radiation. Not only are there multiple sources of radiation, but there are also multiple kinds of radiation and multiple different ranges of energy for that radiation. This makes any kind of effective shield against the radiation for astronauts problematic. However, being difficult to shield against does not mean being impossible to shield against. It is an engineering problem, albeit a rather serious one. Given sufficient time and effort, a solution is likely possible. That will be the topic in a subsequent posting. Now, I wish to talk about the different types of radiation that are in space.
First, I’d like to mention that space radiation can fall into two major categories: electromagnetic radiation and particle radiation. The electromagnetic radiation consists of the whole range of the electromagnetic spectrum, from gamma rays to radio waves. The shorter wavelength (and higher frequency) radiation such as gamma rays and X-rays are considered ionizing radiation, and that is what is a worry to astronauts. There are many different sources of this radiation, just as there are on Earth. However, there are some processes that create ultra-high energy gamma rays. Some of these gamma rays are produced in astrophysical processes far from the Solar System. Gamma ray bursts have been recorded by satellites since the early days of spaceflight. Other ultra-high energy gamma rays can result from interactions of some of the ultra-high energy particles with the Earth’s upper atmosphere. These gamma rays are one type of secondary cosmic rays. As a rule, gamma rays of that energy level are seldom (if ever) produced on Earth in any natural terrestrial process. So, when I was growing up, charts of the electromagnetic spectrum often labeled this ultra-high energy electromagnetic radiation “cosmic rays.” That is how I learned it. Years later, when I was in college, electromagnetic spectrum charts no longer had that label. That ultra-high energy radiation is now included in the overall category of “gamma rays,” unless you look at a old chart or one prepared by someone not familiar with the current usage of the terminology.
Most of what we consider “cosmic rays,” however, rather than being electromagnetic radiation, are high energy particles. Many of these particles are charged particles. There are a few neutral particles in the mix, but they are a very tiny percentage of the cosmic rays. There are three basic sources of cosmic rays. Some are particles originating from the Sun. Others originate from beyond the solar system. And some are believed to originate at the boundary between the solar system and the interstellar medium.
The first that I’ll talk about are the solar cosmic rays. There are always particles streaming away from the Sun. This is the solar wind. Most of these particles are protons (hydrogen nuclei) and electrons. These particles are moving at several hundred kilometers per second. The energy is low enough that ordinary materials of the spacecraft would be effective shielding from the particles themselves. Some of these particles, though, would be problematic for astronauts in spacesuits. However, when charged particles rapidly decelerate, as they would do when they hit the spacecraft, they emit radiation called bremsstrahlung radiation (braking radiation). For most of the solar wind particles, this bremsstrahlung radiation would be soft X-rays — fairly easy to shield against. However, the real danger lies in solar storms. During these storms, generated by a massive explosive release of magnetic energy in the Sun due to a solar flare event, particles can be blasted from the Sun at far higher energies. The radiation storms generated by solar flares can be far more dangerous to astronauts outside of Earth’s magnetosphere. In fact, astronauts exposed outside of a spacecraft would not be sufficient protected by any reasonable spacesuit, and they may experience a lethal dose of radiation during a severe solar radiation storm. Even astronauts inside a spacecraft would experience dangerously high levels of radiation. These solar storms are difficult to predict far in advance. Solar scientists are getting better, but we still don’t do much better than a few days of warning that conditions are right for such storms. That is not sufficient to protect astronauts on an extended mission.
The good news, though, for astronauts in low Earth orbit, though, is that the Earth’s magnetosphere deflects the brunt of these solar radiation storms. However, the blast of particles from the Sun does shift the magnetosphere quite a bit. This has the effect of accelerating charged particles trapped in the magnetosphere (such as in the van Allen radiation belts). These now fast moving particles then cause auroral displays when they crash into Earth’s atmosphere and can pose some radiation problems for astronauts in Earth orbit. For a spacecraft that could be a problem. However, long duration missions, such as on a space station, astronauts will have a lot of water on board. This water makes a decent shield against the trapped particles, thus providing some shielding for astronauts on board the space station during a geomagnetic radiation storm. Still, astronauts will unavoidably be exposed to excess levels of radiation during such an event. Nonetheless, inside of Earth’s magnetosphere, astronauts inside a spacecraft should be sufficiently shielded so that they will not suffer any immediate health problems.
The most important thing about solar radiation storms is that they don’t last long. It may be too expensive to shield a spacecraft outside of Earth’s magnetosphere completely from solar radiation, but it may be possible to make small shielded areas where astronauts can retreat to in the event of a radiation storm. They’d just have to wait out the storm in what may be an uncomfortable setting. But, the storm will subside. A greater threat, though, are the higher energy cosmic rays that originate outside of the solar system. These are much tougher to shield against, and they provide a steady background level of radiation.
The cosmic rays that Hess discovered were largely the byproducts of what we now call galactic cosmic rays. These are extremely high energy particles, most moving at near the speed of light. At such high velocities, when they slam into atoms near the top of Earth’s atmosphere, they create a cascade of subatomic particles that rain down towards Earth. Some of these particles make it to the ground, and others slam into other things creating yet more particles to rain downward. A single high energy cosmic ray can thus create an incredible number of secondary particles that create a shower of particles across hundreds or thousands of square meters on the surface of the Earth. Not all of the particles make it all the way to the ground, so the higher one goes into the atmosphere, the more of these secondary particles that you encounter. That is what airline passengers and crew experience: secondary cosmic rays. Cosmic rays of this energy striking the wall of a spacecraft would likewise create a shower of particles that would hit astronauts inside the spacecraft. Even some of the primary cosmic rays would pass through the wall of the spacecraft to hit the astronauts directly, creating a shower of particles inside the astronaut. As you can see, these primary galactic cosmic rays can be incredibly dangerous. About 89 to 90% of the galactic cosmic rays are high energy protons. About 9 to 10% are helium nuclei. About 1% of the particles are heavier nuclei. High energy electrons are also likely generated in the mechanism that produces galactic cosmic rays, but electrons are much lighter than atomic nuclei and are thus easier to deflect and slow down. The nucleus of practically any fairly stable known element in the periodic table has been found in cosmic rays, though nuclei of elements heavier than iron are rare. The heavier the cosmic ray particle, the more damage it typically does when it hits something.
Even very low levels of galactic cosmic radiation can act like much higher levels of lower energy radiation in terms of biological damage. Worse, how do you shield against these things? A fairly dense and heavy (and thus expensive to launch) shield on a spacecraft would slow the particles, but create bremsstralung radiation of very high energy: hard X-rays and gamma rays. Any effective physical shield would be too heavy to launch. To date, the most effect way of dealing with such high energy radiation is to simply limit exposure. If you are exposed to such radiation for short periods of time, then it would be manageable. NASA regulations limit astronaut radiation exposure to a level that is estimated to subject the astronauts to no more than about a 3% risk of cancer induced by the radiation exposure. Since we don’t really know the exact relative biological effect for this radiation, then we don’t really know how long of an exposure that might be. Estimates range from as short as 100 days exposure to as long at 300 days, but most falling in the 200 day exposure range. That is a little over six months exposure to galactic cosmic rays. That is far longer than the Apollo missions to the Moon, so they were not at great risk due to cosmic rays. However, NASA is talking about long duration missions to the Moon to man a permanent or semi-permanent base there. Those missions would last 4 to 6 months. That puts the length of the proposed missions right in the middle of the exposure limit range. A single mission would thus expose astronauts to their entire career maximum radiation limit, excluding any radiation from solar storms. Worse, a mission to Mars would take 7 to 8 months just to get to Mars, and nearly 3 years round trip (Astronauts cannot simply come back from Mars whenever they want. They have to wait until the planets are aligned in such a way that the return trip is possible. That forces them to wail over a year at Mars until Earth and Mars are back in the proper configuration, making any possible round trip almost 3 years in length). So, just the trip to Mars would exceed the 3% cancer risk that is the NASA imposed limit to astronaut radiation exposure. Then, you have the trip back. And, the thin atmosphere of Mars would make the surface of Mars experience a radiation exposure from secondary cosmic rays similar to what high altitude aircraft experience on Earth, which is near the peak level of secondary radiation. Thus, astronauts on a mission to Mars would experience a significant health risk, perhaps in excess of a 10% risk of cancer, far in excess to NASA’s radiation limits. But, this is also a limit for astronauts working anywhere in space, even on the Moon or in Earth orbit.
Incidentally, you might wonder why Earth’s magnetosphere protects us from galactic cosmic rays. Well, it does, a little. When charged particles move in a magnetic field, they are deflected. This tends to make them move in circular or helical paths. The radius of the circular motion is dependent upon the velocity of the particles and their mass. So the high energy (speed) and mass of the galactic cosmic ray particles makes their circular orbits so large that they still run into Earth. Ironically, until sufficient studies had been done by unmanned spacecraft above Earth’s atmosphere, some scientists had speculated that the trend of increase in radiation levels with altitude observed in balloon and sounding rocket experiments might continue. Had this trend in radiation levels indeed continued, then the environment of outer space would have been far too high in radiation levels for manned spaceflight to even be possible. Fortunately, that trend does not hold. Once you get above the level of peak secondary radiation, then the radiation levels begin to drop to background level that is manageable (though still much higher than experienced on the surface of Earth).
The third type of cosmic rays are what we call anomalous cosmic rays. These anomalous cosmic rays are of mid-energy range between the solar particles and the galactic cosmic rays. The origin of the anomalous cosmic rays are a bit of a mystery, but they are believed to come from the boundary region between the solar system and the interstellar medium, a region known as the heliomagnetosheath, of the heliosphere’s magnetosheath. This is where the solar wind particles are slowing down and bunching up and where the interstellar medium particles are being deflected as they pass by the solar system. It is the edge of the Sun’s magnetic influence. It has been suggested that perhaps particles trapped in the magnetosheath get buffeted and accelerated to high enough energies to escape this region and become the anomalous cosmic rays, but we don’t know that for sure. They are mostly protons.
So, that is a brief primer on cosmic rays. Next, I’d like to talk about how radiation does damage to biological systems, but that will be the topic of my next post.
-Astroprof
Clark on October 18, 2009 at 7:58 pm: 1
Thanks for posting this tutorial series on space radiation. The topic will be of increasing importance as more and more people spend more and more time in space.
I believe you are a bit too pessimistic about shielding spacecraft against CR. A habitat for a deep space mission does not have to follow the standard Apollo capsule/ISS module approach with thin metallic walls. A design optimized for rad shielding would wrap water and fuel tanks, food containers, waste storage, etc. around the living areas. As you are well aware, hydrogen rich materials like water provide excellent rad shielding.
Furthermore, I don’t see any reason the solar storm room can not be the sleeping quarters the rest of the time (assuming the crews are running round-the-clock shifts). That would cut rad exposure significantly for a third of each crew member’s day.
I’ll note that the inflatable modules being built by Bigelow use multiple layers of Kevlar-like materials that handle micro-meteorites as well or better than the hard shell type of modules while producing fewer hard secondaries. The Bigelow team has put considerable effort into rad shielding. My impression is that NASA is currently not putting enough emphasis on rad safety for its deep space mission plans.
There are a lot of variables in rad design, e.g. maybe a layered approach of alternating dense metal and hydrogen rich materials would be best. (i.e. the CRs interact in the metal and the secondaries are absorbed by the intervening materials.) I would suggest a Red/Blue team framework in which the Blue team comes up with designs to minimize rad exposure and the Red team runs particle transport simulations on those designs to determine the crew exposures.
These techniques, of course, won’t reduce exposures to earth ground levels but they might result in cumulative doses on, say, a Mars mission low enough to make it feasible health-wise.
Astroprof on October 19, 2009 at 9:39 am: 2
Clark,
Thanks for you comment. I didn’t mean to sound so pessimistic about cosmic ray shielding. I am going to be getting to more shielding options in a later installment. As you say, we can’t continue to use metal walls as shielding. We need something else. Some promising work is being done, but there needs to be more work.
James on January 4, 2010 at 10:58 am: 3
Curious, if water provides good shielding, would a liquid, frozen, or gas form of water all be equally protective?
Rob on July 2, 2011 at 10:16 pm: 4
Ionic shielding is under investigation in England for shielding astronauts from Alpha and Beta radiation by creating a pocket free from the radiation. Gamma radiation is the only issue for deep space travel. Gamma radiation seems like it could in theory (my own) have a matching frequency of energy to deflect this energy but I am not physics scientist nor a student. I know that gamma radiation doesn’t have mass or electrical charge but could pure energy deflect pure energy?