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Sunday, 1 June 2014

ISU Individual Project: Galactic Cosmic Radiation Review

Hello again, and welcome to another exciting adventure! Today, I'm going to be telling you all about my research into galactic cosmic radiation, and what its existence means for the safety of astronauts. This post discusses the work and findings of my individual project at the ISU, so I hope you learn a lot, and enjoy!

My individual project for my Masters of Space Studies at the International Space University consisted of a literature review of galactic cosmic radiation. This radiation, shortened to GCR, is one of two types of ionizing radiation which poses a danger for astronauts. The other, widely known type is classified as solar event particles (SEP), and originates from the Sun, released during “solar storms”, flares, and coronal mass ejections, and is what gave the Fantastic Four their powers.

While there has been much study into these SEPs, I had not come across much on GCR, and what I had found was fragmented and scattered. I wanted to learn more.

Scouring the literature, I sought to understand the characteristics, biological effects, and shielding practices of GCR. The characteristics would help me understand what GCR is, exactly, and the biological effects would help me determine how much of a problem it posed. Finally, if it did pose a problem, I wanted to know how we are protecting astronauts, and what could be improved.

I learned a lot during this project, creating a 15 minute presentation, which can be viewed here, and a 35 page report. While the details are made in more detail there, I will share a summary of my findings here.

Galactic Cosmic Radiation consists of particles with extremely high energies, coming from outside the solar system.

It is composed of:
  • 85% hydrogen atoms (and protons),
  • 10-14% helium atoms (and electrons) and,
  • 1% heavier atoms (and other sub-atomic particles like neutrons).
These particles have energies ranging from 10 MeV-100 GeV. To compare, SEPs normally have energies of 100 keV-1 GeV, but solar storms hit harder than the slow trickle that is GCR in the solar system.

Why are we not concerned here on Earth?
The Earth’s magnetic field, and atmosphere, provide shielding against incoming GCR. The magnetic field can deflect incoming particles, causing them to miss the Earth, or astronauts in low-Earth orbit (LEO). Earth’s atmosphere causes the particles to interact/impact well before hitting us, thereby losing their energy and danger. This is why we are not concerned with GCR exposure on the Earth, and even the astronauts working on the International Space Station (ISS) are somewhat protected.

Does the amount of GCR change over time?
Yes. An interesting relationship exists between potential GCR exposure and solar activity. The Sun has a solar cycle, a periodic shift in the amount of energy created/deposited throughout the solar system, repeating every 11 years. As the Sun enters solar maximum, when its energy production and deposition is highest, (during the 1989 solar maximum for example) the measured GCR is lower. It is assumed that the Sun’s activity blocks the incoming galactic cosmic radiation. It is a fascinating consequence that if you want to encounter the least amount of GCR, you should travel in space at a time when solar flares are more likely to occur. This makes space mission design that much more complicated.

Where was most of my data coming from?
Now that I understood these characteristics, I wanted to understand how much GCR existed in different environments. I read, compared, and analyzed different articles and databases outlining the radiation exposure of astronauts on different missions. Interestingly, most of the literature I could find was published in the United States, and most of this came from NASA’s Johnson Space Center. I was excited by this as my internship this summer involves working there! (More on this in later posts) A full list of my references can be found here and a great website to consult is here.

How much GCR exists in LEO, on the Moon, and on Mars?
I collected and reviewed the information on astronauts in LEO from Mercury missions (1959) through to ISS missions in the late 1990s. For the Moon, I examined data from the Lunar Reconnaissance Orbiter’s “CRaTER” mission, averaging it over 4 years. On Mars, I compared data from the Mars Odyssey orbiter, and the Mars Curiosity probe. I was very excited in January when data came out from Mars Curiosity! It felt very exciting to be using such new data, being on the frontier of science!

I had to do some work in order to convert all the data into comparable units and measurements. The first unit I looked at was absorbed dose. Measured in Grays, it is a measure of the energy imparted by radiation on an absorbing material. It is a physical property, not necessarily a measure of risk, as other factors must be considered.

The results can be seen in the table below. As can be seen, GCR exposure was higher for the Apollo astronauts than astronauts confined within LEO, which makes sense, as the Apollo astronauts were outside of the shielding effects mentioned earlier. On the Moon itself, GCR exposure is quite high, since the Moon doesn’t benefit from an atmosphere or magnetic field. Mars does not have a magnetic field, but it does have an atmosphere, providing some evidence of shielding as seen when comparing Mars atmosphere and surface measurements.

For those more interested in music than numbers, another interesting feature is “Crater Live Radio”. The CRaTER instrument consists of 6 different sensors, all measuring radiation strikes, and the University of New Hampshire is converting these strikes into music, as seen here. (Best links are in the bottom right of the page).

I incorporated the music into my presentation, with interesting results. When I moved to a certain presentation slide, the music started playing and before I could get a chance to explain it, everyone started checking their phones, worrying that they were interrupting my presentation. It was very amusing, and I smoothly explained the source and meaning of the music, garnering a few laughs of relief.

Biological Effects

How does radiation cause damage to the human body?
The fundamental principle of damage caused by radiation can be imagined as a game of pool/billiards.
The initial impact of particles on a surface, like your skin for example, causes damage of course, but it also dislodges other particles which travel in many directions continuing the process, creating secondary radiation. As the particles travel, they deposit energy which increases the energy of nearby particles. Finally, the particle comes to a stop, after having left a trail of destruction in its wake.

What do we know about different particles and how they affect us?
So far, we have excellent knowledge of the damage caused by smaller particles, such as hydrogen and helium atoms, but we have less concerning the impact of larger atoms, such as silicon and iron. Hydrogen leaves a small, scattered trail, and when compared to a typical mammalian cell, might not be that damaging. However, GCR is composed of heavier nuclei as well, leaving greater tracks of damage, and while iron atoms might be rare in GCR, longer duration missions will encounter them more often.

Where does our understanding of radiation damage come from?
Our understanding of the dangers of ionizing radiation comes mostly from industries and sources here on Earth. Nuclear power production, nuclear accidents, tests, attacks, and radiotherapy have taught us much about how the human body reacts to high energy radiation. However, these sources are all very different from GCR. The first noticeable difference is the energy. GCR energy is incredibly high, but its dosage is low, meaning that its effect is more cumulative than the acute effects of a nuclear weapon. The biological effects of these sources is plenty, and well-known, including: the development of cancer, cardiac/circulatory, and digestive diseases, cataracts, DNA strand/tissue degradation, among others. Owing to the limited exposure, the only symptom to manifest because of space flight is cataracts.

What are astronaut radiation exposure career limits?
The industries mentioned above helped the National Council on Radiation Protection (NCRP) make recommendations for the amount of radiation which could be tolerated by the human body in different environments. These recommendations helped NASA form its annual and career radiation exposure limits.

(NASA, 2008)
How do the limits differ for age/sex, and why?
Interestingly, the limits are different depending on both sex and age. The data on female astronauts and radiation workers is considerably less so the limits are more conservative for them. Additionally, and I’m only speaking statistically, women generally have a higher percentage of body fat, which is more susceptible to radiation damage. Also, since many of the symptoms mentioned above are thought to take a long time to develop, older astronauts have higher exposure limits. These limits are expressed in a unit known as Sieverts, which will be explained in a moment.

Why don’t we know more about damage due to GCR exposure?
Broadening our  understand of GCR exposure is not easily done. We are shielded, mostly, on Earth and in LEO (where all current astronaut activity takes place), and we have nothing on Earth to compare with or test. While equipment is being developed to test this radiation exposure for different materials and electronics, testing on humans is obviously unethical. Testing on animals is ethically easier to accomplish, but there are limits to this as well.

What are the risks for different space missions?
As of 2006, there had been significant research into the risks of human spaceflight, especially for deep space, longer duration missions. The amount of predicted exposure can be seen here, along with something known as an equivalent dose, measured in Sieverts, and chances of radiation exposure induced death, or REID, as it is called.
(Cucinotta, 2006)
The equivalent dose is normally used to outline the risk of exposure. It considers the absorbed dose, then multiplies by a factor based on the type of exposure, the geometry of the situation, the type of particle, and the duration, giving an equivalent dose value. The astronaut career limits, as you may recall, were given in Sieverts.

Now, these estimates were from 2006, before the LRO on the Moon, or Curiosity on Mars.
When I compared the data from these two more recent probes, and extrapolated for the missions outlined in the previous chart, I found that the predictions had underestimated the risks in some cases. The Lunar Base mission assumes a 6 month continuous stay on the Moon and, as you can see here, the estimated dose is lower than the total dose.

Additionally, the Mars Return mission, based on a 200 day trip to/from the planet, and 500 days on the surface, would yield a total dose higher than the estimation.

The difference is small, however, these estimates were intended to be the upper limit of expected exposure. Discovering that the actual dose would be higher is a serious warning to agencies wishing to design for, and conduct, such missions.

Oddly, the Mars Fly-By estimate was much higher than the total dose, as measured by Curiosity’s RAD on the way to Mars, however data revealing the process behind this estimation was limited.

What about a mission to Mars?
Human missions to Mars  has been a hot topic for quite some time, and was the focus of many of us at the ISU this year, as will be discussed in a later post. Mars One is looking to send humans to Mars in 2025, there to live out their lives. However, I personally believe that the risks are still too high to allow missions to Mars.
(Hassier et al., 2014)
When you compare all the data, it can be seen that the exposure due to 500 days on Mars, is roughly the same as the transit to the Red Planet, which is higher than 6 months on the ISS.

The total exposure due to a Mars return mission would be 10 times greater than our current limit of understanding due to ISS missions, and would be similar to receiving an abdominal CT scan every 5 days here on Earth for 900 days.

More work must be done to address this issue if we’re going to push out into this final frontier, and explore strange new worlds.


Can we shield against this radiation?
Perhaps. The definite answer is still unknown. There are two types of radiation shielding: passive and active, both were analyzed in my work.

What is passive shielding?
Passive shielding involves using material to simply block incoming radiation. On Earth, it is relatively easy to provide thick shielding, but in space, where mass is often the currency of space missions, it is not so easy.
(Rapp, 2006)
The above chart illustrates the equivalent dose to be expected from GCR, compared with the density of the material used to block it. The lines represent various materials, as it has been seen that different materials yield a different effect.

Aluminum, the most common of materials used, does not have a great shielding effect. Even increasing its density to 50 g/cm2 would not reduce the radiation levels as significantly as using another material.
Lithium hydride is seen as the best, in theory, and it is used in shielding nuclear power plants, however, it is highly reactive to air and water, and may be difficult to machine/design for human spaceflight habitats.

Water is often suggested, but as seen, it is not the best, nor would it be good to irradiate your water.
One major problem is that the energy level of GCR is so high, that the literature suggests that no material will be able to provide adequate protection. In fact, there is some evidence that some materials cause more radiation than they block, through secondary radiation as I described earlier. Further work must be conducted to test these materials.

What is active shielding?
Active radiation shielding is a long-standing concept, but only recently has it begun to be developed. Seen in science fiction, it is the use of electromagnetic fields to deflect or re-direct radiation, and has been seen in franchises such as Star Trek, known as deflectors.

The concept had been envisioned back in the 1960s, however electronics technology and design were inadequate to handle the requirements of such a system. This graphic here depicts the equivalent dose of radiation versus a typical material shielding thickness.
(Westover et al., 2012)
The lines represent different electromagnetic field strengths, starting from 0 Tesla, all the way to 3 Tesla. As shown, the stronger the field, the greater the ability to deflect incoming radiative particles, and thus, the lower the equivalent dose. Such a shield may be complicated in design and implementation but may offer a solution which material shielding currently lacks.

There are currently two groups seriously looking at active radiation shielding. One is the Space Radiation Superconducting Shield, SR2S, part of the European Union’s 7th Framework Programme. It is being conducted by several companies in partnership with the Italian Space Agency. Surrounding a toroidal habitat with rings of superconducting material, the group hopes to generate electromagnetic fields which combine and enhance each other.

The work is made possible by new innovations in superconducting material production. I attended their workshop in Italy last month, and it seems that they are preparing to move past the theoretical, currently designing the thermal control for such a system.

The other group working on active shielding is NASA’s Innovative Advanced Concepts laboratory. Having worked with SR2S in the initial development, NASA is trying a slightly different design; a long cylindrical system with 6 similarly shaped coils surrounding it. They have chosen this design for mostly mechanical reasons, stating that it will put less stress on the habitat, and be easier to thermally and electrically control. My internship this summer will be with this group, and I look forward to learning more, first hand.

Such technologies are obviously under-developed. They involve complicated mechanical, electrical, and thermal engineering, and while it is prudent to begin planning for the future, these projects will be difficult to fund and support until human deep space missions become a reality.

This project was very valuable to me, and I hope valuable to anyone interested in learning more about this issue. I want to make spaceflight easier and safer and so I have been devoting my time and research to the dangers and engineering involved with sending humans into space. Radiation is a particular threat because its effects are both short and long term. For the most part, space engineering practices have concerned themselves with the short term, SEP/solar flare, effects. This is because these effects are more obvious, and more immediately dangerous. However, as humans push farther into space, for longer duration missions, the cumulative effect of low dose, but continuous and high energy galactic cosmic radiation could cause various health issues and affect the mission.

The intent behind my research was to generate an understanding of what we knew of this issue and how we were dealing with it. I wanted to completely “catch up”, as it were, on the history and practices used by space agencies so that I would be familiar with them and perhaps be able to improve on them.

My findings? I now understand GCR quite well. I know its characteristics, properties, how it changes over time, and I know what humanity knows about its dangers due to exposure and how we can shield against it. I know that the energy of GCR is so high that current materials and practices do little to stop it. I know that other materials may help, but there may be an upper limit to how well current materials can shield against this radiation. I also am excited by the knowledge that science-fiction-sounding ideas, such as the use of electromagnetic shields to block radiation, are being developed.

My presentation was well received, and I was happy that it was well-attended. It marked both an end and a beginning, as I was finishing one project, but hoping to begin more research based on what I have learned. I enjoyed becoming known as “the radiation guy” at our school, the one people would ask concerning the dangers of space radiation. I submitted my abstract to the International Astronomical Conference which is taking place in September, in Toronto, and I was accepted! So, I'll be presenting my findings and research there as well!

This summer, I shall be working on an active radiation shielding project at NASA’s Johnson Space Center and I couldn’t be happier!  I’m looking forward to learning everything I can about NASA and the work involved. I’m awed by the chance to work with NASA, a name and organization which carries a lot of weight in this community, and within my heart as well. And I’m glad to be taking another big step into the future.

The next posts should cover my team project, namely, one-way missions to Mars, as well as my transition from the ISU to Houston, Texas. I am working to improve the rate at which I write blog posts, but either way, I hope you found this enjoyable, pedagogic, and engaging.

I thank you very much for your time, as always.

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