Space Radiation and Humans
Published on Oct 21, 2009 at 2:25 pm.
2 Comments.
Filed under space exploration, space radiation.
As I continue my series on space radiation, the next topic that I want to address is how radiation harms space travelers. I’ve already alluded to this in earlier posts in the series, but I wanted to mention it again.
In an earlier installment, I said that radiation is a process where energy travels from one body to another in essentially straight lines. Since particles (and even light) can be deflected, that is not entirely correct, but it is close enough for our purposes. The radiation when absorbed can heat things, cause electrical currents, or if enough energy is absorbed by an individual atom, ionize that atom (remove an electron). Ions (charged atoms) behave different chemically than neutral atoms. So, ionizing radiation has potential to do more harm than non-ionizing radiation. In the human body, there are vast numbers of atoms and molecules. Most of the atoms are, in fact, part of molecules. It is the chemistry of how these molecules interact with one another, forming new molecules, releasing molecular energy, absorbing energy, etc, that is the basis of life. Modern biology is not just memorization of species classifications or anatomical terms. Rather, modern biologists are deep into organic chemistry and biochemistry. That is sometimes a shock for students in first year biology at my college. Students are surprised to find that their biology professor is talking about the laws of thermodynamics, electrical transport, chemical potentials, and a host of topics that used to be the domain of physicists and chemists. But, it is here, in the realm of molecular biology, that radiation does its damage: by upsetting the chemistry of life. Ionizing radiation can keep atoms and molecules from behaving as they should in a cell, or they can break molecular bonds, altering a molecule.
Most of the time, the damage done by radiation is simply altering a rather minor molecule. There are plenty of other molecules in the cell to take the place of the disrupted one. In rare cases, the altered molecule bonds to some other molecule(s) that it wasn’t supposed to, taking two or more molecules out of commission. But, there are so many molecules in the cell that this is normally of little importance. However, once in a while, the radiation damages a somewhat rarer and more important molecule. Sometimes an amino acid, a protein, or some other important molecule that has an important function in the cell is damaged. It doesn’t work right, and so the cell doesn’t behave properly. New proteins are being made though, so unless the cell is very unlucky, the damaged protein is eventually replaced by a correct one. But, on very rare occasions, the most important of all molecules in a cell, one of the cell’s strands of DNA (deoxyribonucleic acid) is damaged. DNA controls almost everything about how the cell works. When the DNA is damaged, then the cell no longer works properly. In most cases, that means that the cell dies. In some cases, the cell lives but no longer performs the function in the body that it is meant to perform. If such an altered cell reproduces, and its daughters reproduce, and so on, then a tumor can be formed. If the altered cells are particularly invasive and the body’s immune system doesn’t recognize them as a danger, then the altered cells develop into a cancer.
As I said in an earlier post of this series, some forms of radiation are more damaging than others. Some, such as X-rays, may damage one atom, and thus one tiny bit of one base nucleotide of the DNA. Other, more energetic radiation may damage a cluster of atoms, damaging more thane one base pair. The more damage is done, the more likely it will be that the cell will not function properly. In addition to direct damage to DNA, the radiation sometimes can alter some other molecule that can then damage the DNA.
But, biology can be amazing. DNA is very important to cell operation. As a consequence, it is not really surprising that there are biological mechanisms that are at work to limit damage to the DNA. The first of these is simply the structure of the DNA itself. DNA is described in textbooks as a “double helix.” What that means is that it is composed of two matched strands arranged in a helical pattern. By matched strands, each nucleotide on one strand is matched with one on the other strand. There are only four nucleotides: adenine, thymine, cytosine, and guanine. Adenine matches with thymine, and cytosine matches with guanine. So, if one of these molecules is damaged, then it is no longer the correct molecule. It doesn’t fit in the DNA. The cell has enzymes that will detect the miss-matched base pair and substitute the damaged nucleotide with the correct match (adenine, thymine, cytosine, or guanine) that corresponds with the other base nucleotide on the undamaged helical strand. This works wonders, and it happens all of the time in our bodies. The problem comes, though, if both nucleotides in a pair are damaged. In that case, the cellular repair mechanism has problems. It can either fill in the gap with random nucleotides, it can splice a fragment of DNA into the chromosome (viruses can do this), or it can simply spice the undamaged ends of the DNA back together, effectively eliminating whatever the damaged portion of the DNA was. In all cases, the cell will no longer work the way that it did before. That is why the heavier and higher energy particles in galactic cosmic rays are so dangerous.
But, during part of a cell’s cycle, particularly during mitosis when it is replicating, the DNA is more vulnerable to damage. At that time, as the DNA is replicating, damage is difficult to repair. Some of the DNA base pairs are involved in separating and forming new base pairs, and so the enzymes that repair damage are unable to find the correct match for base pairs. The damage thus often is not repaired properly. Cells that are in mitosis a lot are thus more easily damaged by radiation. In a normal human body, these include the hair follicles, the linings of the gut, and bone marrow cells. Thus, radiation exposure can cause someone to lose their hair, get nauseated, and become anemic. The anemia is often the first clinical symptom of extreme radiation exposure. If the exposure is at a low enough level, the nausea may not present, but the anemia will often still occur. Low level chronic exposure often causes the hair to gray rather than completely fall out. Incidentally, cancer cells are often multiplying out of control (which is why they are invasive and eventually get in the way of the body functioning, thus killing the patient), so they are also susceptible to radiation damage. That is how radiation therapy works with cancer patients.
So, the effects of radiation occur at the molecular level. Any sort of realistic strategy for protecting astronauts on long duration space missions requires an understanding of radiation at this level. Unfortunately for astronauts, cosmic rays, particularly the galactic cosmic rays, contain some of the most damaging forms of radiation possible. Thus, very low doses of cosmic rays can act like much larger doses of other forms of radiation. It is quite fortunate that such radiation occurs in such low levels in space that astronauts can be exposed to it for many months before the risk of cancer due to the radiation is significant. But, as I said before, long duration missions, such as an extended stay on the Moon or a mission to Mars, could easily reach or exceed NASA imposed limits on cancer risk. But, even an extended mission to Mars would not expose astronauts to a clinically lethal dosage of radiation. Rather, it may create some health issues and would most certainly create a risk for development of cancer that exceeds the level to which NASA is willing to subject astronauts. Thus, some strategy must be developed to protect astronauts from the danger of space radiation.
Researchers at NASA, in industry, in the military, and in universities are working on the problem of dealing with radiation. The simplest solution is to shield against the radiation. Most of the time, such shielding is accomplished by putting enough absorbing material between the source of the radiation and the person being shielded from the radiation. Sometimes, though, that is not practical. After all, the shielding can be expensive, and it is often very heavy. For aircraft or spacecraft, heavy shielding is not realistic. That means that some other strategy is needed. One possibility is to simply move some of the spacecraft systems so that they shield the occupants. Water turns out to be a good radiation shield. So, putting the water storage and waste water handing systems surrounding the crew compartment provides some shielding. But, there is only so much water carried on board a spacecraft, so water shielding can not be the sole solution. Another idea is to perhaps use some other form of shielding than just a physical shield that absorbs the radiation. One strategy being tossed around is an electromagnetic shield that would deflect the radiation. At present, though, electromagnetic shields exist primarily just on paper. Another strategy, though, is biological. The body does have repair mechanisms for DNA. So, perhaps there is some sort of biological approach that may protect astronauts. This could be in the form of drugs that limit the damage from radiation or that boost the cellular repair mechanisms. Some progress has been made on this front. Perhaps the best solution for an extended space mission would be a combination of different types of shielding and drugs. While current technology and spacecraft construction do not provide much radiation shielding for astronauts, it appears that real progress is being made to come up with strategies for protecting astronauts from radiation exposure.
-Astroprof
Image Credit: NASA
Peta Pendlebury on October 22, 2009 at 6:51 am: 1
Hi
Very interesting article. I am researching radiation effects at a lower level - that experienced by frequent flyers - and the techniques that could be used to protect the body. Your article is the first I have come across that mentions that this could be possible - something that I know from my field is certainly possible. Some people have more ability to protect themselves from radiation and this ability can be improved. I use techniques from my field and external protection from boosting nutritional status - as you state - the protection is at the molecular level and without the basic building blocks of vitamins/minerals/etc our DNA is much more vulnerable. Thanks again - article going in my research file! Peta
SEO Seattle on November 22, 2009 at 9:10 am: 2
Your posts on radiation have been really interesting. I might even be able to talk about it somewhat intelligently now.