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Space Radiation (Part I)

Published on Oct 6, 2009 at 10:36 am. 1 Comment.
Filed under cosmic rays, space exploration, space radiation.

This past weekend, I attended a symposium on space radiation. This is an interesting topic, and I’ve written about it before. But, space radiation is not confined to space. Cosmic rays raining down on Earth also create secondary radiation that impacts air travelers. I have received a number of comments on several of my posts over time regarding radiation in space, so I thought that it was time to revisit the topic. Further, I thought that it might be interesting to stretch this into a series of several posts. So, this is the first of that series.

First of all, there are many different types of radiation in space. A description of that will be in my next posting, along with a description of radiation in general. This radiation can cause physical and biological damage. Engineers can design systems resilient to physical damage from radiation in space. But, the problem is in the biological component of manned spaceflight: humans. The radiation induces damage at the sub-cellular level, in the very chromosomes of the astronauts. The cells then seek to repair the damage. The design of the cells, though is amazing. In many cases, if the damage is not too severe, the damage can, indeed, be repaired. In that case, life goes on, unaffected by the radiation. However, sometimes the damage is too severe, or it comes at an inopportune time in the life cycle of the cell, and it can not be repaired. If the cells are unable to repair the damage, the cell normally dies. Sometimes a partial, or a mistaken repair happens. Usually that, too, is fatal for the cell. Once in a while, though, the cell continues to live, but in a mutant form that no longer behaves like it is supposed to behave in the body. If the cell continues to grow and divide, then a tumor develops. This can lead to the disease that we know as cancer. The human body, though, is amazing in how its own immune system often recognizes cancers and attacks them. In the vast majority of cases, this system works. Once in a while, though, something gets past the immune system, and the patient develops cancer. That cancer often progresses until the patient dies. This is the danger and the worry of space radiation for both astronauts and passengers and crew of high flying aircraft.

By policy, astronauts are limited to no more radiation exposure than that which would be expected to yield a 3% chance of cancer mortality. Now, a 3% chance of death by cancer might seem unacceptably high for the general population, but there are plenty of people who would risk this for a chance to fly in space. In fact, there are many people who would risk a far higher chance of cancer to be an astronaut. The radiation level in space varies with the type of mission, but it always remains fairly tolerable for short duration missions. The radiation absorbed dose is computed by multiplying the radiation level by the time of exposure. Not all radiation has the same effect, so you next have to multiply by a relative biological effect for the particular type of radiation (which can even be different for different body parts). I’ll discuss this in more detail in a later installment of this series.

There are several sources of radiation in space. These different sources result in different radiation levels, and the depend upon a number of factors. The particular radiation exposure is a combination of the mission type (where a spacecraft is going), the duration of the mission, and external factors such as solar activity. That makes predicting radiation exposure complicated, particularly since we lack some data on radiation from certain sources. Again, I intend to discuss this more in a future installment of this series.

We have a lot of data on radiation exposure on Earth, but not so much on radiation exposure in space. We know that the very high energy galactic cosmic rays do different things than the lower energy radiation typical on Earth. So, estimating the risk to astronauts is difficult. There are several different ways of doing this sort of estimation, and they yield different results. Some estimates are that the 3% mortality tolerance may be reached in as few as 100 days outside of Earth’s protective magnetosphere. Other estimates put the danger level at over 200 days. The range comes from the fact that different factors are taken into account (sex of the subject, type of mission, etc). This is quite a wide range of estimates. To be on the safe side, you’d want to keep to the lower end of the estimate. Unfortunately, that level is reached in as few as three months in some cases. Even for the longer estimates, a six month tour of duty on a lunar outpost would reach or exceed safety limits. Even the Earth’s magnetosphere, while protecting astronauts, does not give complete protection. Three tours of duty on the International Space Station might reach the lifetime maximum radiation exposure for astronauts. All of these estimates are for galactic cosmic rays. The Sun also sometimes spits out particles in radiation storms. That complicates the matter.

So, we’ve got space radiation. What do we do about it? On Earth, there are three ways of dealing with radiation. First, you can leave the area of the radiation. That is actually the best solution. But, sometimes you can’t do that. For example, if you work with radioactive materials (such as with a nuclear reactor), then you can’t really get away from the radiation. Likewise, unless you decide simply not to fly in space or in an airplane, then you can not get away from the space radiation. So, in comes the second strategy to deal with radiation: shield against it. With many stationary radioactive sources, this is possible. For example, workers can operate a reactor from behind the safety of a heavy shield. But sometimes, that strategy fails. Once in a while, you simply have to go into the radioactive area to do something. The shielding is too heavy to wear, so workers will simply get exposed to radiation. But, shielding being heavy is another problem for those in vehicles. Radioactive materials can be transported on the ground inside heavy shielded containers by large trucks, but it is much more difficulty to shield an aircraft or spacecraft. Traditional shielding is simply not possible. The craft would be too heavy to get off of the ground. The final strategy for dealing with radiation is to use biological means of protection. There are some drugs that provide some protection to limited radiation exposure. There are different strategies for how these drugs operate. Unfortunately, most of them are pretty toxic, and the side effects are nearly as unpleasant as the radiation exposure itself. Work is progressing, though, on using drugs to combat radiation exposure. I’ll talk about radiation countermeasures in an upcoming post in this series.

I still have plenty of other duties this semester, and some days are pretty booked. So, I won’t be able to post every day, but I’ll try to get several posts per week to complete this series before the month is out.

-Astroprof