Radiation and Long-Term
Space Flight

The biological effects of heavy particle ionizing radiation are approximately proportional to what is called Absorbed Dose (or simply dose). This is measured with instruments which detect the average energy deposited inside a small test volume.

The unit of dose is the gray (abbreviated Gy) which represents the absorbtion of an average of one joule of energy per kilogram of mass in the target material. This new unit has officially replaced the rad, an older unit (but still seen a lot in the radiation literature). One gray equals 100 rads. Absorbed Dose was originally measured for x-rays and gamma radiation but has been extended to describe protons and HZE particles. When used in predicting biological damage, a further distinction must be made as to the "quality" of the radiation. >More on definitions

Although the Absorbed Dose of of some radiation may be measured, another level of consideration must be made before the biological effects of this radiation can be predicted. The problem is that although two different types of heavy charged particle may deposit the same average energy in a test sample, living cells and tissues do not necessarily respond in the same way to these two radiations. This distinction is made via the concept of Relative Biological Effectiveness (RBE) which is a measure of how damaging a given type of particle is when compared to an equivalent dose of x-rays.  The Quality Factor of a given type of radiation is determined in the following way:  A group of RBE measurements are made using a variety of cells and/or tissue (these experiments aren't cheap to perform and the number that are done is driven by the overall interest in the radiation being studied). Basically, the RBE is determined by comparing the damage of the radiation to the cells/tissue of interest to that with an equal dose of gammas or xrays.   Once the RBE data are in hand, a commitee of radiation experts meets and considers all the available data and then assigns a Quality Factor to the radiation.  This may seem a bit unusual to those used to hard formulas or well-defined procedures.

For example, the RBE of alpha particles has been determined (by committee) to be 20 (apparently not very dependent on the energy of these particles).  This means that 1 Gy of alphas is equivalent to 20 Gy of gammas/xrays.  Another way to say this is to use a new unit, the sievert (Sv) which measures Dose Equivalent (the old unit is the rem; 1 sievert = 100 rem).  Click here for info on Mr. Sievert.  Thus 1 Gy absorbed dose of alpha particles is 20 Sv dose equivalent.  The sievert is the unit used in NASA's radiation limits for humans in Low Earth Orbit:


So far, no American astronaut has received doses anywhere near these limits (although details of each astronaut's exposure are private medical data and cannot, by law, be revealed to the public).  But, the longest stay in orbit has been measured only in months.  The possibility of long-term spaceflight makes this issue much more important than it has been in the past.

Here is a description of a typical procedure to determine the RBE (or equivalently, the absorbed dose) of an astronaut during a given exposure in space (thanks to Neal Zapp/NASA/JSC):  First, before the flight, a sample of his/her blood is taken and divided into 4 parts.  At a professional lab, these parts are exposed to 4 different dose levels of gamma radiation.  Later, this blood is processed and photographs are made of the chromosomes from these cells.  These photos are viewed by experts and counts of identifiable damage are recorded.  These data are used to make a simple (roughly linear) graph relating measured chromosomal damage to dose (the Damage/Dose relationship).  Next, the astronaut goes off on her spaceflight mission.  On return, another blood sample is taken and chromosome damage counts are made once again.  Finally, the Damage/Dose curve is used to determine the equivalent dose due of radiation received while in space.
 

Photo set of damaged and undamaged chromosomes used in these studies.  (click to enlarge)

In the image above, note in the upper right a chromosome has been broken up and at upper left (on the right photo) a repair process has put one back together using some bits from the wrong chromosome.  Statistics limit the use of these data, especially for short Shuttle missions.  Better statistics have been obtained from US astronauts who spent months aboard the Russian MIR spacecraft.

The effects of a given dose of  ionizing radiation on humans can be separated into two broad categories: Acute and Long-Term effects.

ACUTE EFFECTS
The acute, or more immediately-seen effects of radiation can affect the performance astronauts.  These effects include skin-reddening, vomiting/nausea and dehydration.  Other tissue and organ effects are possible.  Another term: Acute Radiation Syndrome.

LONG TERM EFFECTS.
Given that only moderate doses of radiation are encountered (and thus acute effects are not seen) the long-term effects of radiation become the most important to consider.  The passage of an energetic charged particle through a cell produces a region of dense ionization along its track.  The ionization of water and other cell components can damage DNA molecules near the particle path but a "direct" effect is breaks in  DNA strands.  Single strand breaks (SSB) are quite common and Double Strand Breaks (DSB) are less common but both can be repaired by built-in cell mechanisms.  "Clustered" DNA damage, areas where both SSB and DSB occur can lead to cell death.  Although "endogenous processes" can lead to DSB,  its occurance due to ionizing radiation (especially the high LET radiation found in space) is an important component of  long-term risk .  For most cell types, the death of a single cell is no big deal -- cells continually die and are replaced by normal processes.  A more dangerous event may be the non-lethal change of DNA molecules which may lead to cell proliferation, a form of cancer. Research topic: The RBE of alpha particles on stem cells. These single and double strand breaks, or lesions, can be studied with the scanning tunneling microscope.

Is there any level of exposure to ionizing radiation which does not produce the risk of long term effects?  This question is controversial and was a topic discussed in an August, 1999 scientific meeting in Ireland.  Read a newspaper report on the meeting.

A related topic of practical importance is the question of how other critical items in space manage under the constant bombardment of ionizating radiation.  It has long been known that electronics equipment can fail due to "short circuits" caused by the passage of cosmic rays through crucial parts.  Other significant problems exist even when no permanent damage is caused.  Errors due to Single Event Upsets in memory locations caused by particles are a constant problem and critical computer systems must therefore have error detection and correction capability.  For example, the Shuttle has 4 computers which vote on each action before making a decision.  This is because at any time one or more of these computers could have made a mistake due to a radiation-caused memory error.  Currently, an Air Force satellite called APEX   has on-board experiments to gather data on the radiation menace to electronics packages in space.  Most of the radiation hits are in the SAA but some are direct GCR  particles.