Radiation and Long-Term
Space Flight
  • What is ionizing radiation?
  • Where does it come from?
  • Can't astronauts simply be shielded from radiation effects?
Atoms and molecules (such as those making up the cells in your body) exist normally in the neutral or uncharged state, the number of positive protons in the nucleus balancing the number of negative orbiting electrons outside the nucleus.  If an electron is lost (due to being struck by an energetic particle)  the resulting atom/molecule is called an ion and its properties are greatly changed. The particles that can cause this type of event are called  Ionizing Radiation    Not only does the ion now appear from a distance as a charged particle, the missing electron causes profound changes in the way the molecule bonds or interacts with other molecules. For this reason, radiations which lack sufficient energy to ionize common molecules (referred to as non-ionizing radiation ) are of much less concern than those higher energy particles which can easily ionize and break chemical bonds. A typical high energy particle of radiation found in the space environment is ionized itself and as it passes through material such as human tissue it disrupts the electronic clouds of the constituent molecules and leaves a path of ionization in its wake. These particles are either singly charged protons or more highly charged nucleii called "HZE" particles. (Z is the symbol for nuclear charge and the disruption caused is proportional to Z squared. Thus a particle with High Z and High Energy is called HZE. )  Particles encountered in space commonly have enough energy to disrupt the nucleus of target atoms  and these collisions can cause nuclear reactions which generate new and potentially more damaging particles.  Nuclear reactions make the analysis of ionizing radiation collisions much more difficult.  More on ionizing radiation

Electromagnetic waves exist as particles and vary according to their energy (proportional to frequency), ranging from low frequency, non-ionizing radio waves, up through the visible light frequencies, and then even higher to x-rays and gammas. It is interesting that the energy of light particles (photons) is just below that required to ionize molecules. At energies just above the visible, ultraviolet photons are able to remove electrons from some of the most easily ionized types of molecules such as those found in and around human cells. Fortunately, these "electromagnetic" types of ionizing radiation are not a great threat to humans in space. This is true because they can either be stopped with thin shields or, as in the case of x-rays and gamma rays, their intensity is fairly low in most volumes of space where humans desire to go. Some have claimed that low frequency electromagnetic fields from power lines are responsible for increased cancer risk but this has been discredited.  This leaves the highly energetic particles which can pass through shielding materials as the most obvious threat to humans in space.
 

There are three major components of ionizing radiation which are of concern to the health of astronauts in space:  GALACTIC COSMIC RAYS, SOLAR PARTICLE EVENTS and the Earth's SOUTH ATLANTIC ANOMALY.

GALACTIC COSMIC RAYS

Galatic Cosmic Rays (GCR) are called galactic because their source is clearly outside the solar system (and thus are assumed to be generated somewhere in our galaxy).  Typically, these particles are highly charged and very energetic.  They pass practically unimpeded through the skin of a typical spacecraft and the skin of the astronauts.  GCR is the dominant radiation to be dealt with on the International Space Station and on Mars missions.


  These particles are affected by the sun's magnetic field  and their average intensity is highest during the period of minimum sunspots when the solar magnetic field is weakest and thus is less able to deflect them. The sun's magnetic field increases as we move towards solar maximum (expected around the year 2000).  As the sun's field increases, GCR particles are more easily deflected and less are seen near the Earth.  One good thing is that the GCR component of radiation is relatively predictable (especially when compared to SPE as explained below) over the short term.

SOLAR PARTICLE EVENTS


Solar Particle Events (SPE) are of special interest to those concerned with radiation health because the particle flux from the sun can change quickly and dramatically with very little notice.  The particles streaming into the solar system along the sun's magnetic field lines are mostly fairly low energy protons and normally this presents an acceptable radiation level to humans, even in rather thin-walled space suits.  Sometimes, with very little warning, an eruption (sometimes seen as  a "flare") near the sun's surface spews out particles which can reach the vicinity of the Earth in less than an hour. The severity of this radiation can easily increase by  factor of 10.  A moderate eruption on 1989 October 22,  just before STS-34 returned to Earth from its orbital mission, did not produce great danger to the astronauts mainly because its low orbit inclination kept the spacecraft under the Earth's protective magnetic field.  A mission at high latitudes would have made this event more serious.  These events that  can be very dangerous to astronauts on a time scale from hours to days are called Coronal Mass Ejections. See a CME in slow motion

Short-term space weather forecasts: NOAA Space Environment Center provides one to three day estimates of the solar flare probability, mostly based on human forecaster judgement. A component of the prediction is related to several indices of solar activity, a principle one being the intensity of a common x-ray band (1-8 angstroms) constantly monitored on the Earth and satellites.  Click here to view a prediction site. Another major consideration is how well the Earth is connected magnetically to the unsettled area of the sun mostly responsible for the x-radiation (which is found with optical telescopes specially designed to monitor the sun).  See Solar Telescopes.

Most people have heard of sunspots which are dark-appearing splotches on the sun and have been observed for a long time.  The ancient Chinese recorded the largest sunspots on the just-setting sun when they could be observed  with the naked eye without harm.  These areas are slightly lower in temperature than the normal surface of the sun and are the result of local disruptions in the magnetic field.  Eruptions which can be dangerous to astronauts are typically associated with the areas around these unsettled sunspots.  Observing sunspot counts over many decades reveals that the number of spots varies widely over a period of approximately 11 years.  Later study has shown that the sun's huge magnetic field reverses direction every 11 years and that the actual period of the solar cycle is 22 years. More detail on the solar cycle
 

SOUTH ATLANTIC ANOMALY
Early in the space age it was found that the Earth's magnetic field acts as a trap to contain energetic charged particles.  These are referred to as the Van Allen belts.  Some of these particles originate from the solar wind but most are produced by the decay products of galactic cosmic rays.  Spacecraft travelling to points far from the Earth must pass through these areas but, in this case, the hazard for humans is low because the passage time through these radiation belts is short.  The passage time is not necessarily short for spacecraft in Low Earth Orbit (LEO).  It is fairly well-known that the Earth's magnetic field axis is significantly out of line with its rotation axis.  Although the north magnetic pole is still in the north it is not very close to the north pole at all.  What is less well-known is that the rotation and magnetic axes are also displaced, meaning that the Earth's field has a significant assymetry as seen on its surface.  The result of this is that there is an area near the coast of Brazil above which particles trapped in the Earth's magnetic field exist at much lower altitudes.  This area is called the South Atlantic Anomaly (SAA).  Lots of SAA graphics   At typical Low Earth Orbit (LEO) altitutes of 300- 500km, the radiation intensity in the SAA is much higher than that found anywhere else in orbit.

Here's a nice color map of the SAA. 


Note that the hottest spot is off the east coast of Brazil.  In Space Shuttle operations, Extra Vehicular Activities (EVA) are prohibited on orbits near any passages through the SAA to avoid this extra radiation risk.  Also, during SAA passage,  equipment is often turned off to minimize the probability of damage due to the ionizing trails of particles through electrically charged components.   (Click to enlarge)

Click for more SAA details



How many particles are going HOW fast? Click here for some graphic details.......


Can't astronauts be shielded from this potentially harmful radiation?  The simple answer is no, not completely.  Shielding provided by the typically-available structural aluminum skin on a spacecraft (around 5mm thick) is significant but it provides very little reduction in the number of energetic ionizing particles. And, the shielding itself produces secondary neutrons  and other energetic particles which pose an additional hazard.  The amount of aluminum shielding required to eliminate the currently-perceived risk from these heavy ions would produce a spacecraft so heavy that it could never be launched.  And, even if this were done, astronauts working outside the spacecraft would still be exposed (especially if a solar event occurred).  Hydrogen rich compounds such as polyethelyne and water are much more effective than aluminum and are being considered for spacecraft use (water, which must be on board for consumption anyway may have a secondary use as shielding in the future). Estimating the risk in any given situation (orbital inclination and altitude if in Earth orbit, type of shielding, current state of the solar wind, etc) is the real challenge. Currently, NASA's plan is reduce the uncertainty of long-term risk to 600% by 2002 and reduce it further to 300% by the year 2008. See NASA's Strategic Plan for more detail.

The overall uncertainty in the risk to humans due to ionizing radiation in space can be attributed to three broad categories:

  1. Uncertainty in the radiation itself; how much of what kind, etc.
  2. Uncertainty in the effects of shielding.  Shields produce a great variety of secondary particles which are generally a hazard too.
  3. The most uncertainty in estimating risk lies in the response of cells and tissues to the radiation they encounter.

See how lunar or Martian dirt can be used as shielding material for space colonizers.

One group which does studies on the general problem of radiation shielding is at Oak Ridge National Laboratory.