The space radiation environment is significantly different from that found
terrestrially. Space radiation primarily consists of high-energy charged particles, such as
protons, alpha and heavier particles, originating from several sources, including galactic
cosmic radiation, energetic solar particles from solar flares and trapped radiation belts.
Some of these high-energy particles inflict greater biological damage than that resulting
from typical terrestrial radiation hazards. Crew exposures can easily exceed exposures
routinely received by terrestrial radiation workers. Increased knowledge of the composition
of the environment and of the biological effects of space radiation is required to assess
health risks to astronaut crews.
Future expeditions into interplanetary space will place crews at increased risk of exposure compared to the current short duration low-Earth orbit (LEO) missions. Long duration exploratory class missions will not benefit from the protection from Galactic Cosmic
Radiation (GCR) and energetic solar flare protons afforded by the Earth's geomagnetic field. Astronaut exposure to GCR within an unprotected or thinly shielded spacecraft during solar minimum
is sufficient to exceed current astronaut exposure guidelines.
Energetic Solar Particle
Events (SPEs) present an additional source
of risk. These events are unpredictable in nature
life threatening to an inadequately protected crew. Unshielded
exposure to large
solar particle events may lead to serious acute
health effects. To minimize the risk to the crew,
and dynamics of the potential radiation environment must be considered
performing spacecraft and mission design.
For terrestrial radiation workers, additional protection against
radiation exposure can be provided through increased shielding.
Additional shielding against space radiation exposure may not
be practical or efficient. Galactic cosmic radiation is extremely
penetrating. Dose equivalent exposure rates behind thin shielding
are reduced rapidly at first, but plateau with increasing thickness.
Thus, thicker shields become less efficient. The additional mass
added purely for reducing radiation exposures becomes a substantial
mass penalty for transportation vehicles and therefore may dramatically
increase mission cost.
The Johnson Space Center (JSC) leads the research and development
activities necessary to address the health effects of space radiation
exposure to astronaut crews. These activities range from quantification
of astronaut exposures to fundamental research into the biological
effects resulting from exposure to high-energy particle radiation.
The Spaceflight Radiation Health Program seeks to balance the
requirements for operational flexibility with the requirement
to minimize crew radiation exposure.
Most manned spaceflight missions have been conducted within the
protection of the Earth's magnetic field. The geomagnetic field
shields the crews from large SPEs and a significant portion of
the GCR. For the low-Earth orbiting missions which have typified
the U.S. manned space program, the largest fraction of the radiation
exposure received has resulted from passage through a region known
as the South Atlantic Anomaly (SAA). The remainder of the exposure
is attributed to high-energy galactic cosmic radiation. During
the Apollo lunar missions, astronauts traversed through the trapped
radiation belts into the unprotected realm of free space outside
the geomagnetosphere. These excursions into cislunar space placed
the astronauts at risk of receiving life threatening radiation
exposures if a large SPE were to occur. Fortunately, no major
solar proton events occurred during these missions.
Trapped RadiationThe Earth's magnetic field is responsible for the formation of the trapped radiation belts that surrounds the Earth. Electrons and protons are trapped in regions starting above the atmosphere and extending to a distance of 10 to 12 Earth radii. Charged particles travel through these zones spiralling around the magnetic field lines, oscillating back and forth between "mirror points" located in opposite hemispheres. Figure 1. illustrates the distribution of high and low energy protons and electrons around the Earth.
The magnetic axis of Earth is tilted approximately 11 degrees from the spin axis and is slightly offset from the center of the Earth. As a result of the shift and tilt of the magnetic field, the trapped proton belts extend down to the atmosphere in a region located over South America and the South Atlantic Ocean. This region is known as the South Atlantic Anomaly (SAA). Low inclination flights typically transit a portion of the SAA during six or seven consecutive orbits each day. Figure 2. shows the exposure rate as a function of orbital position for a 28.5 degree orbit. The SAA is the primary source of radiation exposure for the shuttle and the proposed Space Station Freedom.
The proton spectra and fluence are strong functions of altitude. At the higher altitudes, the greater portion of crew exposures is received during transits through the SAA as a result of greater trapped proton fluence levels. At lower altitudes, the protons in the SAA interact with the residual atmosphere. Some of the protons are lost and contribute to an anisotropic distribution of protons. Over a factor of two difference exists between the proton flux from the east compared to the flux from the west. The anisotrophy in particle flux will be an important factor for Space Station.
In addition to altitude, the integrated dose is a function of
orbital inclination and solar cycle. The dose received by the
crew during low inclination (28.5 degree) low-Earth Orbits (LEO)
is illustrated in Figure 3. The Space
Station is planned to operate at this orbital inclination between
200 and 270 nautical miles (370 to 500 km). Increases in solar
activity expand the atmosphere and increase the losses of protons
in LEO. Therefore, trapped radiation doses in LEO decrease during
solar maximum and increase during solar minimum.
Trajectories of low inclination flights do not pass the regions of maximum intensities within the SAA. Although high inclination flights pass through the SAA maximum intensity regions, less time is spent in the SAA than low inclination flights. Thus crews in high inclination flights receive less net exposure to trapped radiation than in low inclination flights for a given altitude. This is illustrated in Figure 4., which depicts the location of the SAA as measured by internal equipment on the shuttle. Low inclination flights will not transit the SAA south of 28.5 degrees south latitude. High inclination flights transit between North and South 58 degrees.
Galactic Cosmic Radiation (GCR)
Galactic cosmic radiation originates from outside the solar system.
It consists of ionized charged atomic nuclei from hydrogen (87%)
and helium (12%) to uranium (trace) and are characterized by extremely
large kinetic energies (up to several thousand GeV
per atomic mass unit (amu)). The integral flux of four isotopes
at solar minimum conditions is depicted in Figure 5.
These particles are distributed isotropically and found in relatively
The effect of solar activity on the integral GCR flux is also
shown in Figure 5. During periods of
solar maximum activity, the interplanetary magnetic field generated
by the sun provides some protection to the inner solar system,
decreasing GCR integral intensity. Higher energy particles are
not appreciably attenuated, lower energy fluence is significantly
reduced. The integral GCR dose rate in free space is approximately
a factor of 2.5 higher at solar minimum than at solar maximum.
During solar minimum, the unshielded dose to the blood forming
organs (BFO) is approximately 60 rem/year.
The geomagnetic field also deflects many of the lower energy GCR
components. This protection is primarily a function of latitude,
as is shown in Figure 7. Compared to
low inclination orbits, higher inclination orbits are exposed
to increased GCR levels as the spacecraft transits the higher
latitudes. In the proposed Space Station orbit, the magnetic field
provides a factor of 10 reduction in total GCR exposure relative
to the free space environment. During geomagnetic storms, higher
GCR exposures may be experienced at lower latitudes. Little increase
in GCR is realized as the altitude increases.
Solar particle events (SPEs) are injections of energetic electrons,
protons, alpha particles and heavier particles into interplanetary
space during solar flares. During periods of maximum solar activity,
the frequency and intensity of solar flares increase. Most flares
do not present a significant hazard, because they are either too
small to inject significant numbers of energetic solar particles
or they occur at solar longitudinal positions that are unfavorable
for the direct transfer of particles to the Earth along interplanetary
magnetic field lines. However, flares or rapid sequences of large
flares that are orders of magnitude greater in intensity than
most flares, are of particular concern for generating very large
energetic SPEs. These solar proton events generally occur only
once or twice a solar cycle. However, during the 22nd solar cycle,
four comparable extremely large flares occurred in a 4 month period.
The Oct. 19, 1989 event was the largest.
Each solar particle event is characterized by the total number
of particles and the particle energy spectrum. The spectra of
three of the largest proton events are depicted in Figure 8.
The intensity and spectral distribution of SPEs have a significant
impact to shield effectiveness. Figure 9.
represents the shielding effectiveness for these large SPEs.
SPEs pose the greatest threat to unprotected crews in polar, geostationary
or interplanetary orbits. To date, the greatest threat to significant
exposures to astronauts existed during the Apollo Program. Figure 10.
illustrates the variation in timing and magnitude of SPEs that
occurred during the course of the Apollo Program. The calculated
dose for crewmembers within the command module, within the lunar
module or in a space suit performing EVA is represented for each
flare. As can be seen in the figure, it is only fortuitous that
no significant SPEs occurred during the lunar missions.
Fortunately, most SPEs are relatively short-lived (less than 1-2 days), which allows for relatively small volume "storm shelters" to be feasible. To minimize exposure, the crew would be restricted to the storm shelter during the most intense portion of the SPE, which may last for several hours. Storm shelters with shielding of approximately with 20 g/cm2 or more of water equivalent material will provide sufficient shielding to protect the crew.
Additional environmental hazards may be present from the use of
manmade sources. These hazards may be in the form of exposure
resulting from medical investigations, radioisotopic power generators,
or small sources for experiments. Lunar and Martian missions may
include either nuclear reactors for power or propulsion purposes
that will contribute to crew radiation health concerns.
Low Earth Orbit Environment
Current missions are restricted to LEO. Figure 11. shows data taken during the high inclination STS-28 flight and illustrates the contributions of the three natural sources of radiation. The GCR component varies cyclically with maximums at the extreme northern or southern portion of the orbital track. Minimums correspond to transits of the geomagnetic equator, where the spacecraft experiences the maximum geomagnetic protection from GCR. At periodic intervals large spikes in the exposure rates are encountered which correspond to passages through the SAA. The largest spikes are passages through the regions of peak SAA intensity; smaller peaks represent passage through the fringes of the SAA. A unique feature of this data is the effects of a solar particle event measured in LEO. Peaks in the dose rate attributed to the SPE occur at the extreme northern or southern portions of the orbital track. Whereas the GCR and SAA components shown in the figure are "typical" for high inclination flights, the effects of SPEs on dose rates will depend upon a variety of parameters.
NASA has adopted the recommendations that the National Council
on Radiation Protection and Measurements (NCRP) presented in its
Report 98, "Guidance on Radiation Received in Space Activities"
(July, 1989) as the basis for the supplementary standard for spaceflight
crew radiation exposures. The maximum exposure limits are presented
in Tables 1 and 2. Whereas monthly and annual
limits primarily exist to prevent the short term physiological
effects of exposure, career limits exist to contain radiation
risk within a 3% increased lifetime cancer mortality. The recommendations
of the NCRP apply to activities in low-Earth orbit, such as Space
Station. Astronaut exposure limits are greater than those of terrestrial
Recent information from reevaluation of atomic bomb survivor data and other sources has provided impetus for further examination of the acceptable limits of astronaut radiation exposure. Preliminary recommendations from the NCRP evaluation of the new data suggest that even lower career limits for astronauts may be warranted.
During spaceflight, crew exposures are monitored using passive
dosimeters. A new generation of radiation instrumentation is being
developed to assist in interpreting the crew radiation exposures.
Exposure from medical procedures and experiments are also determined
for each astronaut. Records of exposure both to medical examinations
and spaceflight are documented as part of the Spaceflight Radiation
Health Protection Program.
Advanced Radiation Monitors
Longer duration stays in the space radiation environment demand improved dosimetric instrumentation to evaluate the true health hazard to crews. The passive dosimetry currently used for shuttle missions provides only part of the information needed to quantify the crew effective dose equivalent. Two new instruments (TEPC and CPDS) are being developed for use on the Space Station to improve the crew radiation exposure risk estimation. These real-time instruments will continuously telemeter radiation data to Mission Control for monitoring the radiation environment. Prototypes of the new instruments are being tested during shuttle missions. The new Space Station instrumentation includes:
Flight Experiment Support
Radiation flight experiments are frequently flown as part of the
Shuttle Detailed Supplementary Objectives (DSOs) Program. These
experiments include flight testing the radiation monitor prototype
units as well as other monitoring devices.
Data from Space Station prototype spectrometers also have been
used to assist investigations into Single Event Upsets (SEUs)
in shuttle computers. SEUs are events resulting from charged particles
interacting within semiconductor devices that generate enough
charge to change the state of flip-flop circuits, such as computer
memory. These are known as "soft" upsets and can be
corrected by reloading programs or data. "Hard" upsets
result from physical damage to a circuit element from the incident
radiation and is permanent.
Some medical procedures use radioactive tracers to track physiological
changes as crewmembers adapt to the weightless environment. These
hazards are reviewed by the Human Research Procedure and Protocol
Committee. Crew doses are estimated for the procedures and documented
in their health records.
Current models of the trapped radiation environment (AP8, AE8)
are over 20 years old. These are static models and do not take
into account the dynamics that can occur within the trapped radiation
belts. The Space Station platform will provide continuous coverage
over many years to enable data to be collected in which to improve
the models and facilitate improved projections of crew doses.
Data from the Space Station active monitors will be used to update
and verify models of both the trapped radiation environment and
the cosmic radiation environment. Improvements in the models to
accommodate transients as well as to correct for the physical
shift of the SAA will be incorporated. Data gathered from prototype
development flights has been used to provide a better understanding
in the current structure of the LEO trapped radiation hazard.
Health Risk Assessment
The penetration of radiation within the body is being studied.
Human equivalent densimetric phantoms are used to analyze the
dose at different depths within the body. This information is
correlated with transport codes and human anatomical computer
models. These evaluations will lead to the ability of performing
organ specific dose estimates and of estimating the net increased
risk of cancer for the crew.
Fundamental research into the effects of high-LET radiation also
is performed at the Johnson Space Center. A variety of cell tissue
cultures are irradiated with both on-site (gamma radiation, Co-60)
and off-site (proton and heavy particle accelerators) sources.
The transformation toward cancer within these samples is monitored
and the correlation of observed biological effects with received
dose is determined. These types of studies are pivotal in the
development of exposure standards for astronauts. Research in
this arena may lead to the development of a biological dosimeter
and/or countermeasures. Biological dosimeters will use biological
media directly to measure biologically effective doses instead
of relying on extrapolation from physical dosimeter measurements.
Space Station operations will follow similar functional responsibilities.
One of the most significant differences will be the utility of
real-time active radiation monitoring on the Space Station. This
feature will provide significant improvement to operations by
providing exposure monitoring during missions.
Solar System Exploration Division
Nuclear technology may be used to enhance these missions, adding
to the radiation protection concerns. Nuclear power plants on
the lunar surface may be required to overcome the energy storage
challenges resulting from 2 week duration lunar nights. Substantial
savings in travel time and/or in propellant launched into orbit
for Martian missions may necessitate the use of nuclear thermal
propulsion. Although nuclear sources increase radiation risk to
astronauts, overall mission reliability, success and safety may
be increased. Johnson Space Center will be actively involved in
these issues to ensure crew health.
RAD - The absorbed dose, RAD, is the amount of energy absorbed from radiation per mass of material (1 RAD = 100 ergs/g). The SI unit for absorbed dose is the Gray (1 Gray (Gy) = 100 Rad). LET - Linear Energy Transfer quantifies the amount of energy deposited per unit length of particle track. This factor increases with the square of the charge and inversely proportional to the energy of the radiation particle. QUALITY FACTOR - Is a function of the particle LET, which is determined by the charge and energy of radiation particles. This factor accounts for the differences in biological effectiveness of different particles and is used to convert an absorbed dose into a dose equivalent. Currently, values for the quality factor may range from 1 to 20. Quality factor values as high as 100 may be deemed appropriate as additional research continues. REM - The biological equivalent dose, REM (Roentgen equivalent man), is the absorbed dose adjusted for biological effectiveness of the particular type of radiation. It is the product of the absorbed dose and quality factor. The SI unit for biological equivalent dose is Sieverts (1 Sievert (Sv) = 100 Rem).
Benton, E.V., Editor, "Space Radiation", Nuclear Tracks and Radiation Measurements, Vol. 20 No. 1, Pergamon Press New York, January, 1992. Benton, E.V. and J.H. Adams Jr., Editors "Galactic Cosmic Radiation: Constraints on Space Exploration", Nuclear Tracks and Radiation Measurements, Vol. 20 No. 3, Pergamon Press New York, July, 1992. Johnson, A.S., G.D. Badhwar and C. Yang, "Space Station Radiation Dosimetry and Health Risk Assessment", 23rd ICES, SAE 932212, July 1993. McCormack, P.D., C.E. Swenberg and H. Bücker, editors, Terrestrial Space Radiation and Its Biological Effects, NATO ASI Series, Series A: Life Sciences, Vol. 154, Plenum Press, New York, 1988 . Nachtwey, D.S. and T.C. Yang, Radiological Health Risks for Exploratory Class Missions in Space, Acta Astronautica, 23:227-231, 1991. NASA Life Sciences Strategic Planning Study Committee, "Exploring the Living Universe, A Strategy for Space Life Sciences", Final Report, F.C. Robbins, Chairperson, NASA, June 1988. Robbins, D.E. and T.C. Yang, "Radiation And Radiobiology", Chapter 20, in Space Physiology and Medicine, A. Nicogossian, C. Huntoon and S. Pool eds., third edition (in press), Lea and Febiger, Inc., Philadelphia, PA, 1993. Second edition, 1989. Wilson, J.W., L.W. Townsend, W. Shimmerling, G. Khandelwal, F. Khan, J. Nealy, F. Cucinotta, L. Simonsen, J. Shinn and J. Norbury, "Transport Methods and Interactions for Space Radiations, NASA Reference Publication 1257 (NASA-RP-1257), December, 1991.