The space radiation environment: an introduction. Schimmerling W.
https://three-jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf. Date posted: 02-05-2011.
The Space Radiation Environment: An Introduction
Walter Schimmerling, Ph.D.
The components of space radiation that are of concern are the high-energy, charged
nuclei of elements from hydrogen (protons) to iron (high-energy nuclei with charges
greater than 2 are also referred to as “HZE particles”. These particles are part of the
Galactic Cosmic Ray (GCR) background radiation that permeates interplanetary
space. As shown in Figure D. 1(a), the fraction of GCR constituted by the
Differential Flux (m2•sterad•sec•MeV/nucleon)-1
106 10
1
105 H
10-1
RELATIVE ABUNDANCE
104 10-2
C O He
10-3
103 Mg 10-4
Ne Si Fe C
102 10-5
10-6
10
10-7
Fe
10-8
1.0
10-9
0.1 2 3 5 7
4 6
0 5 10 15 20 25 30 10 10 10 10 10 10 10
ATOMIC NUMBER (Z) Kinetic Energy (MeV/Nucleon)
(a) (b)
Figure D. 1. Abundances (a) and energy spectra (b) of GCR.
nuclei of elements heavier than helium is very small; approximately, GCR consist of
85% protons, 14% helium, and 1% heavier particles. As seen in Figure D. 2(b),
showing the distribution in energy of several important HZE nuclei, these particles
have very high energies, sufficient to penetrate many centimeters of tissue or other
materials. In addition, the HZE nuclei are highly charged and, therefore, very
densely ionizing. As a consequence, even though the number of HZE particles is
relatively small, they have a significant biological impact that is comparable to that of
protons.
The space radiation environment: an introduction. Schimmerling W.
https://three-jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf. Date posted: 02-05-2011.
Potential Shock 1011
Enhancement
Integral fluence, protons/cm2
Start of
Increase 1010
Event
Onset
109
Relative
Flux
February 1956
November 1960
108 August 1972
August 1989
Time September 1989
Shock October 1989
Propagation
Delay
Passes 107
Observer 1 10 100 1000
Time From
proton kinetic energy, MeV
Start To Max
Figure D. 2. Time course of a Solar Figure D. 3. Distribution in
Particle Event. energy of proton fluxes
for major past SPEs (free
space)
Solar disturbances occasionally cause much larger fluxes of particles, mainly high
energy protons; these are known as Solar Particle Events (SPE). Peak flux during
an SPE may be two to five orders of magnitude greater than background, within
hours of the event onset, as shown in Figure D. 2. Periods of enhanced flux may
last for days, with successive peaks due to multiple events and enhancements
during shock passage (Figure D. 3), indicating that different physical processes are
involved. However, the number of protons with energies in the region of several
hundred MeV is significant in all cases.
An illustration of the contribution of different components of space radiation outside
the Earth magnetic field is shown in Figure D. 4. This figure shows the calculated
relative contribution of different groups of particles to the dose equivalent (cf.
Appendix B for definition of quantities) behind 3 g/cm2 of aluminum (slightly more
than 1 cm thickness). The left-hand side shows that protons account for almost all of
the SPE radiation, and the right-hand side shows that this is no longer true for GCR,
where HZE particles account for most of the radiation risk.
The space radiation environment: an introduction. Schimmerling W.
https://three-jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf. Date posted: 02-05-2011.
Figure D. 4. Relative contribution of different
components of space radiation to dose equivalent
Protons and electrons of sufficiently low energy can be captured by the Earth’s
magnetic field, schematically indicated in Figure D. 5, as an equivalent bar magnet.
In reality, the magnetic field is more complicated. Its shape is also distorted by the
Sun, so that the magnetic field on the day side is compressed and the magnetic field
on the night side is pushed away. Charged particles entering the Earth’s magnetic
field from space are turned from their path as they traverse the magnetic field lines.
If their energy is sufficiently low, they are trapped into the Van Allen belts (Figure D.
6). These trapped radiation belts surround the Earth at altitudes that depend on the
Earth magnetic field. The belts consist of protons (inner belt) and electrons (inner
and outer belt), spiraling along magnetic field lines from pole to pole. Near the poles,
the trapped radiation belts extend almost down to the surface.
The Earth’s magnetic field is offset and tilted from the Earth’s axis of rotation. Thus,
the radiation belts, centered on the magnetic field, are also not centered on the
Earth axis of rotation. The region where the radiation belts are closest to the Earth’s
surface, near
The space radiation environment: an introduction. Schimmerling W.
https://three-jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf. Date posted: 02-05-2011.
Figure D. 5. The Earth as a magnet Figure D. 6. The Van Allen belts.
showing field lines.
the coast of Brazil, is called the South Atlantic Anomaly (SAA), schematically
shown in the inset of Figure D. 7. The trapped radiation belts are not static: their
altitude distribution and intensity are greatly dependent on solar activity, with hourly,
daily and seasonal changes. Over geological times, the magnetic field of the Earth
has been known to change and reverse itself. The measured long term drift in the
position of the SAA provides continuing evidence of active Earth magnetism. As
shown in Figure D. 7, proton fluxes at energies of 100s of MeV, as measured on
the Mir space station during solar minimum, can still be significant.
The magnetic field of the Earth allows only the fastest, most energetic particles to
penetrate deep into the atmosphere, and the thick atmosphere provides so much
material that most of the incident radiation interacts before it can reach the surface
of the Earth. Thus, space radiation is the source of many of the cosmogenic
nuclides, such as 14C. At the surface of the Earth, only the most energetic, lightest
products of the nuclear interactions of GCR with the atmosphere, mainly µ-mesons,
are still present. This is the radiation background present everywhere on Earth.
The space radiation environment: an introduction. Schimmerling W.
https://three-jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf. Date posted: 02-05-2011.
Earth axis of rotation axis of radiation belts
500 km alt.
Earth’s
Surface
Equator
SAA
1E+09
1E+07
MeVday
1E+05
1E+03
2
p/cm
1E+01
1E-01
1E-03
0 100 200 300 400 500
Proton Energy (MeV)
Figure D. 7. Energy distribution of trapped protons and
South Atlantic Anomaly.
Higher in the atmosphere, at altitudes used by commercial aircraft, radiation is more
intense, and the most hazardous secondary radiation is high energy neutrons
emitted by GCR interactions with the atmosphere. The intensity of the radiation
increases with altitude. At high altitude, large fluxes of high-energy protons from
SPE can cause radiation levels to exceed those permissible for aircraft passengers
or crew. For this reason, radiation levels on high-altitude aircraft must be monitored
and aircraft may be required to descend to safer altitudes during an SPE.
The Space Shuttle and Space Station will be located in low Earth orbit (LEO),
beyond the protection of the atmosphere, but still within the protection of the
magnetic field. In these orbits, the radiation risk will be due to GCR particles too
energetic to be significantly deflected by the magnetic field, and to trapped radiation
belt protons. When the orbit of a spacecraft intersects the SAA, radiation intensity
can increase by an order of magnitude. For this reason, extravehicular activity (EVA)
should be avoided whenever a spacecraft is about to traverse the SAA. Even in the
interior of a spacecraft, exposures could exceed radiation limits during a large SPE.
The space radiation environment: an introduction. Schimmerling W.
https://three-jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf. Date posted: 02-05-2011.
Under such circumstances, crews may be directed to limit activities to the most
highly shielded area of the spacecraft, for use as a “storm shelter”.
Beyond the Earth’s magnetic field, crews are exposed directly to GCR radiation and
to SPE radiation. Spacecraft or planetary habitats thus require their own measures
to avoid radiation overexposures. The most fundamental measures that can be
taken are to ensure that spacecraft and habitat materials are configured to provide
maximum radiation shielding effectiveness; that a “storm shelter” is available; and
that monitoring for SPE provide sufficient warning to crew members involved in EVA.
Other measures are possible, but require a knowledge of biology that is not at
present available.
