ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 40 of 422
energies of particles from these sources also varies, with the cosmic ray energies in excess of 1
GeV/nucleon and trapped particle energies limited to the MeV range.
2.6.1 Trapped particle belts
Energetic electrons and ions are magnetically trapped around the Earth, forming the radiation
belts. The radiation belts consist principally of protons of up to several hundred MeV energy and
electrons of up to a few MeV energy. The inner belt principally contains protons, extends to
about 4 Earth radii, and is reasonably stable in time. The outer belt consists principally of
electrons and extends to about 10 Earth radii and is highly dynamic: being subject to storms and
injection events that follow solar-terrestrial disturbances. The radiation belts are of principal
concern during the low-Earth orbit assembly phase, the Earth escape phase and the Earth return
phase. Mars, lacking a strong magnetic field, is not expected to provide any significant trapped
radiation belts.
2.6.2 Solar proton events
Energetic solar eruptions (Solar Particle Events, SPEs) produce large fluxes of Solar Energetic
Particles (SEPs), which are encountered in interplanetary space and close to the Earth. These
events are rare, occurring primarily during periods of solar maximum activity, which commences
2.5 years before Sun spot maximum and lasting for seven years. The duration of such events is
usually of the order of days, with larger events lasting a week or more. The large fluxes of
energetic particles can contribute a large, even lethal dose over a short period of time and the
mission will be exposed throughout its duration.
Two aspects of the solar proton dose contribution must be considered: the short term and longterm
effects. To ensure that a short-term limit, e.g. the 30-day limit, is not likely to be exceeded,
a storm shelter can be provided that sufficiently shields the astronauts over the duration of the
event. Considering the largest event measured to date, in August 1972, at least 20 g/cm 2 of
shielding would be required to remain below the 30-day limit.
However, to calculate the radiation dose budget for the entire mission, it is more appropriate to
use a statistical model to produce a radiation level based on a confidence level. The ECSS
standard model is the JPL-1991 solar proton model and the confidence level of 90% for a 3-year
mission is used.
2.6.3 Galactic cosmic rays
Galactic Cosmic Rays (GCR) provide a continuous flux of energetic ions from hydrogen to
uranium. Although the flux is low (a few particles per cm 2 per s), GCRs include energetic ions,
which can deposit significant amounts of energy in a small volume and are particularly
damaging to biological materials, e.g. DNA. Because of the high energies of these particles, it is
very difficult to shield against them.
2.6.4 Requirements and design drivers
Radiation limits set by ESA are shown in Table 2-3. Each exposure interval must be addressed in
the mission planning and shielding design. The limits are selected based on a probability of
increased risk to the subject, leading to the career NCRP BFO results ranging from 1 to 4