ESA Document - Emits - ESA
ESA Document - Emits - ESA 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
s HMM Assessment Study Report: CDF-20(A) February 2004 page 41 of 422 depending on age and gender, e.g. an older male is less likely to develop cancer at 1 Sv than a 24-year-old female. Ionising Radiation Exposure Limits Organ Specific Equivalent Dose Limits (Sv) Exposure Interval Blood Forming Organ Eye Skin 30 days (ESA) 0.25 0.5 1.5 Annual (ESA) 0.5 1 3 Career (NCRP-98) 1-4 a 4 6 a varies with gender and age at initial exposure Table 2-3: Radiation exposure limits set by ESA and the NCRP career limit The ECSS-E-10-04 space environment standard provides additional limitations and recommendations. 2.6.4.1 30 Day limit The radiation sources that can contribute to exceeding the 30-day limit are from passages of the trapped particle belts; particularly during a prolonged Earth escape phase and solar proton events. To ensure the dose remains below such a level, it will be necessary to provide a storm shelter in which the astronauts can take refuge. Most events last less than 2 days, however, it is expected that the worst events can last up to a week or more, as shown in Figure 2-15. 30% 25% 20% 15% 10% 5% 0% 60% 19% Event Duration Distribution 4% 5% 4%4% 3% 0%0%0% 2% 0%0% 0 1 2 3 4 5 6 7 8 9 10 11 12 Event duration (Days) Figure 2-15: Distribution of the event duration where the 10 MeV flux exceeds 2 protons/cm 2 /s/ster. 2.6.4.2 Yearly limit The radiation sources that can contribute to exceeding the yearly limit include solar proton events and galactic cosmic rays. Due to the variation of the cosmic rays with the solar cycle, the
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s<br />
HMM<br />
Assessment Study<br />
Report: CDF-20(A)<br />
February 2004<br />
page 41 of 422<br />
depending on age and gender, e.g. an older male is less likely to develop cancer at 1 Sv than a<br />
24-year-old female.<br />
Ionising Radiation Exposure Limits<br />
Organ Specific Equivalent Dose Limits (Sv)<br />
Exposure Interval Blood<br />
Forming<br />
Organ<br />
Eye Skin<br />
30 days (<strong>ESA</strong>) 0.25 0.5 1.5<br />
Annual (<strong>ESA</strong>) 0.5 1 3<br />
Career (NCRP-98) 1-4 a<br />
4 6<br />
a<br />
varies with gender and age at initial exposure<br />
Table 2-3: Radiation exposure limits set by <strong>ESA</strong> and the NCRP career limit<br />
The ECSS-E-10-04 space environment standard provides additional limitations and<br />
recommendations.<br />
2.6.4.1 30 Day limit<br />
The radiation sources that can contribute to exceeding the 30-day limit are from passages of the<br />
trapped particle belts; particularly during a prolonged Earth escape phase and solar proton<br />
events. To ensure the dose remains below such a level, it will be necessary to provide a storm<br />
shelter in which the astronauts can take refuge. Most events last less than 2 days, however, it is<br />
expected that the worst events can last up to a week or more, as shown in Figure 2-15.<br />
30%<br />
25%<br />
20%<br />
15%<br />
10%<br />
5%<br />
0%<br />
60%<br />
19%<br />
Event Duration Distribution<br />
4% 5% 4%4% 3%<br />
0%0%0% 2% 0%0%<br />
0 1 2 3 4 5 6 7 8 9 10 11 12<br />
Event duration (Days)<br />
Figure 2-15: Distribution of the event duration where the 10 MeV flux exceeds 2 protons/cm 2 /s/ster.<br />
2.6.4.2 Yearly limit<br />
The radiation sources that can contribute to exceeding the yearly limit include solar proton<br />
events and galactic cosmic rays. Due to the variation of the cosmic rays with the solar cycle, the
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