ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 42 of 422
yearly limit is not expected to be exceeded during solar maximum by the GCRs alone, but the
limit would be exceeded at solar minimum if inadequate shielding is provided, as shown in
Figure 2-16.
The skin and ocular dose from solar protons, using the ECSS-E-10-04 recommended 95%
confidence level of the dose not being exceeded, with 9 g/cm 2 of shielding is 2 Sv/year.
However, inside a storm shelter providing 25 g/cm 2 of shielding a yearly dose of 0.12 Sv could
be expected in the eyes and skin tissues, assuming a semi-infinite planar shielding geometry. For
the Blood Forming Organs (BFOs) the doses are 5.92 Sv and 0.532 Sv, respectively, assuming a
spherical shell shielding geometry and the muscular tissues providing an additional 5 g/cm 2 of
shielding in addition to the 25 g/cm 2 of the storm shelter.
Dose Eq. Sv/yr
1.000
0.800
0.600
0.400
0.200
0.000
2.6.4.3 Career limit
GCR Dose Equiv. as Fn of Water shield Thickness
Solar Maximum
Solar Minimum
BFO Annual Limit
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Thickness cm or g/cm2
Figure 2-16: GCR dose equivalent (Sv/year) as a function of shielding thickness
All radiation sources contribute to the career limit dose calculation, although the majority of the
dose accrued throughout the mission will be from solar proton events and galactic cosmic rays.
2.6.5 Assumptions and trade-offs
• 500 km orbit about Mars – leads to reduction of about 25% in GCR flux due to planetary
“bulk” shielding.
• A minimum shielding of 16 g/cm 2 to be provided by the Martian atmosphere during
Martian surface activities.
• 25 g/cm 2 of shielding provided by the storm shelter to ensure remaining below the 30 day
and 1 year limits.