Mission Design for the CubeSat OUFTI-1

CHAPTER 55.7 The radiation environmentThe trajectory of charged particles of solar wind, electrons and protons, is modifiedby the interaction with the earth magnetic field: they remain trapped intothe so-called radiations belts, or Van-Allen belts. They are two belts where theradiation environment is therefore extremely hard and the spacecrafts passingthrough them needs to be protected. We can in fact identify two different belts:• the inner belt extending approximately between 1,000 and 15,000 Km. Itcontains high concentrations of energetic protons with energies exceeding100 MeV and electrons in the range of hundreds of kiloelectronvolts• the outer belt extending till 50,000 Km.electrons.It contains mainly energeticThe belts altitude strongly depends also from the solar activity.Anyway OUFTI-1 will have the apogee inside the inner belt and therefore somecares have to be taken. Trapped particles in the radiation bells, as well as solarflare protons and galactic cosmic rays, can cause in fact the so-called SingleEvent Phenomena (SEP) within microelectronic devices. There are three differenttypes of SEP: the Single-Event Upset, SEU, the Single-Event Latchup,LEL, and the Single-Event Burnout, SEB. If the first case neither damages thepart nor interferes with its subsequent operation, the second one causes the partto hang up and to no longer operate until the power to the device is turned offand than back on. The most critical situation is the Single-Event Burnout: inthis case in fact the devices fails permanently.In order to prevent these events, we need to blind somehow the sensible partsbut to do that we need to know the total radiation dose, which represent thesum of the protons, electrons and bremsstrahlung dose produced by the interactionof electrons with the shielding material.The estimation of the total dose has been done with the software SPENVIS,SPace ENVironment Information System, a software developed by the BelgianInstitute for Space Astronomy and funded by the European Space Agency.In figure 5.28 the radiation dose as a function of equivalent aluminium shieldingthickness is represented. The unit for the radiation dose is the rad which isthe amount that deposits 100 ergs (6.25 · 10 7 MeV ) per gram of target material.These values have been calculated for the total mission duration with the hypothesisof solar maximum: they are therefore higher that the real values. Theanalysis has been done for a finite slab with silicon as target material.As expected, the radiation dose of protons and electrons is especially intensebut, already with 2 mm of shielding aluminium, it can be greatly reduced.Once the value of total radiation dose that can be tolerated by the electronicsdevices on board and the frequency of Single Event Phenomena (SEP) will beGalli Stefania 54 University of Liège