## 2 Materials and Methods
### Preliminary Concepts and Simulation Configuration
#### 2.3.2 Shielding  
Radiation and high-energy particles destroy the electronic components of the satellite at a microscopic level (transistor size in 2017 is 7-5 nm [19]). There are several sources of radiation, and different techniques must be applied to protect platforms from their damaging effects:  
1. Galactic Cosmic Rays (GCR) are high-energy charged particles that have originated outside our Solar system. Shielding is not effective to protect the platform against GCR.
2. Solar Energetic Particles (SEP) are electrons, protons, and heavy ions that have originated in the Sun. Also, gradual events accelerated by Coronal Mass Ejections (CME) and impulsive events from Solar flares present a risk to the satellite's electronics.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Object Name** & **Sub Component Object** & **DemiseAltitude (km)** \\ \hline \multirow{6}{*}{Cat-3/MOTS} & Chassis & 73.7 \\  & Solar panels & 77.5 \\ \cline{1-1}  & ADCS subsystem & 71.4 \\ \cline{1-1}  & Power Subsystem & 75.9 \\ \cline{1-1}  & OBC & 77.3 \\ \cline{1-1}  & Camera & 77.1 \\ \cline{1-1}  & Lens & 77.3 \\ \hline \hline \end{tabular}  
* OBC: on-board computer; ADCS: Attitude Control and Determination System.  
\end{table}
Table 2: Reentry survivability analysis has been completed with DAS 2.02 [18]. The main subsystems and materials with their specific shape, thermal mass, size, quantity, and material type have been considered. All parts disintegrate between 71 km and 78 km height, therefore the casualty area and the kinetic energy are equal to 0.  
Figure 1: Apogee/Perigee Altitude history for the studied orbit (Start Year: 2019, Inclination: 55\({}^{\circ}\), RAAN (Right Ascension of the Ascending Node): 8.32\({}^{\circ}\), Arg. Peri.: 0\({}^{\circ}\), Mean Anomaly 0\({}^{\circ}\), and Area-to-Mass: 0.006 kg/m\({}^{2}\)). The initial orbital height for the simulation has been set to 500 km. Reentry will occur after 4.6 years, while the mission has an expected lifespan of 2 years. During this period, the changes in the orbital height are negligible for payload functioning purposes.  
Solar wind: Plasma of charged particles causing disturbances in the magnetosphere.
* Radiation belts: Charged particles (protons and electrons) trapped by the Earth's magnetic field.  
In order to secure the survivability of the satellite for at least 2 years, it is mandatory to implement some shielding around essential components, such as the payload, the OBC (On Board Computer), and the Communication System. The high-energy radiation is mainly blocked by the amount of mass (thickness of the shielding) of the impacted material. Aluminum layers have been thoroughly used as shielding, but Z-graded shields have also proven to perform with a reduced mass compared to Aluminum layers. The NASA Shields-1 technology demonstrator [20] seeks to test Z-graded shielding technology (a laminate of several materials with different atomic numbers, designed to protect against ionizing radiation) [21] in a CubeSat with the corresponding limitations in both mass and available room. SPENVIS [22] has been used to perform the radiation analysis, mainly focusing on trapped proton and electron fluxes, galactic cosmic ray fluxes, and damage equivalent fluencies for solar cells. Both trapped protons and electrons contribute to the Total Ionizing Dose (TID) along with Bremsstrahlung protons and Solar flare protons, but in LEO orbits, protons dominate over electrons in contribution to the TID [23], (p. 30). Shielding, therefore, will be essential to the survival of the mission, but given the high constraints of mass and size of the 6U CubeSat, the thickness of the shielding should be carefully adjusted to comply with its protective task for the duration of the mission with the minimum weight (Figure 2). A 3 mm Aluminum plate shielding (84.3 gr) minimizes both the radiation dose accumulated and the shield mass.