## 3 Results and Discussion  
In Table 1, we report the preliminary experimental results obtained by testing the reaction of the C/C samples to the synergic irradiation of AtOx and UV at five progressive attitudes (Figure 3). Three specimens were exposed to the AtOx/UV combined flux for each attitude position for a total of 15 ground tests in the experimental campaign The samples' mass was measured by means of a high-precision Mettler-Toledo micro-balance of sensitivity \(10^{-6}\) g, as suggested by the practice standard [16]; mass loss values are given to a \(10^{-5}\) g precision as the result of statistical uncertainty by two measurements (sample weight before and after the test), three samples for each position, and three recording operations for each sample. The time of exposure was 39,600 s (11 h) for an AtOx flux of about 1.9 \(\times\) 10\({}^{15}\) n.s./(s\(\cdot\)cm\({}^{2}\)), as estimated by the analyzed fluence effects on the polymide (Kapton HN) reference samples. The measured density of the C/C samples varied around 1.35 g/cm\({}^{3}\), whereas the area exposed to irradiation might be different each time due to unavoidable fluctuations related to the clamping system adopted (Figure 4, images on the left), varying in the 11-17 cm\({}^{2}\) range. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Attitude Test** & **Mass Loss (g)** & **Erosion Yield (cm\({}^{3}\)/n.at.)** \\ \hline \(0^{\circ}\) & 0.01215 & 9.694 \(\times\) 10\({}^{-24}\) \\ \(15^{\circ}\) & 0.01188 & 9.650 \(\times\) 10\({}^{-24}\) \\ \(30^{\circ}\) & 0.01064 & 9.535 \(\times\) 10\({}^{-24}\) \\ \(45^{\circ}\) & 0.00924 & 7.162 \(\times\) 10\({}^{-24}\) \\ \(60^{\circ}\) & 0.00540 & 3.047 \(\times\) 10\({}^{-24}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: AtOx/UV irradiation test over the C/C samples at different attitudes; the values reported represent the average of three measurements for each position. Figure 2: AtOx/UV test: mean detected environmental temperature. The changing attitude clearly resulted in higher erosion at a lower angle of attack from AtOx--i.e., in conditions closer to the normal incidence. Nevertheless, starting from the result obtained in the tests performed at the 0\({}^{\circ}\) attitude condition, the observed drop of AtOx erosion yield was not merely consistent with a typical tilt-angle behavior law, thus suggesting that a more particular effect of AtOx impact orientation on erosion should be envisaged. The erosion may also be increased by simultaneous UV irradiation. A modeling  
Figure 4: C/C sample clamping before the test (**left**) and picture after the test (**right**) for 0\({}^{\circ}\) (**a**) and 60\({}^{\circ}\) (**b**) attitudes; the area visibly affected by the AtOx irradiation is highlighted. Figure 3: C/C samples under AtOx/UV irradiation at different angles of attack with respect to the AtOx beam (the pictures were taken from the window just below the handle system for the holder rotation; see Figure 1). schematization was introduced in order to attain the following: (i) overall, to approach the issue of the AtOx erosion phenomena in generic conditions, and (ii) specifically, to give a physical interpretation of the experimental results obtained. The simplified relationships reported in Equation (1) can be directly arranged to retrieve a differential expression for the mass eroded as a function of time:  
\[\delta M(t)=\rho\cdot A\cdot E_{Y}\cdot f(t)\cdot\delta t \tag{2}\]  
where the generalized flux vs time relation \(f\) (\(t\)) shall be evaluated for the computation of the total mass eroded by the AtOx impacts between two instants of time (for example, the start and the end of a mission) \(t_{A}\) and \(t_{B}\), as:  
\[\Delta M(t_{A}\to t_{B})=\rho\cdot A\cdot E_{Y}\int_{t_{A}}^{t_{B}}f(t)dt \tag{3}\]  
In this framework, the erosion rate is correctly viewed as an intrinsic property of the material, while the surface inclination with respect to the AtOx fluence direction (i.e., the spacecraft motion) is directly included within the generalized function \(f(t)\).