# Carbon Nanocomposites in Aerospace Technology: A Way to Protect Low-Orbit Satellites
## 2 Corrosion Factors and Mechanisms in Low Orbit Environment
### LEO Environmental Challenges and Material Requirements  
The harsh conditions of the low orbit environment typically lead to premature mechanical, thermal and chemical degradation of the spacecraft materials. The effects from space radiation (cosmic rays, solar wind, UV), thermal cycles, vacuum environment and debris need to be considered [26; 27]. Space debris materials and natural micrometeoroids also represent a serious hazard to the external surfaces of spacecrafts, since the relative velocity of these materials can reach tens of kilometres per seconds. Collisions with these space debris could be responsible for complete failure or malfunction of the entire satellite or its subsystems [28]. An example of such damage is shown in Figure 4.  
The UV radiation at low orbit contains two components, namely the vacuum UV at the wavelength of 100-200 nm and the near field UV in the 200-400 nm range. About 8% of the total solar radiation is due to the UV radiation and it causes measurable chemical  
Figure 3: Residual atmosphere at VLEO inside and outside of a satellite. (**a**). Intake device collects the rarefied gas and supplies it to a thruster. The system can also include a thermalization chamber between the intake device and the ionization chamber of the thruster (not shown in this scheme). Reprinted with permission from [PERSON] et al., 2022, [23]. (**b**) Pressure distribution in the intake device at an altitude of 150 km. A case when the focal point of parabola is located inside the discharge channel of the thruster. The pressure rises significantly in some areas around the satellite part, and oxygen could cause significant damage, in particular in hot areas of the intake devices and thrusters. Reprinted with permission from [PERSON] et al., 2021, [24]. External geometry of VLEO satellites also needs to be optimised to decrease drag. At the VLEO altitude range, the gas is rarefied and the free molecular flow should be considered, when the mean free pass of particles exceeds the size of a satellite. (**c**) Illustrates the three types of optimised shapes (marked ‘1’, ‘2’, and ‘3’) of spacecraft, and (**d**) shows the simulated pressure distribution around the body for the typical VLEO conditions. A significant increase in the particle densities over some areas could cause enhanced material damage. Reprinted with permission from [PERSON] et al., 2022, [25]. © 2022 IAA. Published by Elsevier. and mechanical changes in spacecraft materials [29]. UV radiation effects include bond breaking in functional groups such as C-C, C-O, etc., modification of polymers, mass loss of materials by creation of volatile fragments and others. Radiation levels at LEO are around 300 \(\upmu\)Gy and it could go up due to solar events [30]. These effects lead to material degradation and loss of their initial thermal, electrical and mechanical properties [31]. Thermal cycling is one more problem intrinsic to orbital environment; yet, for low orbits, the thermal cycle takes place within a few hours. Thermal fatigue due to this repeated rapid thermal stress ultimately degrades the mechanical properties of spacecraft materials. Therefore, thermal cycling is an important factor to be consider on material selection for LEO environment. Ionizing radiation of the LEO environment is mainly due to galactic cosmic rays, solar activity and radiation belts. This includes alpha particles, high energy beta particles and gamma radiation. Incident ionizing radiation may affect material properties and often results in degradation effects via ionization, atomic displacement, bond breaking, etc. The intensity of these adverse effects depends on the type of radiation, energy and dose. _AO_ (atomic oxygen) is the most abundant gaseous species in LEO which is formed due to photo-dissociation of molecular oxygen at low earth altitudes (Figure 1a). The recombination of dissociated oxygen atoms is unlikely due to the high energy environment and longer mean free paths [33]. Other species include high energy protons (H nuclei), nitrogen, oxygen molecules and argon.