# Carbon Nanocomposites in Aerospace Technology: A Way to Protect Low-Orbit Satellites
## 1 Introduction  
Earth orbits are mainly classified into very low earth orbits (VLEO), low earth orbits (LEO), medium earth orbits (MEO) and high orbits. VLEO orbits are the closest (starting from approximately 100 km) to the Earth, while LEO are currently the most populated orbits located at an altitude range spanning 200 to about 2000 km [1,2] (Figure 1). The LEO satellites have high velocity (>25,000 km/h) because of this low altitude, and it takes them less than two hours to complete a single orbit around the Earth [3]. LEO can be used for communication by networking multiple satellites together [4,5]. These networks are also known as LEO constellations. Iridium, Globalstar and Starlink are examples of such satellite networks [6]. Due to the shorter distance between the ground stations and LEO, the signal latency is comparatively low (5-10 ms) [7]. Transmission power of LEO satellites can also be kept low due to this short distance. Another advantage of LEO is lower launching costs due to a significantly lower impulse needed to launchsatellites to lower orbits. Other uses for LEO include the global positioning system (GPS), space environment studies, space stations and technology demonstrations. The growing commercial space economy is particularly heavily concentrated on LEO, with the proposed expansion of Starlink constellation aiming to deliver up to 42,000 communication satellites for internet-based services [8]. The use of VLEO satellites offers even greater advantages as compared with LEO spacecraft and, for this reason, there is currently an increasing tendency for satellite constellations to occupy lower orbits. To compare, the Iridium telecom constellation (maiden launch in 1997) utilizes the orbits of near 870 km, while SpaceX plans to lower the orbit of newly launched satellites to 340 km. EOI Space plans to occupy an even lower shell, designing its \"Stingrays\" satellites for ultra-high-resolution imagery, and making them capable of operating in a relatively dense atmosphere [9]. In the European Union, a \"Horizon 2020\" program for \"radical redesigning\" of satellites is underway, with the aim of targeting lower orbits [10]. Indeed, very low orbits offer specific advantages including (i) smaller, cheaper optical systems for ultra-high-resolution imagery; (ii) enhanced resolution for the available optical systems; (iii) lower requirements for power for data transmission, (iv) higher signal-to-noise ratio; (v) the possibility of using aerodynamic surfaces for manoeuvring [2]; and (vi) of specific importance, a significantly lower latency in data transfer between satellites and the Earth. Low signal delay enables many important features that can not be readily realized when using high-delay networking. However, nothing comes for free: low orbits are located in the near-Earth layer where the residual atmosphere is much denser compared to higher orbits (Figure 1a). This presents two key problems for the use of low-orbit satellites--the velocity loss due to atmospheric drag and the erosion of materials under action of residual atmosphere. Nevertheless, LEO remains the most occupied orbital range for satellites (Figure 1b allows to directly compare the distribution of satellites by altitudes and the density of residual atmosphere). Apart from other low orbits, there are two more important types of orbits that need to be traced through the high-density atmosphere: the polar and near-polar orbits, and the so-called super low perigee orbit, SuLPO. The polar and near-polar orbits are of great importance, as they allow for Sun-synchronous orbiting, when the satellite passes any specific point at the Earth surface at the same local time. This is imperative for many applications but requires very low orbiting for the precise synchronization. However, along with the higher residual gas density, low polar orbits face one more challenge, namely a significant change in the atmospheric density at the North and South poles, due to the Earth's magnetic field.