## 2 Astrophysical Conditions
However, the GNSS (Global Navigation Satellite Systems) satellites are the exception [10; 20; 22]. Geosynchronous orbits have a period equal to the stellar day (therefore, they are characterized by the lowest satellite velocity). The key type of geosynchronous orbit is the Geostationary Orbit (GEO), which is an equatorial geosynchronous orbit located at an altitude of 35,786 km. Because it has an angular velocity equal to the velocity of the Earth, it gives the impression of a satellite being suspended over a single point above the Earth (above the equator, at a given longitude). This is a convenient property from the point of view of telecommunications. GEO offers very high coverage of the Earth, as it is visible from 43% of its surface [10; 20; 23; 24]. Each of the aforementioned types of orbits offers a specific mix of properties that determine their varying suitability for a variety of applications in fields such as Earth observation, telecommunications, navigation, scientific research, and military applications. LEO satellites circle the Earth in roughly 90 to 105 min, making them well-suited for quick revisit times in Earth observation. Although the residual atmosphere creates drag and shortens orbital lifespans, the proximity to Earth means smaller, lighter equipment can be used, often enabling nano- and microsatellite designs. LEO's accessibility--characterized by the broadest availability of launch providers and the lowest cost per kilogram of payload--fosters a dynamic market. Typical applications include intelligence and surveillance (e.g., Planet Labs - U.S., HawkEye 360 - U.S., ICEYE - Finland), large telecommunications constellations for low-latency internet access (Starlink - U.S., OneWeb - UK), and manned space stations (ISS, Tiangong). MEO satellites must contend with increased radiation exposure compared to LEO. However, these altitudes host key semi-synchronous orbits that fulfill global navigation needs; major GNSS constellations like American GPS or European Galileo reside here. Certain telecommunications networks, such as SES, also operate in MEO to balance global coverage with moderate signal latency. GEO satellites experience heightened exposure to solar wind, demanding robust spacecraft shielding. The principal advantage is vast coverage--one satellite can \"see\" more than a third of the Earth's surface, which is ideal for telecommunications, especially satellite television and broadband internet (however, latency is becoming a problem regarding increasing expectations of internet users). Yet, the requirement for powerful transceivers and large onboard antennas increases satellite size and cost [10; 19; 25]. The source of the Sun's energy is the thermonuclear reactions in our star, involving the fusion of the nuclei of lighter elements into the nuclei of heavier elements, namely the fusion of hydrogen nuclei into helium nuclei [26]. In addition to emitting electromagnetic waves in the full spectrum (including visible light), the Sun is the source of plasma emission in the form of the solar wind and the coronal mass ejections (CMEs) [27]. On the other hand, space is penetrated by cosmic rays from the galaxy. On the Earth's surface, we are secured against this radiation by the magnetic field and the atmospheric layer. However, any payload launched into space has to deal with those challenges. The radiation effects of these particles can not only cause degradation of the surface of satellites or other spacecraft but also failure of electronic systems inside them. The harsh physical conditions of space impose significant challenges that affect every aspect of space-based operations. To understand these challenges, it is reasonable to use a system approach to those operations. Accordingly, they could be divided into three broad subsystems: launch subsystem (getting assets into space and putting payload in the desired place), payload subsystem (spacecraft dedicated to operating in space, mainly satellites), and ground-station subsystem (communicating with space assets) [28].