## 2 Astrophysical Conditions
Each of these subsystems faces constraints due to the realities of space physics, including gravity and orbital mechanics, as well as radiation. Regarding the launch subsystem, to do anything in space, we must first overcome gravity forces, namely, escape from the Earth's gravity well. This issue was theoretically depicted by [PERSON] in 1903 in his rocket equation. Putting it simply, it reveals a fundamental engineering as well as economic constraint: rockets must carry their own fuel to generate thrust. However, the more fuel added, the heavier the rocket becomes, which creates some kind of a vicious cycle. In order to untie this Gordian knot, we need to use multi-stage rockets, which drop the first stage (and sometimes the second stage) with empty tanks to reduce the weight mid-flight of the final stage with the payload. The economic consequences are huge. In contrast to the on-Earth logistics, the payload fraction is extremely limited to only about 2-5% of the total vehicle mass. Although the fuel mass accounts for around 90% of the total rocket mass, it is not the crucial factor determining the huge launch cost [28; 29; 30; 31]. The most important thing is that the stages of the rocket are typically used only once. One could imagine the cost of the terrestrial logistics if it were needed to throw away trucks, trains, or planes just after reaching a destination once. This is why the goal of reusability pursued by the New Space companies is such a game-changer. Regarding the payload subsystem, satellites must contend with multiple environmental challenges once in space. Firstly, they have to stay firmly on an orbital track. Once reaching its target orbit, a satellite needs no further propulsion, while its speed depends on altitude. However, this refers to the ideal conditions of lacking drag. In LEOs, this condition is violated; the more, the lower the altitude of the particular orbit. Orbital decay caused by atmospheric drag in LEOs requires periodic propulsion to keep on track. Otherwise, one has to compromise on mission lifetime. Thus, there is an economic trade-off between the cost of securing the satellite with a propulsion system, and therefore its more complicated, expensive construction as well as higher costs of launching, versus shortening its operational life and therefore replacing satellites frequently, adding costs due to additional replenishments [25; 28; 29; 32]. An additional challenge is created by radiation caused by the Van Allen belts, which trap high-energy particles (mainly in MEOs), as well as by solar activity and cosmic rays from the galaxy (mainly in GEO), posing risks to solar panels and electronics, as well as to astronauts in the manned missions. Therefore, satellites require radiation shielding, which increases weight and cost. Moreover, the electronics and computer components used on satellites must be radiation-hardened, increasing cost. This is true mainly for MEO and GEO satellites. Recently, one could observe a tendency to use commercial off-the-shelf (COTS) electronics and components in LEO satellites. This makes them more fragile as regards the possibility of radiation damage. Therefore, using radiation-hardened components versus off-the-shelf electronics is a matter of economic trade-off between more expensive satellites and more frequent launches of satellites [25; 28; 29; 33; 34]. Regarding the ground-station subsystem, once satellites are operational, they must communicate with Earth. The challenge is to transmit large amounts of data over long distances without degradation and with the shortest latency possible. The first issue is the radio signal's attenuation (loss of power) due to the atmosphere. The second issue is the distance (an altitude is a key differentiating factor between orbits). From a communication point of view, the advantage of GEO is geostationarity (the satellite \"hangs\" over a point on Earth), which allows stable antennas.