Drag is a force acting opposite to the relative motion of an object moving with respect to a surrounding fluid. The higher the density of the fluid, the higher the magnitude of the drag.

Flat-Earthers claim that it is impossible for satellites to have such high speed. Usually, they would compare the speed of satellites to something like the SR-71 Blackbird, which holds the record as the fastest aircraft. They are wrong. At the altitude where satellites reside, the air density is several magnitudes smaller compared to near the surface, and thus, the drag experienced by satellites is also several magnitude smaller.

The reconnaissance aircraft SR-71 Blackbird has the operating speed of 3500 km/h. On the other hand, the ISS is moving at 27500 km/h relative to Earth’s surface, or about eight times greater. Flat-Earthers think this ‘does not make sense’. They claim the ISS should have been destroyed at such high speed, especially as it does not have an aerodynamic shape.

They forget about the air density. The air density at the cruising altitude of SR-71 is 0.04 kg/m³. On the other hand, the air density at the altitude of the ISS is only 0.000000000004 kg/m³. In other words, the density of the air surrounding the ISS is 10000000000× smaller than at the cruising altitude of the SR-71, and thus, the drag experienced by the ISS is also 10000000000× smaller. ISS does not need to have an aerodynamic design even though it is moving at 8× the speed of the SR-71.

Drag will cause the ISS to lose its altitude gradually. To deal with the problem, ISS need to be orbit boosted. Every a few months, there are supply missions to the ISS. The engine of the supply spacecraft can do orbit boost during the time it is docked with the ISS. Or it can be done by the ion thrusters of the ISS itself.

Lower altitude satellites operate in higher air density. For example, the GOCE satellite is operating about 2× closer to the surface than the ISS, and the air density is about 42× higher than at the altitude of the ISS. The GOCE satellite has an aerodynamic body to reduce drag, and thus reduce fuel consumption and increase lifetime. Unlike the ISS, there are no supply missions to this satellite. Once the propellant is spent, it would no longer be able to maintain its orbit and will enter Earth’s atmosphere.