Visibility is a measure of the distance at which an object can be clearly discerned. Visibility depends on the transparency of the air. There is no air in space to disrupt the transmission of light, and therefore, light can travel very far in space practically undisturbed.
The ISS can be seen clearer than objects on Earth’s surface far closer than the ISS. Flat-Earthers use the fact as “evidence” that the ISS is much closer. In reality, only a fraction of the light path from the ISS travels through the dense lower part of the atmosphere that will significantly affect the transmission of light.
On the other hand, light emitted by objects on Earth’s surface has to travel the entire path through Earth’s atmosphere.
One of the farthest observations of an object on Earth is Pic Gaspard, France, from the Finestrelles, Spain, at 443 km, but the resulting photograph is less than stellar. The ISS is at a similar distance, yet we can photograph the ISS and get a far clearer result. The reason is that only the lower part of Earth’s atmosphere is dense enough to degrade the quality of light transmission significantly. And the light from Pic Gaspard to the Finestrelles travels its entire path in the lower atmosphere. On the other hand, only a small fraction of the path of light from the ISS travels through the lower atmosphere.
References
- Visibility – Wikipedia