If you have ever seen a car's headlights from a far distance, you might have noticed that they are not as bright as if they were only a few metres away from you. When the headlights are far away, your eyes are collecting less of the total light that is emitted by them. If you double the distance of the headlights from you, you will see one quarter of the previous brightness. This is known as the inverse-square law of light and it applies to all light sources in the universe, including light reflected off our satellites!

The light that you see from any satellite is simply reflected sunlight. You might already know that you can only see the closest satellites with the naked eye (several hundred kilometers away). The satellites that are located further away are dimmer and might require a visual aid, such as binoculars or a telescope.

For example: A satellite with a range of 40,000 kilometres would appear 10,000 times (100 squared) dimmer than an identical satellite at a 400 kilometre range.

The image below simulates the brightness of a large and highly reflective satellite at ranges of 500km, 1000km, 2000km, 4000km, 8000km and 16000km. The brightness of the 16000km range satellite is 32 times further and 1024 times dimmer than that of the 500km range satellite. Although the camera could see this hypothetical satellite for each range, there exists a range further out in which the camera would finally fail to detect the satellite.