Satellites in orbit around the Earth can sometimes be visible from the ground, especially during certain conditions. However, there are several reasons why satellite lights may not always be visible:
Size and brightness: Satellites are relatively small objects compared to the vastness of space. While some satellites are designed with reflective surfaces or deployable solar panels that can make them appear brighter, they are still much dimmer than most stars and other celestial objects. The reflected sunlight from satellites can be easily overwhelmed by the brightness of the Sun, city lights, or even the Moon.
Altitude and atmospheric interference: Satellites orbit at various altitudes, and their visibility is affected by the Earth's atmosphere. The atmosphere scatters and absorbs light, especially at lower angles near the horizon. As a result, satellites in low-Earth orbit (LEO), which are closer to the Earth's surface, may be more prone to atmospheric interference and less likely to be visible.
Viewing conditions: Observing satellites requires certain conditions. They are most visible in dark, clear skies, away from light pollution. Additionally, the observer needs to know the satellite's position and the direction to look in order to catch a glimpse of it. Satellites can move quickly across the sky, so precise timing and location awareness are crucial for spotting them.
Orientation and light emission: Satellites are not typically designed to emit visible light intentionally. Instead, they primarily reflect sunlight. The angle between the satellite, the Sun, and the observer affects how much sunlight is reflected towards the observer. If the reflective surfaces are not facing the observer or the Sun is at an unfavorable angle, the satellite may not appear visible.
Satellite operational modes: Satellites often have different operational modes, including when they enter Earth's shadow during an eclipse or adjust their orientation to optimize communication or solar power generation. These factors can affect their visibility to observers on the ground.
It's worth noting that there are certain occasions when satellites can become more visible. For example, the SpaceX Starlink satellites, which operate in large constellations, are more noticeable shortly after deployment when they are closely clustered together. These clusters can appear as a line or "train" of lights moving across the sky, making them more easily visible to observers.