They're small but they're also fairly close to Earth. If you're talking about "iridium flares" (satellites visible briefly at certain locations) it's because all of the sunlight is being reflected onto a relatively small area of the Earth: https://en.wikipedia.org/wiki/Satellite_flare

I believe most satellites are still not visible to the naked eye most of the time.

High contrast when they reflect the sun against a pitch-black background, plenty of photons for our eyes to pick up.

There's heaps that you don't see though because they're simply reflecting sunlight in a direction other than towards your eyes.

They can still be a problem for long exposures though, since their rounder parts still reflect a little sunlight in most directions.

A lot will depend on your light pollution levels. The lower the bortle level, the better opportunities you have. I live in a bortle 3/4 and can see plenty of satellites throughout the night

You can see light from stars that are light years away from earth

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