What historically caused the famously bright, brief flares seen from certain satellite constellations like Iridium?

Answer

Large, flat antenna panels perfectly reflecting sunlight toward Earth

The Iridium constellation was historically known for causing incredibly bright, short flares when their large, flat antenna panels aligned perfectly to reflect sunlight directly toward observers on Earth.

What historically caused the famously bright, brief flares seen from certain satellite constellations like Iridium?

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