What caused the famously brilliant, short-lived flashes associated with older Iridium communication satellites?

Answer

Their large, flat antenna panels perfectly angled sunlight toward the observer

Specific older satellite constellations utilized large, flat panels that, when oriented correctly relative to the sun and the ground observer, focused a concentrated beam of reflected sunlight downward.

What caused the famously brilliant, short-lived flashes associated with older Iridium communication satellites?

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