Physics Question #37

Pierre Charbonneau, Eng., a 33 year old male from Jonquière, Quebec, Canada asks on July 24, 1999,

Is there actually a strobe light on the International Space Station? I saw in the sky a sequence of two closed flashes cycled on approximately ten seconds. As I have never seen a protocol like that in airplanes, I want to know if I saw a strobe light mounted on the ISS. A flight of the Space Shuttle passed over where I live in Jonquière, Quebec, Canada recently. On that in that mission, a small satellite mirror was launched - could I have seen that instead?

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The answer

Donald J. Barry answered on July 24, 1999

I do not know whether the ISS has a strobelight attached, but I suspect not. Even if it did, such would be very difficult to see from a distance of several hundred kilometers as you viewed it. The brightness of a satellite is due primarily to its illumination by the sun -- this light is enormous compared to a small lamp or strobe. Satellite flashes are usually due to flat solar panels or the like catching glints of the sun and reflecting them like mirrors to distant observers. As many satellites spin, this could be the cause of the phenomenon you observed. It is also possible that you were witness to a high altitude plane with an unusual strobelight pattern. The mirrored ball released by the shuttle during its recent flight produces a continuously blinking appearance and not the patterned protocol you witnessed.

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