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Wikipedia explains Iridium and other satellite flares very well. The Iridium reflective surfaces are at very particular angles and can flare up to magnitude -9.5, however other satellites don't exceed -5 mag.

I know that solar panels are not as reflective as the Iridium antennae, but there are some huge solar panels up there that I'd expect to reflect higher than that. This report from 2009 claimed a -8 mag flare from the ISS, but looking at the photos, I'm not sure it was that bright.

Soon there won't be any Iridium satellites up there and we'll be back to watching the odd little damp squib.

T.J.L.
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Dave Gremlin
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    If solar panels are directed to the Sun for maximum power, there is no reflection of sunlight down to the surface of Earth. So if a little light is reflected, it goes back to the Sun. Reflectivity of solar panels is very small, otherwise the efficiency would be reduced. – Uwe Mar 16 '19 at 15:40

1 Answers1

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The antenna panels caused bright flares due to a combination of two factors:

  1. antenna panels were oriented at a (roughly) 45 degree angle with respect to the nadir. This positioned them perfectly to reflect sunlight down on to the Earth at dawn and dusk, when the spacecraft were in sunlight but it was getting dark on the ground. Solar panels are usually articulated or rotated to face the Sun as the spacecraft rotates to face the Nadir. This means they are not likely to reflect towards the Earth very often.

Iridium flare GIF GIF Source

  1. Iridium antenna panels are flat and metallic shiny; highly reflective an specular. A huge amount of work and cash has gone into minimizing reflectivity of solar panels because their job is to collect and absorb sunlight.

I don't think there will ever be other satellites with these wide, flat Iridium antenna panels again. They are really characteristic of the old Iridium satellites.

The first images with the shiny metallic surfaces are the old Iridium antenna panels taken from What goes into an Iridium Flare prediction model besides the ephemerides?

The last image is the new Iridium Next satellite and they've moved the antenna array under the body of the spacecraft and it's no longer shiny and reflective. That's from this answer to the question How many communication antennas does a satellite need if it belongs to a constellation?

Old and Busted (original Iridium)

below "View of one of the Main Mission Antenna. The hinged base is on the right side." Cropped. From satobs.org/iridsat.html.

old Iridium antenna panel

below cropped image of a first generation Iridium satellite, showing a bumpy and diffusely reflecting metal antenna panel of the type responsible for producing the flares seen on Earth. From wikimedia.org.

old Iridium antenna panel

New Hotness (Iridium Next)

Iridium Next

above: Conceptual drawing of Iridium-NEXT from harris.com/solution/hosted-payload-solutions.

uhoh
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