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As we can see the Starlink satellites with the naked eye as they pass by. I was wondering about what makes them visible.

Do they have large mirrors that reflect the sunlight making them look as if they have lights on board or do they actually have lights.

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Starlink (and other satellites) categorically do not have exterior lights or illumination, that would be a waste of power for no particular benefit.

The reason that we can sometimes see satellites or other spacecraft at night is because the spacecraft is still exposed to daylight. Here is an image describing this phenomena:

Diagram of spotting satellites (image credit: Gary Meader; from the book "Night Sky With The Naked Eye" by Bob King)

Satellites and other orbiting bodies (ie the Moon) appear so bright because they're reflecting sunlight. Even though they aren't covered in mirrors, the sun is just so bright that white paint or the lunar surface is enough to let us see even tiny satellites from hundreds of kilometers away.

Occasionally, a satellite will "Flare" which is when the angle of the sun on its glass solar panels reflects directly to the observer. This reflection off the solar panels rather than simply the body of the satellite can appear much brighter temporarily.

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I'm going to answer a slightly different question from the other answer: "why are Starlink satellites so visible, compared to other satellites?"

The answer is that they're very close to the earth; the lower edge of what might be considered a viable long-term orbit is 300 km, and some have recently been deployed at 280 km. They are boosting to a higher altitude, and once in their final orbit shouldn't be as visible.

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  • $\begingroup$ There are several different altitudes planned for the fullest implementation of Starlink, but the ones launched in 2019 are targeting only about 550 km 1, 2. They used to talk about 1100 to 1300 km but then reconsidered, because of the $1/r^2$ advantage of being closer. $\endgroup$ – uhoh Dec 21 '19 at 5:06
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    $\begingroup$ The difference in visual brightness between 300 km and 550 km is only about 1.3 magnitudes ($2.5 \log_{10}((550/300)^2)$ so I don't think that "once in their final orbit shouldn't be visible to the naked eye" is true. Also remember that part of the hoopla is about their impact on ground-based astronomical observations, though messing up the night sky for all of humanity is a serious consideration as well. $\endgroup$ – uhoh Dec 21 '19 at 5:07
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    $\begingroup$ I was going by the article: "where they will still be visible to binoculars and telescopes". $\endgroup$ – user3757614 Dec 21 '19 at 7:03
  • $\begingroup$ edit looks great, thanks! $\endgroup$ – uhoh Dec 23 '19 at 8:58
  • $\begingroup$ In a recent webcast, SpaceX stated that the difference in visual brightness once the satellites reach their operational altitude comes primarily from a subsequent change in the orientation of the satellites' solar panels, not from the additional distance created by the new, higher orbit. (Source) $\endgroup$ – Ajedi32 Jan 21 at 15:52

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