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I really enjoy watching the International Space Station fly over head. I subscribe to alerts so I know when it is in my area during the night time. I wouldn't be able to see it if it weren't for the light. Perhaps the light is only so we can see it more easily. I'm not exactly sure why there is a light on it and I've searched for answers.

Why is there a light on the International Space Station facing us on Earth?

It's a very bright white light. It's not a blinking light for collision avoidance. It uses power, maybe not a lot but still a consideration.

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    $\begingroup$ Where do you subscribe to those alerts? It sounds great! $\endgroup$ – TMH Oct 13 '14 at 9:12
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    $\begingroup$ You can subscribe to ISS Alerts here: spotthestation.nasa.gov $\endgroup$ – Scott Oct 13 '14 at 14:00
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    $\begingroup$ "It's a very bright white light" - yup that's our sun alright!! $\endgroup$ – Fattie Oct 14 '14 at 10:09
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    $\begingroup$ en.wikipedia.org/wiki/Satellite_flare $\endgroup$ – Nick T Oct 15 '14 at 7:37
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    $\begingroup$ "It's not a blinking light for collision avoidance." You mean like dodging bullets? $\endgroup$ – a CVn Oct 15 '14 at 13:10
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There are a few positional lights on the visiting spacecraft to the International Space Station (ISS), also doubling as indicators that the visiting spacecraft docked to the station are powered and similar reasons. And the Canadarm2 has lights on it so it can be remotely / CCTV operated also when the station is in the Earth's shadow, and they do have lights inside the station so astronauts / cosmonauts can see, and some of this light would be reflected towards the Earth when someone is in the Cupola pointing towards nadir and its protective shutters are open. But none of these lights are nearly as powerful enough to see them from at least 410 km (254 miles) which is station's current orbital altitude and would be the minimum distance between you and the station when it orbits directly overhead from the observer, and most of them do blink.

What you can see from the ground is however this:

   enter image description here

What's perhaps not best known is that the station's solar panel arrays are double-sided to also collect some of the sunlight reflected off the Earth's albedo (what our planet reflects from the Sun). They generate roughly up to 120 kW of power (on average about 84 kW) needed by the station's large number of equipment, life support, experiments,... to function. It stores excess collected power in batteries for when the station doesn't generate electric power with its photovoltaics (that's where that average comes from, lower than its maximum output), but more importantly to your question, they're roughly the size of a U.S. football field:

                         enter image description here

                       Image source and credit: International Space Station - Facts and Figures

So what you see is indeed as @GWP mentions in his/her answer, even if that started as a rather vague one-line answer. How can we be sure? Simple. The station simply doesn't have sufficient power to run any such lights powerful and large enough to be seen from the ground, as even at 100% efficiency they would consume roughly the same amount of power that the station generates at its own photovoltaics efficiency of well under 50% (yes, they're due for an upgrade if they want to run it till 2024 and run even more experiments). The rest of the incident light from the Sun is mostly reflected, and some absorbed as heat that needs to be radiated to space. But you'd want as little of this heat absorption, since convective heat transfer doesn't really work in the near vacuum in the Low Earth Orbit (LEO), so the solar arrays use coating that matches its efficiency and reflects the rest in wavelengths it's not as efficient in converting incident sunlight into electricity. And if it can utilize less than 50% of what light it has available to it and reflects as much as possible of the rest,... well, you can do the maths.

The light you see is sunlight reflected off the station's enormous solar power arrays, and can only be visible when the observational conditions are dark enough that the bright spot of the station isn't lost in the sunlit background, and the arrays reflect sunlight towards you, so when the station is not in Earth's shadow. When the station is barely entering the Earth's shadow, and if it was visible before it, you can also visually observe it slowly fading into the shadow with your own eyes. That's another proof that it doesn't keep lights strong enough to be seen from the surface on, if the fact that there's absolutely no good reason to do that isn't good enough of a proof on its own.

Here are four frames from the station showing two Soyuz spacecraft docked to it entering the Earth's shadow as seen from one of the ISS HD Earth Viewing Experiment aft facing cams (click on images for larger versions):

enter image description here enter image description here enter image description here enter image description here

This happens about every 93 minutes on the station, or each time it completes one of its orbits. Except during the period of station's high beta angle when it might not enter Earth's shadow at all and its orbit remains more or less aligned with the Earth's day/night terminator for a few days or so. For more on all of this, read e.g.:

OK, so why white if the station's solar arrays reflect brownish-golden color on their own? Simply because of the intensity of reflected sunlight when compared to the darker background of dusk or dawn when the station can be seen with a naked eye. When the station is in the process of entering Earth's shadow and/or its beta angle (the angle between the Sun, the station, and the observer) is shallower, and the station's solar arrays are rotated to collect as much sunlight so towards the Sun, the amount of this reflected light will also be smaller, the light beam will also lose some of its intensity through the atmosphere through a process known as atmospheric diffraction (see the other linked question), and your eyes will be able to distinguish colors more precisely. It will also appear a bit more yellowish at those times.

To somewhat reaffirm my last point, consider this frame capture from the ISS HD Earth Viewing Experiment showing solar arrays on two Soyuz visiting spacecraft docked to the station and reflecting incident sunlight at different angles:

   enter image description here

As you can see, the solar panels are actually not white, but due to the intensity of reflected sunlight, the array of the foremost Soyuz spacecraft appears bright white. It's not exactly white as neither the average wavelength of the light emitted by our Sun by intensity is, and the reflected light does include color component of the materials used on the panels, but for all intents and purposes, it's close enough.

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  • $\begingroup$ If the solar panels are tracking the sun, when the ISS is passing the terminator they will be perpendicular to the ground below and will not reflect light in that direction. What would, are the aluminum radiator panels that cool the solar panels, which have an area of 156 m^2. They wouldn't be directly reflecting sunlight, though - they'd be bouncing back the light from the sunlit portion of the earth below them. I've struggled with this before and that's as near as i can figure it... $\endgroup$ – kim holder Oct 14 '14 at 15:54
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    $\begingroup$ @briligg It's not as simple as that because they rotate the panels to provide as much current as possible from both of its sides. IIRC they actually even made some algorithm competition or alike to solve that. But you're right, it's not just the solar panels that reflect light. The station's truss is also mostly white or otherwise reflective (anodized metal and such) so it would also reflect some of the sunlight. As for radiators, I think the angle would be too steep most of the time. Maybe when the station is low on the horizon to the west and at dawn (Sun to the east) it could. $\endgroup$ – TildalWave Oct 14 '14 at 16:05
  • $\begingroup$ Ooo that does sound like a complicated calculation, i like it. All the same i don't think the solar panels are the main contributors to the light most of the time. For the sake of clarity, i'm going to add an answer that shows some photos taken with telescopes from the ground, and quotes the Hayden Planetarium's commentary on the subject. $\endgroup$ – kim holder Oct 14 '14 at 16:26
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    $\begingroup$ I am surprised this has less votes than the #1 answer. I'd like to reward both of you. Maybe edit them together. $\endgroup$ – Scott Feb 15 '15 at 2:56
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It's not a powered light; what you're seeing is sunlight reflected from the solar panels. That's why you can only see it during near overhead passes around dusk and dawn - the sun has not yet "set" from the space station's altitude.

From http://nasa.gov/vision/space/travelinginspace/f_skywatch.html

The Space Station is one of the most visible man made objects in the sky, because it reflects sunlight and often looks like a slow-moving star.

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    $\begingroup$ @Scott That sounds very similar to the sun reflecting from the moon. It could even be exactly the same mechanism! $\endgroup$ – Gusdor Oct 13 '14 at 14:32
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    $\begingroup$ @Gusdor Good point. I would feel more confident that it is not a powered light with a reputable reference. $\endgroup$ – Scott Oct 13 '14 at 14:54
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    $\begingroup$ @Scott Hopefully this will be reputable enough. nasa.gov/vision/space/travelinginspace/f_skywatch.html The Space Station is one of the most visible man made objects in the sky, because it reflects sunlight and often looks like a slow-moving star. $\endgroup$ – Gusdor Oct 13 '14 at 15:00
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    $\begingroup$ You should do some more stargazing at dusk! In fact, quite a few satellites passing overhead at any given time, visible as a steadily moving 'star'. "At any given time, there are hundreds of satellites in the sky. Most of them are too faint to see, but if you're in an area without much light pollution, and you look carefully enough, there's virtually always a satellite visible. Their rapid motion across the sky and various highly inclined orbits make them unlikely to be anything but artificial." from what-if.xkcd.com/60 $\endgroup$ – Sanchises Oct 13 '14 at 16:18
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    $\begingroup$ @Scott: It's the sun. Source: I work on the space station. $\endgroup$ – Tristan Oct 14 '14 at 18:42
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The ISS is covered with highly reflective material to help regulate its temperature. It isn't so easy to radiate energy away in space, a vacuum is actually the best insulator known to science. So when sunlight hits the station the best thing is to prevent as much of its energy as possible from being absorbed, thus everything is bright white. There are also giant radiators on it for the same reason. They are mostly there to prevent the solar panels from overheating - because they are actually the only things on the ISS that aren't bright white, they are a dark coppery color. The radiators also help cool the rest of the station. Because the solar panels are angled for best capture of light, they rarely reflect much light towards they ground. For the most part, what you see from the ground is light reflected from the body of the space station and from the radiators.

There are a number of photos around online taken with telescopes of the ISS passing overhead near dusk or dawn. The one below is in the public domain and was taken by Ralf Vandebergh. In it, the bright sections are on the main truss and the hab modules.

Ralf Vandebergh ISS photo

And here are links to several more copyrighted ones: One from the European Space Agency

Another from Vandebergh that appeared in Wired

And several by Thierry Legault

When the ISS does reflect sunlight off the solar panels towards the ground, what is seen is a flare, and it can be very bright indeed. Here is what they say about it on the Hayden Planetarium website:

And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly "flare" in brilliance to as bright as magnitude -8; more than 16 times brighter than Venus!

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  • $\begingroup$ It would be nice to have a good measure of apparent magnitude of the ISS on one of these passes and use it to calculate how much light was being reflected towards the observer - if i could figure out how to do that. $\endgroup$ – kim holder Oct 14 '14 at 17:21
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    $\begingroup$ For ISS apparent magnitude best I could find is from Heavens Above: Intrinsic brightness (Magnitude) -1.3 (at 1000 km distance, 50% illuminated), Maximum brightness (Magnitude) -5.1 (at perigee, 100% illuminated). My own rule of a thumb is that it's about as bright as Venus (max –4.89) when "riding the day/night terminator" and about as bright as Mars (max –2.91) otherwise. Both of which also vary in time, that's why it's at least as correct as a clock that doesn't work - two times a day it's spot on. :) $\endgroup$ – TildalWave Oct 14 '14 at 17:28
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    $\begingroup$ There's one problem with what you suggest (that the existing photographs of the station from the ground fairly represent what parts of it reflect the most light). Namely that there's an intrinsic problem to taking stills / video of a brightly lit distant object with optical equipment due to atmospheric diffraction. When the station would be fully lit (or close to it) it would simply be impossible to get much detail of it from ground. So what photos of it you see online taken from the ground are mostly when it reflects less light and not the best representative of what we see with a naked eye. $\endgroup$ – TildalWave Oct 15 '14 at 13:26

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