How can we see the ISS?

[Phlogiston]

I have a friend who I was trying to explain why we can see the ISS. Unfortunately though I don't have a good enough physics background to adequately answer his question, which was:

"Why can I not see a plane once it reaches about 80 km away, yet I can see the ISS, at over 250 km away"

This is my rather un-academic and probably wrong answer:

You cannot always see the ISS very well. Some times it's very dim. The occasion above was 10 mins after last light. In the Western sky it was very bright but as it went East it became dimmer as the amount of light it was getting reduced. This is what differentiates it from an aircraft. Aircraft at night have flashing lights and so have a near constant luminosity. During the day there is a lot more light, and so the atmosphere diffracts a lot of that light, and it gets mixed up with the "noise", especially at the extremes of view, similar to how you cannot see stars during the day. Bare in mind also that the ISS is a bright object. Its almost completely white with reflective metals or panels on it, While planets are not.

Can anyone help me provide a better, more academic, yet understandable answer.

Kind regards

 

[skystare]

That's a good straightforward answer. Another factor is that to see a plane 80 km away requires looking more or less horizontally through 80 km of fairly dense atmosphere, whereas looking straight up the air density drops off rapidly, and the amount of dust, haze, and water vapor even more so as they're mostly confined to the lowest 10 or 15 km.
The ISS is actually so bright under full solar illumination that it can be seen in broad daylight if you know where to look. There's a website called Heavens-Above that shows how to spot an assortment of satellites as well as comets and whatnot.

 

[Phlogiston]

That's a good straightforward answer. Another factor is that to see a plane 80 km away requires looking more or less horizontally through 80 km of fairly dense atmosphere, whereas looking straight up the air density drops off rapidly, and the amount of dust, haze, and water vapor even more so as they're mostly confined to the lowest 10 or 15 km.
The ISS is actually so bright under full solar illumination that it can be seen in broad daylight if you know where to look. There's a website called Heavens-Above that shows how to spot an assortment of satellites as well as comets and whatnot.

Thank you I didn't consider that. But now it seems obvious.

 

[dkotschessaa]

That's a good straightforward answer. Another factor is that to see a plane 80 km away requires looking more or less horizontally through 80 km of fairly dense atmosphere, whereas looking straight up the air density drops off rapidly, and the amount of dust, haze, and water vapor even more so as they're mostly confined to the lowest 10 or 15 km.
The ISS is actually so bright under full solar illumination that it can be seen in broad daylight if you know where to look. There's a website called Heavens-Above that shows how to spot an assortment of satellites as well as comets and whatnot.

The NASA app does this as well and will send you alerts when things are going to show up in your area.

-Dave K

 

[NTL2009]

Isn't contrast and height above the horizon also a factor? A plane is low enough that it can't be in a dark sky while still illuminated by the sun below our horizon. The sun would still be lighting the sky.

I seem to recall seeing planes brightly lit by the reflection of the setting sun, but the sky is still fairly bright, so not a lot of contrast.

And a +1 to the heavens above site. Great for tracking Iridium flares too!

 

[skystare]

Of course, yes, that's probably the greatest factor. Against the black night sky I'd bet that even a little Cessna at ISS height would be a visible spark.

 

[Jamison Lahman]

Isn't contrast and height above the horizon also a factor? A plane is low enough that it can't be in a dark sky while still illuminated by the sun below our horizon. The sun would still be lighting the sky.

I seem to recall seeing planes brightly lit by the reflection of the setting sun, but the sky is still fairly bright, so not a lot of contrast.

And a +1 to the heavens above site. Great for tracking Iridium flares too!

Agreed. The total amount of light reflected by the airplane is similar to the amount reflected by the sky during the day while the ISS reflects a lot more light compared to the night sky. It is similar to why it is hard looking out of a window at night in a room with the lights turned on.