APOD: Saturn and the ISS (2022 Jul 09)

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APOD: Saturn and the ISS (2022 Jul 09)

Post by APOD Robot » Sat Jul 09, 2022 4:05 am

Image Saturn and the ISS

Explanation: Soaring high in skies around planet Earth, bright planet Saturn was a star of June's morning planet parade. But very briefly on June 24 it posed with a bright object in low Earth orbit, the International Space Station. On that date from a school parking lot in Temecula, California the ringed-planet and International Space Station were both caught in this single high-speed video frame. Though Saturn was shining at +0.5 stellar magnitude the space station was an even brighter -3 on the magnitude scale. That difference in brightness is faithfully represented in the video capture frame. In the challenging image, the orbiting ISS was at a range of 602 kilometers. Saturn was about 1.4 billion kilometers from the school parking lot.

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Re: APOD: Saturn and the ISS (2022 Jul 09)

Post by Ann » Sat Jul 09, 2022 4:34 am

Look Ma! One is bright and one is faint! One has rings and one has wings!

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orin stepanek
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Re: APOD: Saturn and the ISS (2022 Jul 09)

Post by orin stepanek » Sat Jul 09, 2022 1:11 pm

ISS_Saturn_TGlenn.jpg
Nice catch Tom!
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Re: APOD: Saturn and the ISS (2022 Jul 09)

Post by De58te » Sat Jul 09, 2022 3:46 pm

Yes, great shot Tom Glenn. Just out of curiosity, is Tom Glenn any relation to astronaut John Glenn?

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Re: APOD: Saturn and the ISS (2022 Jul 09)

Post by VictorBorun » Sat Jul 09, 2022 11:46 pm

Saturn is 10 a. e. from Sun and the ISS is 1 a. e. from Sun.
Sun from Saturn looks 10 times smaller in diameter and 100 times smaller in area.
However in the video frame posted Saturn's pixels are 60/255 gray and the ISS's pixels are 255/255 white.
So the ISS is just 4 times brighter.
And solar panels are 60/277, no brighter than Saturn.

I think: the ISS pixels are saturated and solar panels are quite black, 100 times darker than Saturn's white.
As black as almost whole mirrors reflecting the starfield

Update. Come to think about it, could the ISS be partialy shadowed?
I think normally we don't notice satellites when shadowed; they must be colored and changing the color to red and dark red, but be much dimmer — like Moon looks red at an eclipse, but much dimmer, too.
Tom Glenn however was not taking chanced; he used a tracking software. Could he captured a shadowed moment?

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Re: APOD: Saturn and the ISS (2022 Jul 09)

Post by alter-ego » Sun Jul 10, 2022 11:48 pm

VictorBorun wrote: Sat Jul 09, 2022 11:46 pm ...
Update. Come to think about it, could the ISS be partialy shadowed?
I think normally we don't notice satellites when shadowed; they must be colored and changing the color to red and dark red, but be much dimmer — like Moon looks red at an eclipse, but much dimmer, too.
Tom Glenn however was not taking chanced; he used a tracking software. Could he captured a shadowed moment?
ISS is not in the shadow.
Per Heavens Above and the "challenging image" link, it exited the shadow at 4:25:06am PDT (1m 17s before capture). I used the site coordinates that Tom listed at the link.
 
ISS Orbit - Position Not in Shadow.jpg
 
The visible event UTC timings are:
 
ISS Event Timings_Heavens Above.jpg
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Re: APOD: Saturn and the ISS (2022 Jul 09)

Post by Chris Peterson » Mon Jul 11, 2022 1:55 am

VictorBorun wrote: Sat Jul 09, 2022 11:46 pm Update. Come to think about it, could the ISS be partialy shadowed?
I think normally we don't notice satellites when shadowed; they must be colored and changing the color to red and dark red, but be much dimmer — like Moon looks red at an eclipse, but much dimmer, too.
Not sure if you mean here, or in general. I don't think there are any fully shadowed satellites we could observe from the ground. But partly shadowed ones are observed all the time. Indeed, virtually every ISS pass is partly shadowed at one or both ends, and it's very apparent visually.
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Re: APOD: Saturn and the ISS (2022 Jul 09)

Post by Tom Glenn » Thu Jul 21, 2022 6:32 am

Thanks for all of the comments! Below are a few answers that may be of interest.
De58te wrote: Sat Jul 09, 2022 3:46 pm Just out of curiosity, is Tom Glenn any relation to astronaut John Glenn?
No relation, just coincidental names!
VictorBorun wrote: Sat Jul 09, 2022 11:46 pm However in the video frame posted Saturn's pixels are 60/255 gray and the ISS's pixels are 255/255 white.
So the ISS is just 4 times brighter.
And solar panels are 60/277, no brighter than Saturn.
The tonal values you quote above are correct, with the solar panels and Saturn both at about 60/255, and the brightest parts of the ISS saturated at 255/255. However, the image is not presented as linear (most images are not) and so the relationship between 60 and 255 is not 4x. Instead, a nonlinear curve (gamma of approximately 2.2) was applied globally to the image. This means that the difference between 60 and 255 is actually about 23x. Interestingly, the difference in magnitudes between the two objects would imply an approximate 28x difference in brightness, but that assumes point sources, which these are not. In fact, the ISS is very tricky to expose because it has inconsistent specular highlights that vary considerably from pass to pass. I estimate it was overexposed by about 1 stop (2x) here, so the difference in brightness between Saturn and the ISS as observed in the image is probably about 46x difference. The image had to be exposed for the ISS, which leaves Saturn almost invisible in the raw linear file, but it was revealed upon the gamma transformation, as shown in the comparison set of images below.

Linear data:
ISS-Saturn-TGlenn-linear.jpg
Gamma transformed data
ISS-Saturn-TGlenn-gamma.jpg
VictorBorun wrote: Sat Jul 09, 2022 11:46 pm could the ISS be partialy shadowed?
The ISS was completely out of Earth's shadow. You may be thinking of images in which you can see the ISS exhibiting shadows that are cast by the spacecraft itself, which are quite common, and very interesting to observe.
alter-ego wrote: Sun Jul 10, 2022 11:48 pm
ISS is not in the shadow.
Per Heavens Above and the "challenging image" link, it exited the shadow at 4:25:06am PDT (1m 17s before capture). I used the site coordinates that Tom listed at the link.
You are correct, but out of interest for readers I would point out that when retrospectively analyzing orbits using Heavens Above, the results become less accurate as time passes because the program uses the current orbital elements (at the time of your query) to produce the charts, and those are only accurate near the epoch. So even in your star chart, which was produced well after the event, we can see some slight discrepancies with respect to both position and time for the ISS if we compare to the chart I produced at the time of the event.
Heavens-above.jpg
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