How can I tell whether a straight trail in my night photo is the ISS or an airplane?

Asked 12/5/2017

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I photographed the southwest sky from Paros, Greece on 21/08/2017 around 2:00am EST and noticed a short straight trail near the lower-left part of the image. I wondered if it might be the International Space Station. Is there a reliable way to verify whether a trail like this is from the ISS, and based on the appearance of the trail itself, is it more likely to be something else such as an airplane?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

8y ago

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As the others answered, it's not the ISS, but I wanted to answer about the posibility of it being the ISS anyway.

At first I thought it can't be the ISS, because I like to observe the ISS at night and it's moving much faster than in your picture, but then I made some more precise calculations.

Your 18mm lens on a crop body has a 66°×47°; 76° (H×V; D) field of view. Your trail is 68 pixels long, and your image diagonal is 961 pixels, so your trail occupies 68/961 × 76° = 5.37°. At 30s exposure this means an average angular velocity of 0.179°/s.

Your trail is approximately 215 pixels over the horizon, and the image is 533 pixels tall, so 215/533 × 47° = 18.9° over the horizon. Let's assume the ISS is α=18.9° over the horizon relative to your position. Assuming the height of the ISS (h = 405km) is much smaller than the earth radius, some basic trigonometry put its distance relative to you to D = h/sin α = 1253km. Now its observed angular velocity depends on its motion relative to you, but let's go for the average scalar velocity, which is ωₐ = 1/(2π) * Integral[ω*|sin[x]|, 0, 2π], where ω is the best case (tangential motion) velocity. ω = v/D, where v = 7.76km/s, the orbital speed of the ISS, so ω = 6.19 × 10⁻³ rad/s. Evaluating the integral we get ωₐ = 3.94 × 10⁻³ rad/s. In 30s it would have moved `0.118 rad = 6.76°.

This number is comparable to your trail, which surprised me, because I know the ISS can move much faster. But of course, I observe it in the best circumstances, when it's directly above. If the ISS is directly above, D=405km and ω = 19.16 × 10⁻³ rad/s = 1.097°/s.

Originally by user49699. Source · Licensed CC BY-SA 4.0

user49699

8y ago

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AI Answer

Generated from our catalog & community — verify before relying on it.

It’s most likely an airplane, not the ISS.

Why: the trail appears slightly dotted/broken, which is typical of an aircraft’s flashing lights during a longer exposure. The ISS normally appears as a steady moving point of light, not a regularly flashing dashed line. It can brighten from reflections, but not usually in a consistent blink pattern like an aircraft.

A good way to verify this is to check a satellite-prediction site such as Heavens-Above. Set your exact location, date, and time, then look up past passes under “Daily predictions for brighter satellites.” If the ISS was not crossing that part of the sky then, the trail was something else.

You can also estimate from the photo: compare the trail length to your exposure time and field of view to get angular speed. If that speed doesn’t match a plausible ISS pass for that altitude and direction, that’s another clue it isn’t the ISS.

So based on the trail’s dashed appearance and the usual behavior of the ISS, airplane is the better explanation.

UniqueBot

AI

8y ago

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