Question: We enjoy going out at night and watching the International Space Station (ISS) pass overhead. It's always such an incredibly bright object, and it's cool knowing people are up there. How do you know in advance exactly when and where to spot the ISS? And could I see any details through my binoculars? — DE, Algoma, WI
Answer: I'm also an avid ISS-gazer. The reason it's so amazingly bright is that the entire station is about the size of a football field, and many of its components (especially the solar panels) are highly reflective. As you can see in the animation, there's a "visibility window" just after sunset and just before dawn. An observer on the night side of Earth (the yellow dot is Algoma) can still see the ISS shining in the sunlight until it passes into Earth's shadow. Note that this view of Earth is from above the North Pole.
It should be noted this animation is not to scale. The ISS orbits at an average altitude of 380 km (240 miles), but that's still high enough to catch the sunlight long after sunset in Algoma. From the time the ISS first appears over your horizon, till the time it passes into Earth's shadow, it's visible on average for around 3 minutes. The maximum under perfect geometric conditions would be about 9 minutes.
The ISS is moving extremely fast: 7.7 km/s (4.8 miles/s). At that speed it circles the Earth every 93 minutes. So in the interest of simplicity, I didn't bother to show the Earth rotating. It's not really relevant to this discussion.
Also for simplicity, I chose to keep the ISS solar panels facing the Sun. The panels are mounted on gimbals that can rotate to adjust for anything from low drag mode (parallel to the ISS motion) to maximum power mode (perpendicular to the Sun). When they need maximum power, those panels can generate 84 kW of electrical energy.
To predict when and where to spot the ISS, I use Spot The Station. You'll find it on my Links page. It defaults to the ISS, but there's also a link to locate other satellites. Another nice online simulator is I.S.S. Tracker, which looks much like the main video display at NASA Mission Control.
Finally, you ask if binoculars would allow you to see more detail. Maybe — if you have good eyesight, clear sky, and can keep the binoculars focused on that rapidly moving dot of light. Using my 10x80 binocs, I was just able to make out the shape as rectangular, but not much more. This is a "football field" 380 km away, so the magnification afforded by typical binoculars (10x to 30x) just won't let you see much detail.
But if you have a telescope that can automatically track or time lapse the ISS, you can get some really nice details at higher magnifications. Arizona photographers Tom and Jennifer Polakis have some stunning images of the ISS passing in front of the Sun and Moon. Their website is also on my Links page.
For those who have never observed the ISS, I recommend you give it a try. Spot The Station makes it easy to know exactly when and where to look. Just enter your location, and you'll get dates, times, and directions.
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