What I Watched Last Night: The International Space Station

No, I didn’t go see Gravity. I watched the actual International Space Station fly by my house; it’s become part of my daughter’s bedtime routine over the last couple of weeks. And it’s awesome! I tried to spot it a few times over the summer, but the long daylight hours limited my opportunities to see it (and there was no way I was keeping her up that late). Now that it’s getting dark earlier, it’s in a position for us to watch it almost every evening before the kiddo goes to bed. She’s a little bit obsessed with saying goodnight to the astronauts, and I love it. “Mommy, it’s a freakin’ space station!” Yes it is, kiddo.

How in the world can such a small satellite be seen from Earth, much less with the naked eye? The ISS is powered by enormous solar panels that, due to its altitude, reflect sunlight for a few hours after sunset and a few hours before sunrise. While the habitable space with crew living quarters and laboratories is only a bit larger than an average-sized 5-bedroom house, the solar array makes the total size about the same dimensions as a football field. That means it reflects a lot of light; it’s the brightest man-made object in the sky and is about as bright as Venus during normal conditions. If the arrays are lined up just right to reflect sunlight directly at your location, it can “flare” with 8-16 times the brightness of Venus. If you know exactly where to look, you can even spot it when the sun is up, though it’s a lot trickier. From personal experience, it’s about as bright as a fairly low-flying aircraft, but you can tell the difference because the ISS doesn’t blink.

There are a few different ways to find out when the ISS will be visible from your location. NASA launched a “Spot the Station” program in November 2012 that allows you to choose your location and receive email alerts about 12 hours before it will be visible. However, it only sends out alerts if the station will be at least 40° above the horizon, so you may miss some viewing opportunities if this is the only service you use. I also use a free app called ISS Spotter that lets you set alerts at altitudes down to 10° above the horizon. It’s also helpful because you can track the station on their map to get a better idea of exactly where to look for the station. (The NASA emails tell you in which directions the station will appear and disappear, but it’s easier for me to visualize from the map.) There are also many other apps and websites that will give you similar predictions for the ISS and other satellites that are visible from Earth. AstroViewer has a cool ISS page that shows updated satellite imagery every second to show you exactly where the satellite is passing over, along with a world map to show the groundtrack of the current full orbit and that same orbit positioned on a globe so that you can see why it looks like the station is moving along a wavy line. Its orbit is inclined 51.6° off the equator to ensure that it could be reached not only by shuttles (formerly) launched from the US but also by Russian spacecrafts (which means that if you live in the far north, the station will never pass directly overhead and may not even be visible at all).

Of course, if you want to watch the station, you need to pay close attention to the time it’s supposed to pass over, because it moves fast. The station’s average speed is 17,227 miles per hour, meaning it completes an orbit in just over 90 minutes. Depending on the exact path past your location, it’ll probably only be visible for 2-6 minutes. I live about 90 miles north of NYC; if I check the location tracker on the app as I’m putting my shoes on five minutes before I go out to watch the station, it’s usually just crossing over Mexico. That’s a long way to travel in a very short time! It’s also amazing to see just how far away the ISS can be relative to the ground and still be visible. On a lot of nights we see the ISS on the northernmost part of its orbit as it starts to swing southward. At that point it’s roughly crossing the southern tip of Hudson Bay, about 750 miles north of me. It’s not very high in the sky at that point, barely skimming the top of the house across the street, but that’s just astonishing. This ISS orbits at an average altitude of 205 miles (but can go as high as 255 miles), which is why it’s visible from so far away.

Have you ever watched the ISS fly over? It’s a surreal, peaceful, and humbling experience, and I highly recommend it.