The annual International Space Station marathon viewing season begins later this week, when skywatchers in the Northern Hemisphere can watch up to five ISS passes in one night.
I've seen the International Space Station (ISS) pass over my house a hundred times yet never tire of the sight. Inside that bright light, a crew of several astronauts looks earthward with the same sense of wonder. Now in its 21st year in orbit, the ISS is the brightest, most recognizable satellite in the sky. Few naked-eye sky sights elicit more wows at public star parties than the Venus-bright "star" speeding through the constellations.
Thanks to the steep 51.6° inclination of its orbit, the station passes overhead for anyone living between latitudes 51.6°N and 51.6°S. It's visible at least 10° above the horizon between 63°N and 63°S as well, guaranteeing that nearly every person on Earth has a chance to see it.
ISS "viewing seasons" are cyclic, with each lasting several weeks. You might first spot it in the evening sky, very low in the south, as it makes a single pass a night. The altitude and number of passes increases nightly and then tapers off, to be followed by a period of daytime-only passes until it reappears at dawn. From dawn the station transitions back to the evening sky to begin a new cycle.
Some mornings or evenings we see only a single appearance, but if you're diligent you can often catch two: a long, bright pass preceded or followed by a fainter partial pass. "Partials" occur when the space station glides into in the cover of Earth’s shadow and fades from view.
Like the Moon does on occasion, the ISS passes into eclipse, but does so routinely because our planet's shadow covers much more sky at the station's 406-km altitude. The ship's orbital period is also more than 400 times shorter than the Moon's, so it's in and out of shadow 16 times a day.
Watching the ISS move into eclipse makes for exciting viewing. Have a pair of binoculars at the ready to watch the ship fade and redden as it slides into Earth's shadow. From the perspective of the astronauts, the Sun is setting at the station, so the ISS takes on a sunset hue for a few seconds as it races into shadow.
Each year within a few weeks of summer solstice in either hemisphere, the space station’s orbit and Earth’s day–night terminator nearly align. For a few days and nights the Sun never sets on the ISS. Like seeing the "midnight Sun" at the Arctic Circle on the first day of summer, astronauts and their craft bask in sunlight 24/7. The Northern Hemisphere season happens between May and July; southern observers get their turn between November and January. This year's brief season begins on Friday, May 17th, and wraps up on the 20th.
In late May–early June near the summer solstice, the Sun doesn’t set on the International Space Station
Depending on your location you may see up to five successive passes of the ISS during that time. Go out at dusk to catch the easy early ones and stay up till dawn for the rest. Someone should really be giving out certificates to anyone who achieves this sleep-defying feat. Maybe something for the Astronomical League to consider? Some places get six passes during a 24-hour period: 3–4 in the early morning followed by 2–3 appearances that evening.
Of course you'll need to know exactly when and where to look. You can find the space station and get a map showing its path for your location at Heavens Above. Click the link, select your city, and then tap the ISS link for a list of passes in the coming nights. Click on a date to see a map and timeline. Or go to NASA’s Spot the Station site for times, where you can also sign up to get e-mail or text alerts whenever there's a favorable pass for your city. Apps are great, too, especially when they're free. Try ISS Spotter for iPhone and ISS Detector for Android.
What to Anticipate
The ISS orbits the Earth from west to east to take advantage of the planet's rotation, so it always rises in the west and tracks east. When low, the ISS is fainter than when overhead because of the added line-of-sight distance. It shines brightest east of the zenith, when the station is both close and also nearly opposite the Sun and lit up like a little full Moon. Here are some cool things to watch for in addition to the aforementioned sunsets and sunrises:
- Flares — Occasionally, sunlight will strike the arrays or an area on the station at just the right angle to reflect back to your eyes, causing the ISS to briefly flare in brightness.
- Jerky Movements — If you watch the ISS or any satellite closely it will appear to move jerkily as it crosses the sky. What you’re really seeing are your own jerky eye movements transposed on the sky. Our peepers are not well-oiled machines.
- Conjunctions — At Heavens Above you can get a map of any pass and note when the station crosses in front of or passes very close to a bright star or planet. Jupiter's a favorite and now gleams in the southeast around 10 p.m. local time.
- Beautiful Dumps — Astronauts have to occasionally dump wastewater outside the station which immediately crystallizes and expands into a cloud. If you ever look up to discover the ISS has a tail like a comet, you're witnessing a dump.
- Telescopic Revelations — With only a little practice you can center and follow the ISS in a low-power telescope. I anticipate its path while looking through my finderscope then quickly move to the eyepiece to track it. Once held in view, the solar arrays are easy to see at 60×, and you can even make out shapes in the station proper.
- Shareworthy Photos — You'll need a tripod and the ability to take a 15-second or longer exposure. Get a map from Heavens Above so you know where to point the camera. Set your lens and exposure to manual (M). Pre-focus on a bright star using the camera's Live View feature. If you don't know how to use it, click here for a great tutorial. Next, set the ISO for 800 or 1600 and open your lens up to f/2.8 or f/4. Most cameras cut off at 30 seconds of exposure, so you'll only get a segment of the ISS's complete path. For exposures longer than 30 seconds, either carefully hold down the shutter button or use a remote release that will keep the shutter open with a simple click. When using the release, don't forget to turn your shutter speed knob to “B," which allows for unlimited time exposures. Do a test exposure in advance to check the result.
Even if you get hit with cloudy weather this week, the space station will continue to make passes through early June. The next series of 24/7 sunlit passes begins in mid-July.