Many of us have seen the the International Space Station (ISS) pass overhead at night. Maybe once a night. How would you like to see it five times in in the span of one evening? This weekend, you’ll have your chance.
Passes starts a typical 6-minute run with an appearance low in the western sky traveling east. As the ISS climbs in altitude, it approaches the observer and gets brighter and brighter. After reaching a maximum altitude in either the northern or southern sky, it speeds off to the east and fades.
We see the space station normally only during mid to late twilight, when darkness is gathering on the ground, but the sun still shines on satellites high overhead. I like to compare it to seeing a tall mountaintop still catching the sun’s rays when it’s dark in the valley below.
One complete and one partial pass are normal for an evening’s skywatching. ‘Partials’ occur when the space station disappears early from view before it’s completed its circuit to the eastern horizon. Why? Its orbit finally takes it inside Earth’s shadow, which eclipses the station from view as surely as the shadow covers the moon during a lunar eclipse.
But in late May to early June each year, the space station’s orbit and Earth’s day-night terminator nearly align, making it possible to see four or even five ISS passes from dusk till dawn. The terminator is the line separating daylight from darkness. From the astronauts’ viewpoint, it’s the time of the midnight sun. Back down on the planet between latitudes 40-55° North, the ISS remains in sunlight during every 92 minute pass. 92 minutes is how long it takes the station to orbit the Earth traveling at approximately 17,150 mph.
In late May-early June, the sun doesn’t set on the International Space Station, so we get to see it during every pass it makes overnight.
This special “ISS marathon” will be in full swing tonight through early next week from many locations across the northern hemisphere. For example, in Duluth, Minn., if you’re willing to sacrifice sleep tonight, you’ll see four circuits of the space station starting at 10:48 p.m. in late twilight and ending at 3:38 a.m. tomorrow morning. Saturday night (May 28) is even better with five passes beginning at 9:55 p.m. Skywatchers in Philadelphia can catch five orbital loops tonight beginning at 10:13 p.m. local time and finishing Saturday morning at 4:42 a.m.
How do I know this? I always visit Heavens Above to find out when to look, where to look, how bright and other details for every ISS pass visible from and location on Earth. Head on over, find your city, then click the ISS link under the Satellites heading for a table of times when it’s visible. You’ll also find other particulars including brightness: the higher the negative number, the brighter the space station will appear. For comparison, Mars currently shines at magnitude –2.0, Vega at 0 and Venus, the brightest star-like object in the sky, at –4.
If you click the date link, a useful map will pop up with the space station’s track across the sky — most useful for anticipating where to begin looking and to get a feel for the direction and altitude of the pass.
Remember that the ISS always appears in the western sky first, contrary to the movement of the stars, and travels to the east. Low altitude passes put a lot of lateral distance between you and the station, making them fainter. Not by much though. Even on a low arc, the ISS shines as bright as Vega. Overhead passes means the ISS is as close as it can get – straight up at about 250 miles away with the station only a magnitude shy of the planet Venus. Absolutely stunning!
If you closely watch the ISS as it moves against the starry sky, it will appear to move jerkily. This would be very bad orbital maneuvering if true. What you’re really seeing are your own jerky eye movements transposed on the sky. Some of my favorite passes are those when the space station fades from view mid-track as it passes into Earth’s shadow. I always keep binoculars handy for these passes so I can watch the station turn orange and red as it experience one of its many orbital sunsets. Try it yourself.
While I really don’t expect many of you to stay up all night, it’s fun to know you can go out any time of night for the next week and catch a look at the football stadium-sized spaceship silently passing over your home or apartment. And if you want a gander at what the crew of astronauts are seeing when they look down to Earth, check out the live view site.