The annual International Space Station Marathon is underway with multiple passes visible each night. Here are some fun and unique ways to see and share it.
The International Space Station (ISS) will be your constant companion the next few weeks. The ship and crew of six astronauts will make visible passes for skywatchers in the Northern Hemisphere every hour and a half from dusk till dawn.
If you're game, you can even participate in what satellite watchers informally call an ISS Marathon. Clear skies and a crazed determination to stay awake all night, a trait common among our tribe, are all you need.
Every year in May and June, the space station remains in sunlight, or nearly so, throughout its orbit. From an astronaut’s point of view, the Sun never sets, the orbital equivalent of the Land of the Midnight Sun. For observers in the Northern Hemisphere, the ISS avoids eclipse by Earth's shadow and remains in full sunlight pass after pass. As the ISS circles the planet every 90 minutes, punctual skywatchers can witness up to 5–6 passes each night, the first in evening twilight and the last at dawn.
Normally, only one or two passes are visible at dusk or dawn with the others eclipsed by Earth's shadow. But around the time of solstice the station's orbit and Earth’s day–night terminator nearly align, and the shiny craft comes round again and again like a ball on a tether.
Around the time of the summer solstice for the Northern Hemisphere and winter solstice for the southern hemisphere, the Sun never sets on the ISS as seen in this video made on board.
The same situation plays out for skywatchers in the Southern Hemisphere in November–December when that hemisphere is tilted toward the Sun. On Sunday night (May 20th) I caught two ISS passes, the first at 10:17 p.m. and the second at 11:54 p.m. I could have stayed up for the 1:31 a.m., 3:07 a.m. and 4:44 a.m. passes, but work loomed on the 9:00 a.m. horizon.
To find out when and where to see the space station, try Heavens Above, one of my favorite sites. Select a city under the Configuration heading and you'll get a list of passes for each night. Click on the date for a map showing the station's path across the sky. Lots of people like the email alerts from NASA's Spot the Station. Or you can use a free app for your mobile phone like ISS Spotter for iPhone and ISS Detector for Android. The Sky & Telescope Satellite Tracker gives predictions for upcoming passes, while the Transit Tracker will let you know when the space station comes within 10° of any naked-eye planet, the Moon, or the Sun (or more rarely, when it crosses either Sun or Moon).
The ISS always appears in the western sky first and travels east. Look for a bright, pale yellow star, with a steady light. When seen at low altitude, the station appears fainter because there's more lateral or horizontal distance between you and the craft. Even then it still shines as bright as Vega. But when it makes a high or overhead pass, it’s only about 250 miles away, as close as it gets, and outshines everything in the night sky except Venus and the Moon.
Peak brightness occurs not when the station arrives at the zenith, but some distance beyond it to the east, when the Sun (on the opposite side of Earth) directly illuminates the satellite similar to what happens during a full Moon. For a short time, full illumination compensates for its increasing lateral distance. Similarly, when the ship first appears in the west, its angle to the Sun is slight, so we see a "crescent" ISS. This contributes to its fainter appearance at that time.
Jack be Quick
With so many opportunities for seeing the ISS, you've got time to try out some new tricks. If you've never seen it through a telescope, it's easier than you think. The amazing thing is that you can discern its shape, especially those golden, kapton-insulated solar arrays. I use a wide-field eyepiece with a magnification of 65×. Catch it first when it's low in the west and moving with a slower apparent speed. A low-power finderscope is key here. Aim the finder at the ISS and note its direction of travel, then set the crosshairs at the spot you anticipate the station to reach in a few seconds and quickly return to the eyepiece. Like cupping a leaping frog with your hand, you'll get the knack of it after a couple tries.
This video of the ISS through a telescope hints at the visual impression.
With the satellite in view, pull the scope along to follow, keeping the ISS centered in the field of view as best you can. Sure, it'll be a little herky-jerky, but you'll thrill to the sight during those seconds when you're locked on.
Once you've succeeded alone, get your friends or family to queue up at the eyepiece during another pass. Your job will now be to keep the ISS centered as best you can in the finder while moving the scope to follow. That gives each person an opportunity to look. I've had up to six people spot the ISS during a single pass. It makes for great fun at star parties.
If you like playing catch, you're going to love tossing the International Space Station around. Last year, I wondered how far apart two observers could be and still view the ISS simultaneously, so I e-mailed my friend Rick, who lives in Port Angeles, Washington, and we hatched a plan.
Our two cities are about 2,350 km apart as the crow flies. Because the station orbits at an altitude of about 400 km, an observer (at sea level) can see it up to about 2,300 km away at setting or rising. We figured that when the ISS stood midway between us — about 1,650 kilometers give or take — Rick should see it dropping off in the eastern sky the same time I'd see it rise in the west.
Next, we identified a suitable pass, one we could both see despite being two time zones apart. Since Rick lives in the far west, we selected an early pass for him so that by the time the station arrived over Duluth, it would still be in sunlight. The pass we chose had the ISS reaching maximum altitude at 9:34 p.m. Pacific Time for Port Angeles and 11:39 p.m. Central Time (9:39 p.m. Pacific) for Duluth, just 5 minutes later.
We got on the phone together at 9:30 p.m. PDT. A couple minutes later when my friend spotted the station, it was game on. At 9:35, when the ISS had crossed into the eastern sky for Port Angeles, I caught sight of it clearing the western horizon in Duluth. For 60–90 seconds we watched it simultaneously from 2,350 km apart!
About the time the ISS crossed my meridian, Rick caught a last glimpse in the east. I found the experience exhilarating. For a few moments my distant friend and I attended a live performance together on an Earth-sized stage.
I encourage you to play space station "catch" with a friend or family member. Fire up two Heavens Above tabs — one for each city — and comb through the list to find pass times that occur around the same time (deducting the time zone factor) for the two locations. Call a friend and share an out-of-this-world experience!