Observing Alert – Space Station ‘Marathon’ Starts This Week

What’s your favorite satellite? For me it’s the space station. Not only is it the brightest spacecraft in the sky, but it’s regularly visible from so many places. It’s also unique. Most satellites are either spent rocket stages or unmanned science and surveillance probes. The ISS is inhabited by a crew of astronauts. Real people.

Every time I see that bright, moving light I think of the crew floating about the cabin with their microgravity hair, performing experiments and pondering the meaning of it all while gazing out the cupola windows at the rolling blue Earth below. Starting Friday, the station will make up to 5 flybys a night from dusk till dawn. Marathon anyone?

The ISS’s orbit is inclined 51.6 degrees to the equator and passes overhead for anyone living between 51.6 degrees north and 51.6 degrees south latitude. It’s visible well beyond this zone also but never passes through the zenith outside of these limits. Traveling at a little more than 17,000 mph (27,350 kph) the station completes an orbit in 93 minutes.

Diagram showing the Earth in late May when the space station’s orbital track is closely aligned with the day-night terminator. The astronauts see the sun 24-hours a day (midnight sun effect) while we on the ground get to watch repeated passes. Credit: Bob King

Most of the time we get one easy-to-see bright pass preceded or followed by a fainter partial pass. ‘Partials’ occur when the space station glides into Earth’s shadow and disappears from view during an appearance. But in late May-early June each year, the space station’s orbit and Earth’s day-night terminator nearly align. From the astronauts’ viewpoint, the sun never sets, much like seeing the midnight sun from the Arctic Circle. From down on the planet between latitudes 40-55 degrees north, the ISS remains in sunlight during repeated 90 minute-long orbits.

Instead of once or twice a night, we’ll see passes all night long from dusk till dawn starting about May 30. For instance, on May 31 from Minneapolis, Minn., skywatchers will be treated to four flybys at 12:12 a.m, 1:44 a.m., 3:20 a.m. and 11:23 p.m. The best nights are June 4 and 6 with five passes. By the 10th, the space station ‘marathon’ winds down and we return to 2-3 passes a night.

In late May-early June near the summer solstice, the sun doesn’t set on the International Space Station

The ISS always appears in the western sky first and travels east opposite to the movement of the stars. Low altitude flybys are fainter because there’s more lateral distance between you and the station. Even then the it still shines as bright as Vega. But when the ISS flies overhead, it’s only about 250 miles away, as close as it gets. Then it outshines everything in the night sky except Venus and the moon. Absolutely stunning.

The track of the ISS near Vega in Lyra. From right to left, the station is passing from sunlight into Earth’s shadow. Its color transitions from white to red. Credit: Bob King

Have you ever noticed that satellites, including the ISS, appear to move in a jerky or zigzag fashion if you watch them closely? What you’re really are your own eyes not moving smoothly as you follow the satellite across the starry sky. My favorite passes are those where the space station fades away mid-flyby as it encounters Earth’s shadow. I always keep binoculars handy for these passes so I can watch the ISS turn color from pale yellow (caused by the gold Mylar plastic used in its many solar panels) to orange and red as it experiences one of its many orbital sunsets.

The phenomenon is easy to capture on camera too. Find out when the station will cross into shadow using the maps from Heavens-Above (see below) and point your tripod-mounted camera in that direction. I typically use a 35mm lens wide open to f/2.8 and a 30-second exposure at either ISO 400 – if still twilight – or 800 in a darker sky.

The multiple solar panels on the ISS give it the shape of the letter ‘H’ when viewed through a telescope. Other modules are visible too but hard to see as clearly.  Credit: NASA

There are many ways to find out when the ISS will pass over your city. My favorite are the listings in Heavens-Above. Login with your city and you’ll see a complete list with links to create maps of the station’s track across the sky. There’s also Spaceweather’s Satellite Flyby tracker. Type in your zip code and hit enter. Couldn’t be easier. You can also have NASA send you an e-mail when the most favorable (highest, brightest) passes occur by adding your e-mail to the Spot the Station site. Be aware though that you won’t be notified of some of the less favorable passes.

Half-minute video of the space station tracked through a telescope

One last pleasure of space station watching is seeing it in a telescope. Notoriously tricky to track when magnified, after minimal research I’ve come up with a method that allows at least a half dozen people to see it up close during a good flyby. One person mans the finderscope, keeping the station in the center of the crosshairs, while one happy observer after another takes their turn for a look through the eyepiece. Sure, it’s a little herky-jerky, but you’d be surprised how much you can see at magnifications as low as 60x. The solar panels really jump out. Observing solo might mean a couple tries positioning the moving target  ahead of where you think it will cross the field of view and then being ready to lock on and follow.

Well, I’m going to prep for the upcoming marathon. See you in spirit on the course!

Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

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