Image: Space station flare
Martin Popek
In a time-exposure photograph, the international space station can be seen streaking across the sky over the town of Nydek in the Czech Republic in June 2007.
By
updated 11/5/2007 8:47:49 PM ET 2007-11-06T01:47:49

With the space shuttle Discovery having successfully undocked from the international space station early Monday, skywatchers across much of the United States and southern Canada are now in for a real treat on Tuesday and Wednesday mornings.

Weather permitting, there will be opportunities to see both the Discovery orbiter and the space station independently flying across the sky from many locations.

The sight should easily be visible to anyone, even from brightly lit cities.

The appearance of either the space shuttle or the space station moving across the sky is not in itself unusual. On any clear morning within a couple of hours of local sunrise and with no optical aid, you can usually spot several orbiting Earth satellites creeping across the sky like moving stars. Satellites become visible only when they are in sunlight and the observer is in deep twilight or darkness. This usually means shortly after dusk or before dawn.

What makes the prospective upcoming passages so interesting is that you'll be able to see the two largest orbiting space vehicles in the sky at the same time.

Shuttle Discovery undocked from the station at 5:32 a.m. ET on Monday.  Discovery then flew around the ISS before finally pulling away from the station at 7:18 a.m.  The shuttle is scheduled to return to the Kennedy Space Center in Florida Wednesday, Nov. 7, at 1:02 p.m.

What to expect
On Tuesday and Wednesday mornings, both vehicles will pass across North America three times at intervals spaced at 91.6 minutes and in each case moving on southwest-to-northeast trajectories

Appearing as a pair of very "bright 'stars," the space station should appear as the somewhat brighter object and will be trailing Discovery.  Early Monday morning, the two vehicles had just undocked as they began their series of passes over North America and were still very close together — so close that from many locations in the eastern and central United States they appeared not as separate objects, but rather as a single bright starlike object to the unaided eye.

It will be a different story on Tuesday and Wednesday mornings, however, as the two vehicles will have separated to a distance of 26 miles (42 kilometers). 

As seen from the ground, the apparent separation distance between the two vehicles should not appear to exceed 20-degrees.  (Your clenched fist, held at arm's length will measure roughly 10 of those degrees.)  So the maximum apparent distance between the ISS and the Discovery orbiter should not exceed "two fists" as they move across the sky.

Brighter than the stars
A telescope would be needed to make out details of the sprawling station, now orbiting at 220 miles (354 kilometers) above Earth's surface. Traveling in their respective orbits at about 17,188 mph (27,661 kilometers per hour), both should be visible on each pass for a duration of about 1 to 5 minutes.

Because of its size and configuration of highly reflective solar panels, the space station is now, by far, the brightest human-made object currently in orbit around the planet.  On favorable passes, it approaches magnitude -3 in brightness, which would rival the planet Jupiter and is four times brighter than Sirius, the brightest star in the night sky.

On future missions, additional solar arrays will be deployed, making the station appear even brighter. When completed in 2010, it might even rival Venus, the brightest planet.  And as a bonus, sunlight glinting directly off the solar panels can sometimes make the station appear to briefly flare in brilliance.

When and where to look
The shuttle-station tandem will be visible across southern Canada and most of the 48 contiguous United States.  Unfortunately, neither Hawaii nor Alaska will have favorable viewing passes during this week. 

Viewing times and locations are available at these sites:

Each will ask for either your city or your latitude and longitude, and respond with a list of suggested spotting times. Predictions that have been computed a few days ahead of time will usually be quite accurate to within less than a minute. However, they can change due to the slow decay of the space station's orbit and periodic reboosts to higher altitudes, so it is wise to check frequently for updates.

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