With the Space Shuttle Endeavour scheduled to undock from the International Space Station (ISS) on Friday evening, skywatchers across much of the United States and southern Canada are in for a real treat early on Saturday and Sunday morning.
Weather permitting on these mornings, there will be opportunities to see both the Endeavour orbiter and the ISS flying across the sky from many locations.
Appearing as a pair of very bright "stars," the ISS should appear as a somewhat brighter object and will be trailing Endeavour as they move across the sky. The pair will appear only a few degrees apart on Saturday morning, but the gap between the two will likely widen to perhaps 20-degrees or more by Sunday morning (your clenched fist held at arm's length measures roughly 10-degrees; so on Sunday morning expect Endeavour and the ISS to separated by about two fist-lengths).
The sight should easily be visible to anyone, even from brightly lit cities. Considering that after this mission there will be only be four left before the shuttle program ends (tentatively in September) the view of a shuttle orbiter and the Space Station flying in tandem will soon be a sight that will pass into history.
New York's View
The Tri-State Area will get two opportunities to see Endeavour flying with the ISS. The first will come on Saturday morning, just over 9 and a half hours after both vehicles have undocked; so they will still be relatively close to each other. The two spacecraft will emerge from out of the Earth's shadow at 5:21 a.m. EST, at an altitude of 18-degrees above the north-northwest horizon. They will attain a maximum altitude of 25-degrees above the north-northeast horizon at 5:23 a.m. and will drop toward the eastern horizon at 5:25 a.m. If you're familiar with the constellations, you'll see them glide between Cepheus and the "W" of Cassiopeia and pass under the bright star Deneb in Cygnus. Overall, this pass should last about 3 minutes and 50 seconds.
Your other opportunity will come early on Sunday. By then, the two spacecraft will be more widely separated (recall the "two fists" rule), but they will trace a much higher arc across the sky. Emerging from the Earth's shadow, about 15-degrees above the northwest horizon at 5:43 a.m. EST, Endeavour and the ISS will climb to a maximum altitude of 63-degrees above the northeast horizon at 5:45 a.m, then drop down toward the east-southeast horizon by 5:48. Overall, this pass should last 5 minutes 10 seconds! At their closest point (when highest in the sky), they should be about 240 miles away. If you're familiar with the constellations, they will appear to pass between the Big Dipper and Polaris (the North Star), then streak past the lozenge-shaped head of Draco, then, as they descend they will pass close to two of the brightest stars in the sky: Vega in Lyra and Altair in Aquila. Interestingly, the ground track for both vehicles shows that they will appear to pass directly overhead (or very nearly so) as seen from Catskill, NY in Columbia County.
Region of visibility
Generally speaking, the tandem will be visible across southern Canada and most of the 48 contiguous United States (Hawaii and Alaska will not have favorable viewing passes during this upcoming week).
Across southern Canada and the northern half of the United States there will be two or three morning viewing opportunities. For some favored locations, like Chicago and Milwaukee there will be as many as four opportunities. Over the southern United States, the viewing opportunities will be reduced to just one. Much of Florida (save for the Panhandle), central and southern Georgia, and parts of western and southern Texas will unfortunately be denied a view of the "dynamic duo" because they'll appear too low in the sky and too near to sunrise to be easily visible.
For other locations . . .
So what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these three web sites:
Each will ask for your zip code or city, and respond with a list of suggested spotting times. Predictions computed a few days ahead of time are usually accurate within a few minutes. However, they can change due to the slow decay of the space station's orbit and periodic reboosts to higher altitudes. Check frequently for updates.
Another great site is Real Time Satellite Tracking, which shows where the ISS or Space Shuttle are at any given moment during the day or night.
What to expect
Both vehicles will be traveling across North America on northwest-to-southeast trajectories.
A large telescope is needed to make out details of the sprawling station. Traveling in their respective orbits at approximately 18,000 mph (29,000 kph), both should be visible anywhere from about one to five minutes (depending on the particular viewing pass) as they glide with a steady speed across the sky.
Because of its size and configuration of highly reflective solar panels, the space station is now, by far, the brightest man-made object currently in orbit around the Earth.
On favorable passes, it approaches magnitude -5 in brightness, which would rival the planet Venus and is more than 25 times brighter than Sirius, the brightest star in the night sky. Some have even caught a glimpse of the ISS just prior to sunset or shortly after sunrise. And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly flare in brilliance.
Other satellites too
The appearance of either the space shuttle or the space station moving across the sky is not in itself unusual. On any clear evening within a couple of hours of local sunset 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 Endeavour is expected to undock from the ISS at 7:54 p.m. Eastern Standard Time on Friday. Endeavour will fly around the ISS before finally pulling away from the Station at 9:38 p.m. EST, although it should still remain at a relatively close distance to it until its scheduled return to the Kennedy Space Center in Florida Sunday evening.