When the space shuttle Endeavour leaves the International Space Station skywatchers across much of the United States and southern Canada are in for a real treat early on Saturday and Sunday morning.
Weather permitting, there will be opportunities to see the space station and Endeavour and the space station flying across the sky from many locations once the shuttle undocks from the orbiting lab Friday night.
The sight should easily be visible to anyone, even from brightly lit cities. Considering that after this mission there will be only be four flights left before the space shuttle program ends (tentatively in September) the view of a shuttle orbiter and space station flying in tandem will soon be a sight that will pass into history.
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 Earth-orbiting 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 space station at 7:54 p.m. ET on Friday. Endeavour will fly around the space station before finally pulling away from the Station at 9:38 p.m. ET, 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.
What to expect
Both vehicles will be traveling across North America on northwest-to-southeast trajectories.
Appearing as a pair of very "bright stars," the station should appear as the somewhat brighter object and will be trailing Endeavour as they move across the sky. The pair should 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 station to separated by about "two fists").
A large telescope would be 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 station just prior to sunset or shortly after sunrise. And as a bonus, sunlight glinting directly off the solar panels can sometimes make the station appear to briefly flare in brilliance.
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 be 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.
In contrast. some localities will be favored with exceptionally good views. From Milwaukee, for instance, the space station and the shuttle will appear to soar to an altitude of 86-degrees — virtually overhead — during a 4-minute over fly beginning at 5:53 a.m. CT.
Europeans will also be favored with views in their pre-sunrise skies. London, for instance will have four opportunities to see the Endeavour and the space station: two early on Saturday morning and two more on Sunday morning.
The best of these will come on Saturday beginning at around 05:47 GMT. The two spacecraft will appear to emerge from the Earth's shadow low in the west-southwest, quickly reaching a maximum altitude of 28 degrees up in the south-southwest, then taking another two minutes to glide over toward the south-southeast horizon before finally disappearing from view.
When and where to look
So what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these four popular Web sites: Chris Peat's Heavens Above, Science@NASA's J-Pass, NASA's SkyWatch and Spaceweather.com.
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 this one, which provides real-time satellite tracking and shows you at any given moment during the day or night over what part of the Earth the space station or shuttle happens to be.
Endeavour's six-astronaut crew is wrapping up a 14-day mission to the space station. The astronauts performed three spacewalks and tricky robotic arm work to install a brand-new room called Tranquility and a stunning observation deck to the orbiting laboratory.
The shuttle is due to land Sunday night at 10:16 p.m. ET.